- Working with Query Expressions
- Placing Conditions on Queries
- Learning How to Use Operators
- Summary
- Q&A
This chapter is from the book
This chapter is from the book
This chapter is from the book
Learning How to Use Operators
Operators are the elements you use inside an expression to articulate how you want specified conditions to retrieve data. Operators fall into six groups: arithmetic, comparison, character, logical, set, and miscellaneous. MySQL utilizes three types of operators: arithmetic, comparison, and logical.
Arithmetic Operators
The arithmetic operators are plus (+), minus (–), divide (/), multiply (*), and modulo (%). The first four are self-explanatory. Modulo returns the integer remainder of a division. Here are two examples:
5 % 2 = 1 6 % 2 = 0
The modulo operator does not work with data types that have decimals, such as Real or Number.
If you place several of these arithmetic operators in an expression without any parentheses, the operators are resolved in this order: multiplication, division, modulo, addition, and subtraction. For example, the expression
2*6+9/3
equals
12 + 3 = 15
However, the expression
2 * (6 + 9) / 3
equals
2 * 15 / 3 = 10
Watch where you put those parentheses! Sometimes the expression does exactly what you tell it to do, rather than what you want it to do. The same holds true for MySQL.
The following sections examine the arithmetic operators in some detail and give you a chance to write some queries.
Plus (+)
You can use the plus sign in several ways. Type the following statement to display the PRICE table:
SQL> SELECT * FROM PRICE; mysql> select * from price; ITEM WHOLESALE --------------- --------- TOMATOES .34 POTATOES .51 BANANAS .67 TURNIPS .45 CHEESE .89 APPLES .23 6 rows selected. +----------+-----------+ | item | wholesale | +----------+-----------+ | TOMATOES | 0.34 | | POTATOES | 0.51 | | BANANAS | 0.67 | | TURNIPS | 0.45 | | CHEESE | 0.89 | | APPLES | 0.23 | +----------+-----------+ 6 rows in set (0.11 sec)
Now type
SQL> SELECT ITEM, WHOLESALE, WHOLESALE + 0.15 FROM PRICE; mysql> select item, wholesale, wholesale+.15 -> from price;
Here the + adds 15 cents to each price to produce the following:
ITEM WHOLESALE WHOLESALE+0.15 --------------- --------- --------------- TOMATOES .34 .49 POTATOES .51 .66 BANANAS .67 .82 TURNIPS .45 .60 CHEESE .89 1.04 APPLES .23 .38 6 rows selected. +----------+-----------+---------------+ | item | wholesale | wholesale+.15 | +----------+-----------+---------------+ | TOMATOES | 0.34 | 0.49 | | POTATOES | 0.51 | 0.66 | | BANANAS | 0.67 | 0.82 | | TURNIPS | 0.45 | 0.60 | | CHEESE | 0.89 | 1.04 | | APPLES | 0.23 | 0.38 | +----------+-----------+---------------+ 6 rows in set (0.00 sec)
What is this last column with the unattractive column heading WHOLESALE+0.15? It's not in the original table. SQL allows you to create a virtual or derived column by combining or modifying existing columns.
Retype the original entry:
SQL> SELECT * FROM PRICE;
The following table results:
ITEM WHOLESALE --------------- --------- TOMATOES .34 POTATOES .51 BANANAS .67 TURNIPS .45 CHEESE .89 APPLES .23 6 rows selected. +----------+-----------+ | item | wholesale | +----------+-----------+ | TOMATOES | 0.34 | | POTATOES | 0.51 | | BANANAS | 0.67 | | TURNIPS | 0.45 | | CHEESE | 0.89 | | APPLES | 0.23 | +----------+-----------+ 6 rows in set (0.11 sec)
The output confirms that the original data has not been changed and that the column heading WHOLESALE+0.15 is not a permanent part of it. In fact, the column heading is so unattractive that you should do something about it.
Type the following:
SQL> SELECT ITEM, WHOLESALE, (WHOLESALE + 0.15) RETAIL FROM PRICE;
Here's the result:
ITEM WHOLESALE RETAIL --------------- --------- ------ TOMATOES .34 .49 POTATOES .51 .66 BANANAS .67 .82 TURNIPS .45 .60 CHEESE .89 1.04 APPLES .23 .38 6 rows selected. mysql> select item, wholesale, wholesale+.15 retail -> from price; +----------+-----------+--------+ | item | wholesale | retail | +----------+-----------+--------+ | TOMATOES | 0.34 | 0.49 | | POTATOES | 0.51 | 0.66 | | BANANAS | 0.67 | 0.82 | | TURNIPS | 0.45 | 0.60 | | CHEESE | 0.89 | 1.04 | | APPLES | 0.23 | 0.38 | +----------+-----------+--------+ 6 rows in set (0.01 sec)
This is wonderful! Not only can you create new output columns, but you can also rename them on the fly. You can rename any of the columns using the syntax column_name alias. (Note the space between the column_name and alias.)
For example, the query
SQL> SELECT ITEM PRODUCE, WHOLESALE, WHOLESALE + 0.25 RETAIL FROM PRICE;
renames the columns as follows:
PRODUCE WHOLESALE RETAIL --------------- --------- ------- TOMATOES .34 .59 POTATOES .51 .76 BANANAS .67 .92 TURNIPS .45 .70 CHEESE .89 1.14 APPLES .23 .48 mysql> select item Produce, wholesale, wholesale+0.25 -> from price; +----------+-----------+----------------+ | Produce | wholesale | wholesale+0.25 | +----------+-----------+----------------+ | TOMATOES | 0.34 | 0.59 | | POTATOES | 0.51 | 0.76 | | BANANAS | 0.67 | 0.92 | | TURNIPS | 0.45 | 0.70 | | CHEESE | 0.89 | 1.14 | | APPLES | 0.23 | 0.48 | +----------+-----------+----------------+ 6 rows in set (0.00 sec)
NOTE
Some implementations of SQL use the syntax <column name = alias>. The preceding example would be written as follows:
SQL> SELECT ITEM = PRODUCE, WHOLESALE, WHOLESALE + 0.25 = RETAIL, FROM PRICE;
NOTE
Check your implementation for the exact syntax.
MySQL allows you to present your column alias in mixed case.
You might be wondering what use aliasing is if you are not using command-line SQL. Fair enough. Have you ever wondered how report builders work? Some day, when you are asked to write a report generator, you'll remember this and not spend weeks reinventing what Dr. Codd and IBM have wrought.
In some implementations of SQL, the plus sign does double duty as a character operator. You'll see that side of the plus sign a little later today.
Minus (-)
Minus also has two uses. First, it can change the sign of a number. You can use the table HILOW to demonstrate this function.
SQL> SELECT * FROM HILOW; STATE HIGHTEMP LOWTEMP ------ -------- ------- CA -50 120 FL 20 110 LA 15 99 ND -70 101 NE -60 100
For example, here's a way to manipulate the data:
SQL> SELECT STATE, -HIGHTEMP LOWS, -LOWTEMP HIGHS FROM HILOW; STATE LOWS HIGHS ----- ---- ----- CA 50 -120 FL -20 -110 LA -15 -99 ND 70 -101 NE 60 -100 mysql> select state, -highs Lows, -lows Highs -> from hilow; +-------+------+-------+ | state | Lows | Highs | +-------+------+-------+ | FL | -110 | -20 | | LA | -99 | -15 | | ND | -101 | 0 | | NE | -100 | 0 | +-------+------+-------+ 4 rows in set (0.04 sec)
NOTE
Notice that the minus sign was reversed on the temperatures.
The second (and obvious) use of the minus sign is to subtract one column from another—for example,
SQL> SELECT STATE, 2 HIGHTEMP LOWS, 3 LOWTEMP HIGHS, 4 (LOWTEMP - HIGHTEMP) DIFFERENCE 5 FROM HILOW; STATE LOWS HIGHS DIFFERENCE ----- ---- ----- ---------- CA -50 120 170 FL 20 110 90 LA 15 99 84 ND -70 101 171 NE -60 100 160 mysql> select state, -> highs, -> lows, -> highs - lows Difference -> from hilow; +-------+-------+------+------------+ | state | highs | lows | Difference | +-------+-------+------+------------+ | FL | 110 | 20 | 90 | | LA | 99 | 15 | 84 | | ND | 101 | 0 | 101 | | NE | 100 | 0 | 100 | +-------+-------+------+------------+ 4 rows in set (0.25 sec)
Notice the use of aliases to fix the data that was entered incorrectly. This remedy is merely a temporary patch, though, and not a permanent fix. You should see to it that the data is corrected and entered correctly in the future. On Bonus Day 1, "Debugging Your SQL Statements," you'll learn how to correct bad data.
This query not only fixed (at least visually) the incorrect data but also created a new column containing the difference between the highs and lows of each state.
If you accidentally use the minus sign on a character field, you get something like this:
SQL> SELECT -STATE FROM HILOW; ERROR: ORA-01722: invalid number no rows selected
The exact error message varies with implementation. Here is an example using MySQL:
mysql> select -state -> from hilow; +--------+ | -state | +--------+ | 0 | | 0 | | 0 | | 0 | +--------+ 4 rows in set (0.00 sec)
MySQL evaluated the SELECT statement, but as you can see, the results are rather meaningless.
Divide (/)
The division operator has only the one, obvious meaning. Using the table PRICE, type the following:
SQL> SELECT * FROM PRICE; ITEM WHOLESALE -------- --------- TOMATOES .34 POTATOES .51 BANANAS .67 TURNIPS .45 CHEESE .89 APPLES .23 6 rows selected. mysql> select * from price; +----------+-----------+ | item | wholesale | +----------+-----------+ | TOMATOES | 0.34 | | POTATOES | 0.51 | | BANANAS | 0.67 | | TURNIPS | 0.45 | | CHEESE | 0.89 | | APPLES | 0.23 | +----------+-----------+
You can show the effects of a two-for-one sale by typing the next statement:
SQL> SELECT ITEM, WHOLESALE, (WHOLESALE/2) SALEPRICE 2 FROM PRICE; ITEM WHOLESALE SALEPRICE --------------- --------- --------- TOMATOES .34 .17 POTATOES .51 .255 BANANAS .67 .335 TURNIPS .45 .225 CHEESE .89 .445 APPLES .23 .115 6 rows selected. mysql> SELECT ITEM, WHOLESALE, WHOLESALE/2 Saleprice -> from price; +----------+-----------+-----------+ | ITEM | WHOLESALE | Saleprice | +----------+-----------+-----------+ | TOMATOES | 0.34 | 0.1700 | | POTATOES | 0.51 | 0.2550 | | BANANAS | 0.67 | 0.3350 | | TURNIPS | 0.45 | 0.2250 | | CHEESE | 0.89 | 0.4450 | | APPLES | 0.23 | 0.1150 | +----------+-----------+-----------+ 6 rows in set (0.26 sec)
The use of division in the preceding SELECT statement is straightforward (except that coming up with half pennies can be tough).
Multiply (*)
The multiplication operator is also straightforward. Again, using the PRICE table, type the following:
SQL> SELECT * FROM PRICE; ITEM WHOLESALE --------------- --------- TOMATOES .34 POTATOES .51 BANANAS .67 TURNIPS .45 CHEESE .89 APPLES .23 6 rows selected.
The output from this query reflects an across-the-board 10% discount. The actual data in the table has not changed.
SQL> SELECT ITEM, WHOLESALE, WHOLESALE * 0.9 NEWPRICE FROM PRICE; ITEM WHOLESALE NEWPRICE --------------- --------- -------- TOMATOES .34 .306 POTATOES .51 .459 BANANAS .67 .603 TURNIPS .45 .405 CHEESE .89 .801 APPLES .23 .207 6 rows selected. mysql> select Item, -> Wholesale, Wholesale * 0.9 "New Price" -> from price; +----------+-----------+-----------+ | Item | Wholesale | New Price | +----------+-----------+-----------+ | TOMATOES | 0.34 | 0.31 | | POTATOES | 0.51 | 0.46 | | BANANAS | 0.67 | 0.60 | | TURNIPS | 0.45 | 0.41 | | CHEESE | 0.89 | 0.80 | | APPLES | 0.23 | 0.21 | +----------+-----------+-----------+ 6 rows in set (0.00 sec)
NOTE
One last thing about aliases. By the use of quotes surrounding your aliases, you can give your column a two-word heading.
These operators enable you to perform powerful calculations in a SELECT statement.
Modulo (%)
The modulo operator returns the integer remainder of the division operation. Using the table REMAINS, type the following:
SQL> SELECT * FROM REMAINS;
NUMERATOR DENOMINATOR
--------- -----------
10 5
8 3
23 9
40 17
1024 16
85 34
6 rows selected.
mysql> select * from remains;
+-----------+-------------+
| numerator | denominator |
+-----------+-------------+
| 10 | 5 |
| 8 | 3 |
| 23 | 9 |
| 40 | 17 |
| 1024 | 16 |
| 85 | 34 |
+-----------+-------------+
6 rows in set (0.43 sec)
You can also create a new output column, REMAINDER, to hold the values of NUMERATOR % DENOMINATOR:
SQL> SELECT NUMERATOR,
DENOMINATOR,
NUMERATOR%DENOMINATOR REMAINDER
FROM REMAINS;
NUMERATOR DENOMINATOR REMAINDER
--------- ----------- ---------
10 5 0
8 3 2
23 9 5
40 17 6
1024 16 0
85 34 17
6 rows selected.
mysql> select numerator, denominator, numerator%denominator
-> from remains;
+-----------+-------------+-----------------------+
| numerator | denominator | numerator%denominator |
+-----------+-------------+-----------------------+
| 10 | 5 | 0 |
| 8 | 3 | 2 |
| 23 | 9 | 5 |
| 40 | 17 | 6 |
| 1024 | 16 | 0 |
| 85 | 34 | 17 |
+-----------+-------------+-----------------------+
6 rows in set (0.01 sec)
Some implementations of SQL implement modulo as a function called MOD (see Day 4, "Molding Data with Built-in Functions"). The following statement produces results that are identical to the results in the preceding statement:
SQL> SELECT NUMERATOR, DENOMINATOR, MOD(NUMERATOR,DENOMINATOR) REMAINDER FROM REMAINS; mysql> select numerator, denominator, -> mod(numerator,denominator) -> from remains; +-----------+-------------+----------------------------+ | numerator | denominator | mod(numerator,denominator) | +-----------+-------------+----------------------------+ | 10 | 5 | 0 | | 8 | 3 | 2 | | 23 | 9 | 5 | | 40 | 17 | 6 | | 1024 | 16 | 0 | | 85 | 34 | 17 | +-----------+-------------+----------------------------+ 6 rows in set (0.00 sec)
Precedence
Precedence is the order in which an implementation will evaluate different operators in the same expression. This section examines the use of precedence in a SELECT statement. Using the table PRECEDENCE, type the following:
SQL> SELECT * FROM PRECEDENCE;
N1 N2 N3 N4
------- -------- -------- --------
1 2 3 4
13 24 35 46
9 3 23 5
63 2 45 3
7 2 1 4
mysql> select * from precedence;
+----+----+----+----+
| n1 | n2 | n3 | n4 |
+----+----+----+----+
| 1 | 2 | 3 | 4 |
| 13 | 24 | 35 | 46 |
| 9 | 3 | 23 | 5 |
| 63 | 2 | 45 | 3 |
| 7 | 2 | 1 | 4 |
+----+----+----+----+
5 rows in set (0.00 sec)
Use the following code segment to test precedence:
SQL> SELECT
2 N1+N2*N3/N4,
3 (N1+N2)*N3/N4,
4 N1+(N2*N3)/N4
5 FROM PRECEDENCE;
N1+N2*N3/N4 (N1+N2)*N3/N4 N1+(N2*N3)/N4
----------- ------------- -------------
2.5 2.25 2.5
31.26087 28.152174 31.26087
22.8 55.2 22.8
93 975 93
7.5 2.25 7.5
mysql> select n1+n2*n3/n4,
-> (n1+n2)*n3,n4,
-> n1+(n2*n3)/n4
-> from precedence;
+-------------+------------+----+---------------+
| n1+n2*n3/n4 | (n1+n2)*n3 | n4 | n1+(n2*n3)/n4 |
+-------------+------------+----+---------------+
| 2.50 | 9 | 4 | 2.50 |
| 31.26 | 1295 | 46 | 31.26 |
| 22.80 | 276 | 5 | 22.80 |
| 93.00 | 2925 | 3 | 93.00 |
| 7.50 | 9 | 4 | 7.50 |
+-------------+------------+----+---------------+
5 rows in set (0.00 sec)
Notice that the first and last columns are identical. If you added a fourth column N1+N2*(N3/N4), its values would also be identical to those of the current first and last columns.
Comparison Operators
True to their name, comparison operators compare expressions and return one of three values: TRUE, FALSE, or UNKNOWN. Wait a minute! Unknown? TRUE and FALSE are self-explanatory, but what is UNKNOWN?
To understand how you could get an UNKNOWN, you need to know a little about the concept of NULL. In database terms, NULL is the absence of data in a field. It does not mean that a column has a zero or a blank in it. A zero or a blank is a value. NULL means nothing is in that field.
If you make a comparison such as Field = 9 and the only acceptable value for Field is NULL, the comparison will come back UNKNOWN. Because UNKNOWN is an uncomfortable condition, most flavors of SQL change UNKNOWN to FALSE and provide a special operator, IS NULL, to test for a NULL condition.
Here's an example of NULL: Suppose an entry in the PRICE table does not contain a value for WHOLESALE. The results of a query might look like this:
SQL> SELECT * FROM PRICE; ITEM WHOLESALE --------- --------- TOMATOES .34 POTATOES .51 BANANAS .67 TURNIPS .45 CHEESE .89 APPLES .23 ORANGES
Notice that no value appears in the WHOLESALE field position for ORANGES. The value of the field WHOLESALE for ORANGES is NULL. The NULL is noticeable in this case because it is in a numeric column. However, if the NULL appeared in the ITEM column, it would be impossible to tell the difference between NULL and a blank.
Try to find the NULL:
SQL> SELECT * 2 FROM PRICE 3 WHERE WHOLESALE IS NULL; ITEM WHOLESALE -------- --------- ORANGES
As you can see by the output, ORANGES is the only item whose value for -WHOLESALE is NULL, or does not contain a value. What if you use the equal sign (=) instead?
SELECT * FROM PRICE WHERE WHOLESALE = NULL; no rows selected
You wouldn't find anything because the comparison WHOLESALE = NULL returned a FALSE—the result was unknown. It would be more appropriate to use an IS NULL instead of =, changing the WHERE statement to WHERE WHOLESALE IS NULL. In this case, you would get all the rows where a NULL existed.
This example also illustrates both the use of the most common comparison operator (=) and the playground of all comparison operators, the WHERE clause. You already know about the WHERE clause, so here's a brief look at the equal sign.
Equal Sign (=)
Earlier today you saw how some implementations of SQL use the equal sign in the SELECT clause to assign an alias. In the WHERE clause, the equal sign is the most commonly used comparison operator. Used alone, the equal sign is a very convenient way of selecting one value out of many. Try this:
SQL> SELECT * FROM FRIENDS; LASTNAME FIRSTNAME AREACODE PHONE ST ZIP -------- --------- -------- -------- -------- BUNDY AL 100 555-1111 IL 22333 MEZA AL 200 555-2222 UK MERRICK BUD 300 555-6666 CO 80212 MAST JD 381 555-6767 LA 23456 BULHER FERRIS 345 555-3223 IL 23332
Let's find JD's row. (On a short list this task appears trivial, but you may have more friends than we do—or you may have a list with thousands of records.)
SQL> SELECT * FROM FRIENDS WHERE FIRSTNAME = 'JD'; LASTNAME FIRSTNAME AREACODE PHONE ST ZIP -------- --------- -------- -------- -- ----- MAST JD 381 555-6767 LA 23456 mysql> select * from friends -> where firstname = 'JD'; +----------+-----------+----------+----------+----+-------+ | lastname | firstname | areacode | phone | st | zip | +----------+-----------+----------+----------+----+-------+ | MAST | JD | 381 | 555-6767 | LA | 23456 | +----------+-----------+----------+----------+----+-------+ 1 row in set (0.37 sec)
We got the result that we expected. Try this:
SQL> SELECT * FROM FRIENDS WHERE FIRSTNAME = 'AL'; LASTNAME FIRSTNAME AREACODE PHONE ST ZIP -------- --------- -------- -------- -- ----- BUNDY AL 100 555-1111 IL 22333 MEZA AL 200 555-2222 UK
NOTE
Here you see that = can pull in multiple records. Notice that ZIP is blank on the second record. ZIP is a character field (you learn how to create and populate tables on Day 9, "Creating and Maintaining Tables"), and in this particular record, the NULL demonstrates that a NULL in a character field is impossible to differentiate from a blank field.
Here's another very important lesson concerning case sensitivity:
SQL> SELECT FIRSTNAME FROM FRIENDS WHERE FIRSTNAME = 'BUD'; FIRSTNAME --------------- BUD 1 row selected. mysql> select firstname from friends where firstname = 'BUD'; +-----------+ | firstname | +-----------+ | BUD | +-----------+ 1 row in set (0.00 sec)
Now try this:
SQL> select FIRSTNAME from friends where firstname = 'Bud'; no rows selected. mysql> select firstname -> from friends -> where firstname = 'bud'; +-----------+ | firstname | +-----------+ | BUD | +-----------+ 1 row in set (0.01 sec)
Even though SQL syntax is not case sensitive, data within it is, at least in some implementations. As you can see in the preceding examples, data stored in an Oracle database (SQL*Plus) is case sensitive while the MySQL example demonstrates the opposite.
Most companies prefer to store data in uppercase to provide data consistency. I recommend that you always store data either in all uppercase or in all lowercase, regardless of what type or database you are working in. Mixing case might create difficulties when you try to retrieve accurate data through comparisons in the WHERE clause.
Greater Than (>) and Greater Than or Equal To (>=)
The greater than operator (>) works like this:
SQL> SELECT * FROM FRIENDS WHERE AREACODE > 300; LASTNAME FIRSTNAME AREACODE PHONE ST ZIP -------- --------- -------- -------- -- ------ MAST JD 381 555-6767 LA 23456 BULHER FERRIS 345 555-3223 IL 23332
This example found all the area codes greater than (but not including) 300. To include 300, type this:
SQL> SELECT * 2 FROM FRIENDS 3 WHERE AREACODE >= 300; LASTNAME FIRSTNAME AREACODE PHONE ST ZIP -------- --------- -------- -------- -- ----- MERRICK BUD 300 555-6666 CO 80212 MAST JD 381 555-6767 LA 23456 BULHER FERRIS 345 555-3223 IL 23332 mysql> select * from friends -> where areacode >= 300; +----------+-----------+----------+----------+----+-------+ | lastname | firstname | areacode | phone | st | zip | +----------+-----------+----------+----------+----+-------+ | MERRICK | BUD | 300 | 555-6666 | CO | 80212 | | MAST | JD | 381 | 555-6767 | LA | 23456 | | BULHER | FERRIS | 345 | 555-3223 | IL | 23332 | +----------+-----------+----------+----------+----+-------+ 3 rows in set (0.34 sec)
With this change you get area codes starting at 300 and going up. You could achieve the same results with the statement AREACODE > 299.
NOTE
Notice that no quotes surround 300 in this SQL statement. Number-defined fields do not require quotes.
Less Than (<) and Less Than or Equal To (<=)
As you might expect, these comparison operators work the same way as > and >= work, only in reverse:
SQL> SELECT * 2 FROM FRIENDS 3 WHERE ST < 'LA'; LASTNAME FIRSTNAME AREACODE PHONE ST ZIP -------- --------- -------- -------- -- ----- BUNDY AL 100 555-1111 IL 22333 MERRICK BUD 300 555-6666 CO 80212 BULHER FERRIS 345 555-3223 IL 23332 mysql> select * from friends where st < 'LA'; +----------+-----------+----------+----------+----+-------+ | lastname | firstname | areacode | phone | st | zip | +----------+-----------+----------+----------+----+-------+ | BUNDY | AL | 100 | 555-1111 | IL | 22333 | | MERRICK | BUD | 300 | 555-6666 | CO | 80212 | | BULHER | FERRIS | 345 | 555-3223 | IL | 23332 | +----------+-----------+----------+----------+----+-------+ 3 rows in set (0.00 sec)
NOTE
In an Oracle database, if the column has only two characters, the column name is shortened to two characters in the returned rows. If the column name had been COWS, it would come out CO. The widths of AREACODE and PHONE are wider than their column names, so they are not truncated.
Wait a minute. Did you just use < on a character field? Of course you did. You can use any of these operators on any data type. The result varies by data type. For example, use lowercase in the following state search:
SQL> SELECT * 2 FROM FRIENDS 3 WHERE STATE < 'la'; LASTNAME FIRSTNAME AREACODE PHONE ST ZIP -------- --------- -------- -------- -- ----- BUNDY AL 100 555-1111 IL 22333 MEZA AL 200 555-2222 UK MERRICK BUD 300 555-6666 CO 80212 MAST JD 381 555-6767 LA 23456 BULHER FERRIS 345 555-3223 IL 23332 mysql> select * from friends where st < 'la'; +----------+-----------+----------+----------+----+-------+ | lastname | firstname | areacode | phone | st | zip | +----------+-----------+----------+----------+----+-------+ | BUNDY | AL | 100 | 555-1111 | IL | 22333 | | MEZA | AL | 200 | 555-2222 | UK | | | MERRICK | BUD | 300 | 555-6666 | CO | 80212 | | MAST | JD | 381 | 555-6767 | LA | 23456 | | BULHER | FERRIS | 345 | 555-3223 | IL | 23332 | +----------+-----------+----------+----------+----+-------+ 3 rows in set (0.00 sec)
Uppercase is usually sorted before lowercase; therefore, the uppercase codes returned are less than 'la'. Again, to be safe, check your implementation.
TIP
To be sure of how these operators will behave, check your language tables. Most PC implementations use the ASCII tables.
To include the state of Louisiana in the original search, type
SQL> SELECT * 2 FROM FRIENDS 3 WHERE STATE <= 'LA'; LASTNAME FIRSTNAME AREACODE PHONE ST ZIP -------- --------- -------- -------- -- ----- BUNDY AL 100 555-1111 IL 22333 MERRICK BUD 300 555-6666 CO 80212 MAST JD 381 555-6767 LA 23456 BULHER FERRIS 345 555-3223 IL 23332 mysql> select * from friends where st <= 'LA'; +----------+-----------+----------+----------+----+-------+ | lastname | firstname | areacode | phone | st | zip | +----------+-----------+----------+----------+----+-------+ | BUNDY | AL | 100 | 555-1111 | IL | 22333 | | MERRICK | BUD | 300 | 555-6666 | CO | 80212 | | MAST | JD | 381 | 555-6767 | LA | 23456 | | BULHER | FERRIS | 345 | 555-3223 | IL | 23332 | +----------+-----------+----------+----------+----+-------+ 4 rows in set (0.00 sec)
Inequalities (< > or !=)
When you need to find everything except for certain data, use the inequality symbol, which can be either < > or !=, depending on your SQL implementation. For example, to find everyone who is not AL, type this:
SQL> SELECT * 2 FROM FRIENDS 3 WHERE FIRSTNAME <> 'AL'; LASTNAME FIRSTNAME AREACODE PHONE ST ZIP -------- --------- -------- -------- -- ----- MERRICK BUD 300 555-6666 CO 80212 MAST JD 381 555-6767 LA 23456 BULHER FERRIS 345 555-3223 IL 23332 mysql> select * from friends where st <> 'AL'; +----------+-----------+----------+----------+----+-------+ | lastname | firstname | areacode | phone | st | zip | +----------+-----------+----------+----------+----+-------+ | BUNDY | AL | 100 | 555-1111 | IL | 22333 | | MERRICK | BUD | 300 | 555-6666 | CO | 80212 | | MAST | JD | 381 | 555-6767 | LA | 23456 | | BULHER | FERRIS | 345 | 555-3223 | IL | 23332 | +----------+-----------+----------+----------+----+-------+ 4 rows in set (0.00 sec)
To find everyone not living in California, type this:
SQL> SELECT * 2 FROM FRIENDS 3 WHERE STATE != 'CA'; LASTNAME FIRSTNAME AREACODE PHONE ST ZIP -------- --------- -------- -------- -- ----- BUNDY AL 100 555-1111 IL 22333 MEZA AL 200 555-2222 UK MERRICK BUD 300 555-6666 CO 80212 MAST JD 381 555-6767 LA 23456 BULHER FERRIS 345 555-3223 IL 23332 mysql> select * from friends where st != 'CA'; +----------+-----------+----------+----------+----+-------+ | lastname | firstname | areacode | phone | st | zip | +----------+-----------+----------+----------+----+-------+ | BUNDY | AL | 100 | 555-1111 | IL | 22333 | | MERRICK | BUD | 300 | 555-6666 | CO | 80212 | | MAST | JD | 381 | 555-6767 | LA | 23456 | | BULHER | FERRIS | 345 | 555-3223 | IL | 23332 | +----------+-----------+----------+----------+----+-------+ 4 rows in set (0.00 sec)
NOTE
Notice that both symbols, <> and !=, can express "not equal" in the two implementations we have shown you.
Character Operators
You can use character operators to manipulate the way character strings are represented, both in the output of data and in the process of placing conditions on data to be retrieved. This section describes two character operators: the LIKE operator and the || operator, the latter of which conveys the concept of character concatenation.
I Want to Be Like LIKE
What if you wanted to select parts of a database that fit a pattern but weren't quite exact matches? You could use the equal sign and run through all the possible cases, but that process would be boring and time consuming. Instead, you can use LIKE. Consider the following:
SQL> SELECT * FROM PARTS; NAME LOCATION PARTNUMBER ----------- ----------- ---------- APPENDIX MID-STOMACH 1 ADAMS APPLE THROAT 2 HEART CHEST 3 SPINE BACK 4 ANVIL EAR 5 KIDNEY MID-BACK 6
How can you find all the parts located in the back? A quick visual inspection of this simple table shows that it has two parts, but unfortunately the locations have slightly different names. Try this:
SQL> SELECT * 2 FROM PARTS 3 WHERE LOCATION LIKE '%BACK%'; NAME LOCATION PARTNUMBER ------- -------- ---------- SPINE BACK 4 KIDNEY MID-BACK 6
You can see the use of the percent sign (%) in the statement after LIKE. When used inside a LIKE expression, % is a wildcard. What you asked for was any occurrence of BACK in the column location. If you queried
SQL> SELECT * FROM PARTS WHERE LOCATION LIKE 'BACK%';
you would get any occurrence that started with BACK:
NAME LOCATION PARTNUMBER ----- -------- ---------- SPINE BACK 4 mysql> select * from parts where location like 'BACK%'; +-------+----------+------------+ | name | location | partnumber | +-------+----------+------------+ | SPINE | BACK | 4 | +-------+----------+------------+ 1 row in set (0.00 sec)
If you queried
SQL> SELECT * FROM PARTS WHERE NAME LIKE 'A%';
you would get any name that starts with A:
NAME LOCATION PARTNUMBER ----------- ----------- ---------- APPENDIX MID-STOMACH 1 ADAMS APPLE THROAT 2 ANVIL EAR 5
Is LIKE case sensitive in both Oracle and MySQL? Try the next query to find out.
SQL> SELECT * FROM PARTS WHERE NAME LIKE 'a%'; no rows selected mysql> select * from parts where name like 'a%'; +-------------+-------------+------------+ | name | location | partnumber | +-------------+-------------+------------+ | APPENDIX | MID-STOMACH | 1 | | ADAMS APPLE | THROAT | 2 | | ANVIL | EAR | 5 | +-------------+-------------+------------+ 3 rows in set (0.00 sec)
The answer is yes in Oracle and no in MySQL. References to data are dependent upon the implementation you are working with.
What if you want to find data that matches all but one character in a certain pattern? In this case you could use a different type of wildcard: the underscore.
Underscore (_)
The underscore is the single-character wildcard. Using a modified version of the table FRIENDS, type this:
SQL> SELECT * FROM FRIENDS; LASTNAME FIRSTNAME AREACODE PHONE ST ZIP -------- --------- -------- -------- -- ----- BUNDY AL 100 555-1111 IL 22333 MEZA AL 200 555-2222 UK MERRICK BUD 300 555-6666 CO 80212 MAST JD 381 555-6767 LA 23456 BULHER FERRIS 345 555-3223 IL 23332 PERKINS ALTON 911 555-3116 CA 95633 BOSS SIR 204 555-2345 CT 95633
To find all the records where STATE starts with C, type the following:
SQL> SELECT * 2 FROM FRIENDS 3 WHERE ST LIKE 'C_'; LASTNAME FIRSTNAME AREACODE PHONE ST ZIP -------- --------- -------- -------- -- ----- MERRICK BUD 300 555-6666 CO 80212 PERKINS ALTON 911 555-3116 CA 95633 BOSS SIR 204 555-2345 CT 95633 mysql> select * from friends where st like 'C_'; +----------+-----------+----------+----------+----+-------+ | lastname | firstname | areacode | phone | st | zip | +----------+-----------+----------+----------+----+-------+ | MERRICK | BUD | 300 | 555-6666 | CO | 80212 | | PERKINS | ALTON | 911 | 555-3116 | CA | 95633 | | BOSS | SIR | 204 | 555-2345 | CT | 95633 | +----------+-----------+----------+----------+----+-------+ 1 row in set (0.00 sec)
You can use several underscores in a statement:
SQL> SELECT * 2 FROM FRIENDS 3 WHERE PHONE LIKE'555-6_6_'; LASTNAME FIRSTNAME AREACODE PHONE ST ZIP -------- --------- -------- -------- -- ----- MERRICK BUD 300 555-6666 CO 80212 MAST JD 381 555-6767 LA 23456
The previous statement could also be written as follows:
SQL> SELECT * 2 FROM FRIENDS 3 WHERE PHONE LIKE '555-6%'; LASTNAME FIRSTNAME AREACODE PHONE ST ZIP -------- --------- -------- -------- -- ----- MERRICK BUD 300 555-6666 CO 80212 MAST JD 381 555-6767 LA 23456
Notice that the results are identical. These two wildcards can be combined. The next example finds all records with L as the second character:
SQL> SELECT * 2 FROM FRIENDS 3 WHERE FIRSTNAME LIKE '_L%'; LASTNAME FIRSTNAME AREACODE PHONE ST ZIP -------- --------- -------- -------- -- ----- BUNDY AL 100 555-1111 IL 22333 MEZA AL 200 555-2222 UK PERKINS ALTON 911 555-3116 CA 95633
Concatenation (||)
The || (double pipe) symbol concatenates two strings. Try this:
SQL> SELECT FIRSTNAME || LASTNAME ENTIRENAME 2 FROM FRIENDS; ENTIRENAME ---------------------- AL BUNDY AL MEZA BUD MERRICK JD MAST FERRIS BULHER ALTON PERKINS SIR BOSS 7 rows selected.
Notice that || is used instead of +. If you use + to try to concatenate the strings, -the SQL interpreter used for this example (Personal Oracle) returns the following error:
SQL> SELECT FIRSTNAME + LASTNAME ENTIRENAME FROM FRIENDS; ERROR: ORA-01722: invalid number
It is looking for two numbers to add and throws the error invalid number when it doesn't find any.
NOTE
Some implementations of SQL use the plus sign to concatenate strings. Check your implementation.
MySQL can be set up to allow the || for concatenation; however, this is not the default when MySQL is installed. concat() is the default. Any number of variables may be passed to the function concat() and it is quite easy to use. Should you desire to change the parameters in MySQL to allow the use of the || for concatenation, please research the documentation provided with MySQL on the subject first.
mysql> select concat(firstname," ",lastname)Entirename from friends; +---------------+ | Entirename | +---------------+ | AL BUNDY | | BUD MERRICK | | JD MAST | | FERRIS BULHER | +---------------+ 4 rows in set (0.00 sec)
Here's a more practical example using concatenation:
SQL> SELECT LASTNAME || ',' || FIRSTNAME NAME FROM FRIENDS; NAME ---------------------------------------------- BUNDY , AL MEZA , AL MERRICK , BUD MAST , JD BULHER , FERRIS PERKINS , ALTON BOSS , SIR 7 rows selected. mysql> select concat(lastname,","," ",firstname)Name from friends; +----------------+ | Name | +----------------+ | BUNDY, AL | | MEZA, AL | | MERRICK, BUD | | MAST, JD | | BULHER, FERRIS | | PERKINS, ALTON | | BOSS, SIR | +----------------+ 4 rows in set (0.00 sec)
The Oracle statement inserted a comma between the last name and the first name. This was done because Oracle (and other implementations) accounts for the entire length that a column may be when it concatenates to the another string. This creates a natural spacing between the values of the columns/strings. The MySQL statement inserted a comma and a space between the two columns. MySQL automatically runs the values of the columns/strings into one, thus any "natural" spacing between the values is lost.
NOTE
More on this space issue: Notice the extra spaces between the first name and the last name in the Oracle examples. These spaces are actually part of the data. With certain data types, spaces are right-padded to values less than the total length allocated for a field. See your implementation. Data types will be discussed on Day 9.
So far you have performed the comparisons one at a time. That method is fine for some problems, but what if you need to find all the people at work with last names starting with P who have less than three days of vacation time? Logical operators can help in this case.
Logical Operators
Logical operators separate two or more conditions in the WHERE clause of a SQL statement.
Vacation time is always a hot topic around the workplace. Say you designed a table called VACATION for the accounting department:
SQL> SELECT * FROM VACATION; LASTNAME EMPLOYEENUM YEARS LEAVETAKEN --------------- ----------- ----- ---------- ABLE 101 2 4 BAKER 104 5 23 BLEDSOE 107 8 45 BOLIVAR 233 4 80 BOLD 210 15 100 COSTALES 211 10 78 6 rows selected.
Suppose your company gives each employee 12 days of leave each year. Using what you have learned and a logical operator, find all the employees whose name starts with B and who have more than 50 days of leave coming.
SQL> SELECT LASTNAME, 2 YEARS * 12 - LEAVETAKEN REMAINING 3 FROM VACATION 4 WHERE LASTNAME LIKE 'B%' 5 AND 6 YEARS * 12 - LEAVETAKEN > 50; LASTNAME REMAINING -------- --------- BLEDSOE 51 BOLD 80 mysql> select lastname, -> years*12 - leavetaken remaining -> from vacation -> where lastname like 'B%' -> and years*12 - leavetaken > 50; +----------+-----------+ | lastname | remaining | +----------+-----------+ | BLEDSOE | 51 | | BOLD | 80 | +----------+-----------+ 2 rows in set (0.00 sec)
This query is the most complicated you have done to date. The SELECT clause (lines 1 and 2) uses arithmetic operators to determine how many days of leave each employee has remaining. The normal precedence computes YEARS * 12 - LEAVETAKEN. (A clearer approach would be to write (YEARS * 12) - LEAVETAKEN.)
LIKE is used in line 4 with the wildcard % to find all the B names. Line 6 uses the > to find all occurrences greater than 50.
The new element is on line 5. You used the logical operator AND to ensure that you found records that met the criteria in lines 4 and 6.
AND
AND requires that both expressions on either side be true to return TRUE. If either expression is false, AND returns FALSE. For example, to find out which employees have been with the company for 5 or fewer years and have taken more than 20 days leave, try this:
SQL> SELECT LASTNAME 2 FROM VACATION 3 WHERE YEARS <= 5 4 AND 5 LEAVETAKEN > 20 ; LASTNAME -------- BAKER BOLIVAR mysql> select lastname from vacation -> where years <= 5 -> and leavetaken > 20; +----------+ | lastname | +----------+ | BAKER | | BOLIVAR | +----------+ 2 rows in set (0.00 sec)
If you want to know which employees have been with the company for 5 years or more and have taken less than 50 percent of their leave, you could write:
SQL> SELECT LASTNAME WORKAHOLICS 2 FROM VACATION 3 WHERE YEARS >= 5 4 AND 5 ((YEARS *12)-LEAVETAKEN)/(YEARS * 12) < 0.50; WORKAHOLICS ----------- BAKER BLEDSOE mysql> select lastname Workaholics -> from vacation -> where years >= 5 -> and ((years * 12) - leavetaken) / (years * 12) < 0.50; +-------------+ | Workaholics | +-------------+ | BOLD | | COSTALES | +-------------+ 2 rows in set (0.00 sec)
Check these people for burnout. Also check out how we used the AND to combine these two conditions.
OR
You can also use OR to sum up a series of conditions. If any of the comparisons are true, OR returns TRUE. To illustrate the difference, run the last query with OR instead of with AND:
SQL> SELECT LASTNAME WORKAHOLICS 2 FROM VACATION 3 WHERE YEARS >= 5 4 OR 5 ((YEARS *12)-LEAVETAKEN)/(YEARS * 12) >= 0.50; WORKAHOLICS ----------- ABLE BAKER BLEDSOE BOLD COSTALES mysql> select lastname -> from vacation -> where years >= 5 -> OR ((years*12)-leavetaken)/(years*12) < 0.50; +----------+ | lastname | +----------+ | BAKER | | BLEDSOE | | BOLD | | COSTALES | +----------+ 5 rows in set (0.00 sec)
The original names are still in the list, but you have three new entries (who would probably resent being called workaholics). These three new names made the list because they satisfied one of the conditions. OR requires only that one of the conditions be true in order for data to be returned.
NOT
NOT means just that. If the condition it applies to evaluates to TRUE, NOT makes it FALSE. If the condition after the NOT is FALSE, it becomes TRUE. For example, the following SELECT returns the only two names not beginning with B in the table:
SQL> SELECT * 2 FROM VACATION 3 WHERE LASTNAME NOT LIKE 'B%'; LASTNAME EMPLOYEENUM YEARS LEAVETAKEN -------- ----------- ----- ---------- ABLE 101 2 4 COSTALES 211 10 78 mysql> select * from vacation -> where lastname not like 'B%'; +----------+-------------+-------+------------+ | lastname | employeenum | years | leavetaken | +----------+-------------+-------+------------+ | ABLE | 101 | 2 | 4 | | COSTALES | 211 | 10 | 78 | +----------+-------------+-------+------------+ 2 rows in set (0.00 sec)
NOT can also be used with the operator IS when applied to NULL. Recall the PRICE table where we put a NULL value in the WHOLESALE column opposite the item ORANGES.
SQL> SELECT * FROM PRICE; ITEM WHOLESALE --------------- --------- TOMATOES .34 POTATOES .51 BANANAS .67 TURNIPS .45 CHEESE .89 APPLES .23 ORANGES 7 rows selected.
To find the non-NULL items, type this:
SQL> SELECT * 2 FROM PRICE 3 WHERE WHOLESALE IS NOT NULL; ITEM WHOLESALE --------------- --------- TOMATOES .34 POTATOES .51 BANANAS .67 TURNIPS .45 CHEESE .89 APPLES .23 6 rows selected.
Set Operators
On Day 1 you learned that SQL is based on the theory of sets. The following sections examine set operators. Set operators are used to combine different sets of data returned by different queries into one query, and ultimately, one data set. There are various set operators available in SQL that allow you to combine different data sets to meet your data processing needs.
UNION and UNION ALL
UNION returns the results of two queries minus the duplicate rows. As stated previously, this text uses MySQL version 3.23.49, which is the most recent, stable version of MySQL. Starting with MySQL version 4.0, there will be support for unions. Examples and discussion of unions will be strictly in Oracle. The following two tables represent the rosters of teams:
SQL> SELECT * FROM FOOTBALL; NAME ---------------- ABLE BRAVO CHARLIE DECON EXITOR FUBAR GOOBER 7 rows selected. SQL> SELECT * FROM SOFTBALL; NAME ----------------- ABLE BAKER CHARLIE DEAN EXITOR FALCONER GOOBER 7 rows selected.
How many different people play on one team or another?
SQL> SELECT NAME FROM SOFTBALL 2 UNION 3 SELECT NAME FROM FOOTBALL; NAME ---------------- ABLE BAKER BRAVO CHARLIE DEAN DECON EXITOR FALCONER FUBAR GOOBER 10 rows selected.
UNION returns 10 distinct names from the two lists. How many names are on both lists (including duplicates)?
SQL> SELECT NAME FROM SOFTBALL 2 UNION ALL 3 SELECT NAME FROM FOOTBALL; NAME ---------------- ABLE BAKER CHARLIE DEAN EXITOR FALCONER GOOBER ABLE BRAVO CHARLIE DECON EXITOR FUBAR GOOBER 14 rows selected.
The combined list—courtesy of the UNION ALL statement—has 14 names. UNION ALL works just like UNION except that it does not eliminate duplicates. Now show me a list of players who are on both teams. You can't do that with UNION—you need to learn INTERSECT.
INTERSECT
INTERSECT returns only the rows found by both queries. The next SELECT statement shows the list of players who play on both teams:
SQL> SELECT * FROM FOOTBALL 2 INTERSECT 3 SELECT * FROM SOFTBALL; NAME ---------------- ABLE CHARLIE EXITOR GOOBER
In this example, INTERSECT finds the short list of players who are on both teams by combining the results of the two SELECT statements.
MINUS (Difference)
MINUS returns the rows from the first query that were not present in the second. For example:
SQL> SELECT * FROM FOOTBALL 2 MINUS 3 SELECT * FROM SOFTBALL; NAME ----------------- BRAVO DECON FUBAR
The preceding query shows the three football players who are not on the softball team. If you reverse the order, you get the three softball players who aren't on the football team:
SQL> SELECT * FROM SOFTBALL 2 MINUS 3 SELECT * FROM FOOTBALL; NAME ------------------ BAKER DEAN FALCONER
Miscellaneous Operators: IN and BETWEEN
The two operators IN and BETWEEN provide a shorthand for functions you already know how to do. If you wanted to find friends in Colorado, California, and Louisiana, you could type the following:
SQL> SELECT * 2 FROM FRIENDS 3 WHERE STATE= 'CA' 4 OR 5 STATE ='CO' 6 OR 7 STATE = 'LA'; LASTNAME FIRSTNAME AREACODE PHONE ST ZIP -------- --------- -------- -------- -- ----- MERRICK BUD 300 555-6666 CO 80212 MAST JD 381 555-6767 LA 23456 PERKINS ALTON 911 555-3116 CA 95633
Or you could type this:
SQL> SELECT *
2 FROM FRIENDS
3 WHERE ST IN('CA','CO','LA');
LASTNAME FIRSTNAME AREACODE PHONE ST ZIP
-------- --------- -------- -------- -- -----
MERRICK BUD 300 555-6666 CO 80212
MAST JD 381 555-6767 LA 23456
PERKINS ALTON 911 555-3116 CA 95633
mysql> select * from friends
-> where st in ('CA','CO','LA');
+----------+-----------+----------+----------+----+-------+
| lastname | firstname | areacode | phone | st | zip |
+----------+-----------+----------+----------+----+-------+
| MERRICK | BUD | 300 | 555-6666 | CO | 80212 |
| MAST | JD | 381 | 555-6767 | LA | 23456 |
| PERKINS | ALTON | 911 | 555-3116 | CA | 95633 |
+----------+-----------+----------+----------+----+-------+
2 rows in set (0.20 sec)
The second example is shorter and more readable than the first. You never know when you might have to go back and work on something you wrote months ago. IN also works with numbers. Consider the following, where the column AREACODE is a number:
SQL> SELECT * 2 FROM FRIENDS 3 WHERE AREACODE IN(100,381,204); LASTNAME FIRSTNAME AREACODE PHONE ST ZIP -------- --------- -------- -------- -- ----- BUNDY AL 100 555-1111 IL 22333 MAST JD 381 555-6767 LA 23456 BOSS SIR 204 555-2345 CT 95633
If you needed a range of data from the PRICE table, you could write the following:
SQL> SELECT * 2 FROM PRICE 3 WHERE WHOLESALE > 0.25 4 AND 5 WHOLESALE < 0.75; ITEM WHOLESALE -------- --------- TOMATOES .34 POTATOES .51 BANANAS .67 TURNIPS .45
Or using BETWEEN, you would write this:
SQL> SELECT * 2 FROM PRICE 3 WHERE WHOLESALE BETWEEN 0.25 AND 0.75; ITEM WHOLESALE -------- --------- TOMATOES .34 POTATOES .51 BANANAS .67 TURNIPS .45 mysql> select * from price -> where wholesale between .25 and .75; +----------+-----------+ | item | wholesale | +----------+-----------+ | TOMATOES | 0.34 | | POTATOES | 0.51 | | BANANAS | 0.67 | | TURNIPS | 0.45 | +----------+-----------+ 4 rows in set (0.08 sec)
Again, the second example is a cleaner, more readable solution than the first.
NOTE
If a WHOLESALE value of 0.25 existed in the PRICE table, that record would have been retrieved also. Parameters used with BETWEEN are inclusive.