- Selecting Data from Multiple Tables
- Types of Joins
- Join Considerations
- Summary
- Q&A
- Workshop
This chapter is from the book
This chapter is from the book
This chapter is from the book
Types of Joins
While different implementations have many ways of joining tables, you concentrate on the most common joins in this lesson. The types of joins that you learn are
Component Locations of a Join Condition
As you have learned from previous hours, the SELECT and FROM clauses are both required SQL statement elements; the WHERE clause is a required element of a SQL statement when joining tables. The tables being joined are listed in the FROM clause. The join is performed in the WHERE clause. Several operators can be used to join tables, such as =, <, >, <>, <=, >=, !=, BETWEEN, LIKE, and NOT; they can all be used to join tables. However, the most common operator is the equal symbol.
Joins of Equality
Perhaps the most used and important of the joins is the EQUIJOIN, also referred to as an INNER JOIN. The EQUIJOIN joins two tables with a common column in which each is usually the primary key.
The syntax for an EQUIJOIN is
SELECT TABLE1.COLUMN1, TABLE2.COLUMN2... FROM TABLE1, TABLE2 [, TABLE3 ] WHERE TABLE1.COLUMN_NAME = TABLE2.COLUMN_NAME [ AND TABLE1.COLUMN_NAME = TABLE3.COLUMN_NAME ]
NOTE
Take note of the sample SQL statements. Indentation is used in the SQL statements to improve overall readability. Indentation is not required, but is recommended.
Look at the following example:
SELECT EMPLOYEE_TBL.EMP_ID,
EMPLOYEE_PAY_TBL.DATE_HIRE
FROM EMPLOYEE_TBL,
EMPLOYEE_PAY_TBL
WHERE EMPLOYEE_TBL.EMP_ID = EMPLOYEE_PAY_TBL.EMP_ID;
This SQL statement returns the employee identification and the employee's date of hire. The employee identification is selected from the EMPLOYEE_TBL (although it exists in both tables, you must specify one table), whereas the hire date is selected from the EMPLOYEE_PAY_TBL. Because the employee identification exists in both tables, both columns must be justified with the table name. By justifying the columns with the table names, you tell the database server where to get the data.
Data in the following example is selected from tables EMPLOYEE_TBL and EMPLOYEE_PAY_TBL tables because desired data resides in each of the two tables. An equality join is used.
SELECT EMPLOYEE_TBL.EMP_ID, EMPLOYEE_TBL.LAST_NAME,
EMPLOYEE_PAY_TBL.POSITION
FROM EMPLOYEE_TBL, EMPLOYEE_PAY_TBL
WHERE EMPLOYEE_TBL.EMP_ID = EMPLOYEE_PAY_TBL.EMP_ID;
EMP_ID LAST_NAM POSITION --------- -------- ------------- 311549902 STEPHENS MARKETING 442346889 PLEW TEAM LEADER 213764555 GLASS SALES MANAGER 313782439 GLASS SALESMAN 220984332 WALLACE SHIPPER 443679012 SPURGEON SHIPPER 6 rows selected.
Notice that each column in the SELECT clause is preceded by the associated table name in order to identify each column. This is called qualifying columns in a query. Qualifying columns is only necessary for columns that exist in more than one table referenced by a query. You usually qualify all columns for consistency and to avoid any questions when debugging or modifying SQL code.
Natural Joins
A NATURAL JOIN is nearly the same as the EQUIJOIN; however, the NATURAL JOIN differs from the EQUIJOIN by eliminating duplicate columns in the joining columns. The JOIN condition is the same, but the columns selected differ.
The syntax is as follows:
SELECT TABLE1.*, TABLE2.COLUMN_NAME
[ TABLE3.COLUMN_NAME ]
FROM TABLE1, TABLE2 [ TABLE3 ]
WHERE TABLE1.COLUMN_NAME = TABLE2.COLUMN_NAME
[ AND TABLE1.COLUMN_NAME = TABLE3.COLUMN ]
Look at the following example:
SELECT EMPLOYEE_TBL.*, EMPLOYEE_PAY_TBL.SALARY FROM EMPLOYEE_TBL, EMPLOYEE_PAY_TBL WHERE EMPLOYEE_TBL.EMP_ID = EMPLOYEE_PAY_TBL.EMP_ID;
This SQL statement returns all columns from EMPLOYEE_TBL and SALARY from the EMPLOYEE_PAY_TBL. The EMP_ID is in both tables, but is retrieved only from the EMPLOYEE_TBL because both contain the same information and do not need to be selected.
The following example selects all columns from the EMPLOYEE_TBL table and only one column from the EMPLOYEE_PAY_TBL table. Remember that the asterisk (*) represents all columns of a table.
SELECT EMPLOYEE_TBL.*, EMPLOYEE_PAY_TBL.POSITION FROM EMPLOYEE_TBL, EMPLOYEE_PAY_TBL WHERE EMPLOYEE_TBL.EMP_ID = EMPLOYEE_PAY_TBL.EMP_ID;
EMP_ID LAST_NAM FIRST_NA M ADDRESS CITY ST ZIP PHONE
---------- -------- -------- - ------------- ------------ -- ----- ----------
PAGER POSITION
---------- --------------
311549902 STEPHENS TINA D RR 3 BOX 17A GREENWOOD IN 47890 3178784465
MARKETING
442346889 PLEW LINDA C 3301 BEACON INDIANAPOLIS IN 46224 3172978990
TEAM LEADER
213764555 GLASS BRANDON S 1710 MAIN ST WHITELAND IN 47885 3178984321
3175709980 SALES MANAGER
313782439 GLASS JACOB 3789 RIVER BLVD INDIANAPOLIS IN 45734 3175457676
8887345678 SALESMAN
220984332 WALLACE MARIAH 7889 KEYSTONE INDIANAPOLIS IN 46741 3173325986
SHIPPER
443679012 SPURGEON TIFFANY 5 GEORGE COURT INDIANAPOLIS IN 46234 3175679007
SHIPPER
6 rows selected.
NOTE
Notice how the output has wrapped in the previous example. The wrap occurred because the length of the line has exceeded the limit for the line (which is usually 80 characters per line by default).
Using Table Aliases
The use of table aliases means to rename a table in a particular SQL statement. The renaming is a temporary change. The actual table name does not change in the database. As you will learn later in this hour, giving the tables aliases is a necessity for the SELF JOIN. Giving tables aliases is most often used to save keystrokes, which results in the SQL statement being shorter and easier to read. In addition, fewer keystrokes means fewer keystroke errors. Also, programming errors are typically less frequent if you can refer to an alias, which is often shorter in length and more descriptive of the data with which you are working. Giving tables aliases also means that the columns being selected must be qualified with the table alias. The following are some examples of table aliases and the corresponding columns:
SELECT E.EMP_ID, EP.SALARY, EP.DATE_HIRE, E.LAST_NAME FROM EMPLOYEE_TBL E, EMPLOYEE_PAY_TBL EP WHERE E.EMP_ID = EP.EMP_ID AND EP.SALARY > 20000;
The tables have been given aliases in the preceding SQL statement. The EMPLOYEE_TBL has been renamed E. The EMPLOYEE_PAY_TBL has been renamed EP. The choice of what to rename the tables is arbitrary. The letter E is chosen because the EMPLOYEE_TBL starts with E. Because the EMPLOYEE_PAY_TBL also begins with the letter E, you could not use E again. Instead, the first letter (E) and the first letter of the second word in the name (PAY) are used as the alias. The selected columns were justified with the corresponding table alias. Note that SALARY was used in the WHERE clause and must also be justified with the table alias.
Joins of Non-Equality
NON-EQUIJOIN joins two or more tables based on a specified column value not equaling a specified column value in another table. The syntax for the NON-EQUIJOIN is
FROM TABLE1, TABLE2 [, TABLE3 ] WHERE TABLE1.COLUMN_NAME != TABLE2.COLUMN_NAME [ AND TABLE1.COLUMN_NAME != TABLE2.COLUMN_NAME ]
An example is as follows:
SELECT EMPLOYEE_TBL.EMP_ID, EMPLOYEE_PAY_TBL.DATE_HIRE FROM EMPLOYEE_TBL, EMPLOYEE_PAY_TBL WHERE EMPLOYEE_TBL.EMP_ID != EMPLOYEE_PAY_TBL.EMP_ID;
The preceding SQL statement returns the employee identification and the date of -hire for all employees who do not have a corresponding record in both tables. The following example is a join of non-equality:
SELECT E.EMP_ID, E.LAST_NAME, P.POSITION FROM EMPLOYEE_TBL E, EMPLOYEE_PAY_TBL P WHERE E.EMP_ID <> P.EMP_ID;
EMP_ID LAST_NAM POSITION --------- -------- ------------- 442346889 PLEW MARKETING 213764555 GLASS MARKETING 313782439 GLASS MARKETING 220984332 WALLACE MARKETING 443679012 SPURGEON MARKETING 311549902 STEPHENS TEAM LEADER 213764555 GLASS TEAM LEADER 313782439 GLASS TEAM LEADER 220984332 WALLACE TEAM LEADER 443679012 SPURGEON TEAM LEADER 311549902 STEPHENS SALES MANAGER 442346889 PLEW SALES MANAGER 313782439 GLASS SALES MANAGER 220984332 WALLACE SALES MANAGER 443679012 SPURGEON SALES MANAGER 311549902 STEPHENS SALESMAN 442346889 PLEW SALESMAN 213764555 GLASS SALESMAN 220984332 WALLACE SALESMAN 443679012 SPURGEON SALESMAN 311549902 STEPHENS SHIPPER 442346889 PLEW SHIPPER 213764555 GLASS SHIPPER 313782439 GLASS SHIPPER 443679012 SPURGEON SHIPPER 311549902 STEPHENS SHIPPER 442346889 PLEW SHIPPER 213764555 GLASS SHIPPER 313782439 GLASS SHIPPER 220984332 WALLACE SHIPPER 30 rows selected.
You may be curious why 30 rows were retrieved when only 6 rows exist in each table. For every record in EMPLOYEE_TBL, there is a corresponding record in EMPLOYEE_PAY_TBL. Because non-equality was tested in the join of the two tables, each row in the first table is paired with all rows from the second table, except for its own corresponding row. This means that each of the 6 rows are paired with 5 unrelated rows in the second table; 6 rows multiplied by 5 rows equals 30 rows total.
In the previous section's test for equality example, each of the six rows in the first table were paired with only one row in the second table (each row's corresponding row); six rows multiplied by one row yields a total of six rows.
CAUTION
When using NON-EQUIJOINs, you may receive several rows of data that are of no use to you. Check your results carefully.
Outer Joins
An OUTER JOIN is used to return all rows that exist in one table, even though corresponding rows do not exist in the joined table. The (+) symbol is used to denote an OUTER JOIN in a query. The (+) is placed at the end of the table name in the WHERE clause. The table with the (+) should be the table that does not have matching rows. In many implementations, the OUTER JOIN is broken down into joins called LEFT OUTER JOIN, RIGHT OUTER JOIN, and FULL OUTER JOIN. The OUTER JOIN in these implementations is normally optional.
NOTE
You must check your particular implementation for exact usage and syntax of the OUTER JOIN. The (+) symbol is used by some major implementations, but is non-standard.
The general syntax is
FROM TABLE1
{RIGHT | LEFT | FULL} [OUTER] JOIN
ON TABLE2
The Oracle syntax is
FROM TABLE1, TABLE2 [, TABLE3 ] WHERE TABLE1.COLUMN_NAME[(+)] = TABLE2.COLUMN_NAME[(+)] [ AND TABLE1.COLUMN_NAME[(+)] = TABLE3.COLUMN_NAME[(+)]]
NOTE
The OUTER JOIN can only be used on one side of a join condition; however, you can use an OUTER JOIN on more than one column of the same table in the join condition.
The concept of the OUTER JOIN is explained in the next two examples. In the first example, the product description and the quantity ordered are selected; both values are extracted from two separate tables. One important factor to keep in mind is that there may not be a corresponding record in the ORDERS_TBL table for every product. A regular join of equality is performed:
SELECT P.PROD_DESC, O.QTY FROM PRODUCTS_TBL P, ORDERS_TBL O WHERE P.PROD_ID = O.PROD_ID;
PROD_DESC QTY -------------------------------- --- WITCHES COSTUME 1 PLASTIC PUMPKIN 18 INCH 25 PLASTIC PUMPKIN 18 INCH 2 LIGHTED LANTERNS 10 FALSE PARAFFIN TEETH 20 KEY CHAIN 1 6 rows selected.
Only six rows were selected, but there are 10 distinct products. You want to display all products, whether the products have been placed on order or not.
The next example accomplishes the desired output through the use of an OUTER JOIN. Oracle's syntax is used for the OUTER JOIN.
SELECT P.PROD_DESC, O.QTY FROM PRODUCTS_TBL P, ORDERS_TBL O WHERE P.PROD_ID = O.PROD_ID(+);
PROD_DESC QTY -------------------------------- --- WITCHES COSTUME 1 ASSORTED MASKS FALSE PARAFFIN TEETH 20 ASSORTED COSTUMES PLASTIC PUMPKIN 18 INCH 25 PLASTIC PUMPKIN 18 INCH 2 PUMPKIN CANDY PLASTIC SPIDERS CANDY CORN LIGHTED LANTERNS 10 KEY CHAIN 1 OAK BOOKSHELF 12 rows selected.
All products were returned by the query, even though they may not have had a quantity ordered. The outer join is inclusive of all rows of data in the PRODUCTS_TBL table, whether a corresponding row exists in the ORDERS_TBL table or not.
Self Joins
The SELF JOIN is used to join a table to itself, as if the table were two tables, temporarily renaming at least one table in the SQL statement. The syntax is as follows:
SELECT A.COLUMN_NAME, B.COLUMN_NAME, [ C.COLUMN_NAME ] FROM TABLE1 A, TABLE2 B [, TABLE3 C ] WHERE A.COLUMN_NAME = B.COLUMN_NAME [ AND A.COLUMN_NAME = C.COLUMN_NAME ]
The following is an example:
SELECT A.LAST_NAME, B.LAST_NAME, A.FIRST_NAME FROM EMPLOYEE_TBL A, EMPLOYEE_TBL B WHERE A.LAST_NAME = B.LAST_NAME;
The preceding SQL statement returns the employees' first name for all the employees with the same last name from the EMPLOYEE_TBL. Self joins are useful when all of the data you want to retrieve resides in one table, but you must somehow compare records in the table to other records in the table.
Another common example used to explain a self join is as follows. Suppose you have a table that stores an employee identification number, the employee's name, and the employee identification number of the employee's manager. You may want to produce a list of all employees and their managers' names. The problem is that the manager name does not exist in the table, only the employee name:
SELECT * FROM EMP; ID NAME MGR_ID ---- --------- ------ 1 JOHN 0 2 MARY 1 3 STEVE 1 4 JACK 2 5 SUE 2
In the following example, we have included the table EMP twice in the FROM clause of the query, giving the table two aliases for the purpose of the query. By providing two aliases, it is as if you are selecting from two distinct tables. All managers are also employees, so the join condition between the two tables compares the value of the employee identification number from the first table with the manager identification number in the second table. The first table acts as a table that stores employee information, whereas the second table acts as a table that stores manager information:
SELECT E1.NAME, E2.NAME FROM EMP E1, EMP E2 WHERE E1.MGR_ID = E2.ID; NAME NAME --------- --------- MARY JOHN STEVE JOHN JACK MARY SUE MARY
Joining on Multiple Keys
Most join operations involve the merging of data based on a key in one table and a key in another table. Depending on how your database has been designed, you may have to join on more than one key field to accurately depict that data in your database. You may have a table that has a primary key that is comprised of more than one column. You may also have a foreign key in a table that consists of more than one column, which references the multiple column primary key.
Consider the following Oracle tables that are used here for examples only:
SQL> desc prod Name Null? Type ----------------------------------------- -------- ---------------------------- SERIAL_NUMBER NOT NULL NUMBER(10) VENDOR_NUMBER NOT NULL NUMBER(10) PRODUCT_NAME NOT NULL VARCHAR2(30) COST NOT NULL NUMBER(8,2) SQL> desc ord Name Null? Type ----------------------------------------- -------- ---------------------------- ORD_NO NOT NULL NUMBER(10) PROD_NUMBER NOT NULL NUMBER(10) VENDOR_NUMBER NOT NULL NUMBER(10) QUANTITY NOT NULL NUMBER(5) ORD_DATE NOT NULL DATE
The primary key in PROD is the combination of the columns SERIAL_NUMBER and VENDOR_NUMBER. Perhaps two products can have the same serial number within the distribution company, but each serial number is unique per vendor.
The foreign key in ORD is also the combination of the columns SERIAL_NUMBER and VENDOR_NUMBER.
When selecting data from both tables (PROD and ORD), the join operation may appear as follows:
SELECT P.PRODUCT_NAME, O.ORD_DATE, O.QUANTITY FROM PROD P, ORD O WHERE P.SERIAL_NUMBER = O.SERIAL_NUMBER AND P.VENDOR_NUMBER = O.VENDOR_NUMBER;