MySQL SQL Syntax and Use

This chapter is from the book

MySQL, 2nd Edition

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This chapter is from the book

This chapter is from the book 

MySQL, 2nd Edition

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Using FULLTEXT Searches

Versions of MySQL from 3.23.23 on include the capability for performing full text searches. The full text search engine allows you to look for words or phrases without using pattern-matching operations. This capability is enabled for a given table by creating a special kind of index and has the following characteristics:

• Full text searches are based on FULLTEXT indexes, which may be created only for MyISAM tables, and only for TEXT columns and non-BINARY CHAR and VARCHAR columns.

• FULLTEXT searches are not case sensitive. This follows as a consequence of the column types for which FULLTEXT indexes may be used.

• Common words are ignored for FULLTEXT searches, where "common" means "present in at least half the records." It's especially important to remember this when you're setting up a test table to experiment with the FULLTEXT capability. (Be sure to insert at least three records into your test table. If the table has just one or two records, every word in it will occur at least 50 percent of the time and you'll never get any results!) Certain very common words, such as "the," "after," and "other," are stop words that are always ignored. Words that are too short also are ignored. By default, "too short" is defined as less than four characters, but with a recent enough server may be set lower.

• Words are defined as sequences of characters that include letters, digits, apostrophes, and underscores. This means that a string like "full-blooded" is considered to contain two words—"full" and "blooded." Normally, a full text search matches whole words, not partial words, and the FULLTEXT engine considers a record to match a search string if it includes any of the words in the search string. A variant form of search called a boolean full text search allows you to impose the additional constraint that all the words must be present (either in any order, or, to perform a phrase search, in exactly the order listed in the search string). With a boolean search, it's also possible to match records that do not include certain words or to add a wildcard modifier to match all words that begin with a given prefix.

• A FULLTEXT index can be created for a single column or multiple columns. If it spans multiple columns, searches based on the index look through all the columns simultaneously. The flip side of this is that when you perform a search, you must specify a column list that corresponds exactly to the set of columns that matches some FULLTEXT index. For example, if you want to search col1 sometimes, col2 sometimes, and both col1 and col2 sometimes, you should have three indexes: one for each of the columns separately, and one that includes both columns.

Some of these features require more recent versions of MySQL than others. The following table shows the versions at which FULLTEXT features were introduced:

The following examples show how to use full text searching by creating FULLTEXT indexes and then performing queries on them using the MATCH operator.

A FULLTEXT index is created the same way as other indexes. That is, you can define it with CREATE TABLE when creating the table initially or add it afterward with ALTER TABLE or CREATE INDEX. Because FULLTEXT indexes require you to use MyISAM tables, you can take advantage of one of the properties of the MyISAM handler if you're creating a new table to use for FULLTEXT searches: Table loading proceeds more quickly if you populate the table and then add the indexes afterward rather than loading data into an already indexed table. Suppose you have a data file named apothegm.txt containing famous sayings and the people to whom they're attributed:

Aeschylus               Time as he grows old teaches many lessons
Alexander Graham Bell   Mr. Watson, come here. I want you!
Benjamin Franklin       It is hard for an empty bag to stand upright
Benjamin Franklin       Little strokes fell great oaks
Benjamin Franklin       Remember that time is money
Miguel de Cervantes     Bell, book, and candle
Proverbs 15:1           A soft answer turneth away wrath
Theodore Roosevelt      Speak softly and carry a big stick
William Shakespeare     But, soft! what light through yonder window breaks?

If you want to search by phrase and attribution separately or together, you need to index the columns separately and also create an index that includes both columns. You can create, populate, and index a table named apothegm as follows:

CREATE TABLE apothegm (attribution VARCHAR(40), phrase TEXT);
LOAD DATA LOCAL INFILE 'apothegm.txt' INTO TABLE apothegm;
ALTER TABLE apothegm
  ADD FULLTEXT (phrase),
  ADD FULLTEXT (attribution),
  ADD FULLTEXT (phrase, attribution);

After setting up the table, perform searches on it using MATCH to name the column or columns to search and AGAINST() to specify the search string. For example:

mysql> SELECT * FROM apothegm WHERE MATCH(attribution) AGAINST('roosevelt');
+--------------------+------------------------------------+
| attribution        | phrase                             |
+--------------------+------------------------------------+
| Theodore Roosevelt | Speak softly and carry a big stick |
+--------------------+------------------------------------+
mysql> SELECT * FROM apothegm WHERE MATCH(phrase) AGAINST('time');
+-------------------+-------------------------------------------+
| attribution       | phrase                                    |
+-------------------+-------------------------------------------+
| Benjamin Franklin | Remember that time is money               |
| Aeschylus         | Time as he grows old teaches many lessons |
+-------------------+-------------------------------------------+
mysql> SELECT * FROM apothegm WHERE MATCH(attribution,phrase)
  -> AGAINST('bell');
+-----------------------+------------------------------------+
| attribution           | phrase                             |
+-----------------------+------------------------------------+
| Alexander Graham Bell | Mr. Watson, come here. I want you! |
| Miguel de Cervantes   | Bell, book, and candle             |
+-----------------------+------------------------------------+

In the last example, note how the query finds records that contain the search word in different columns, which demonstrates the FULLTEXT capability of searching multiple columns at once. Also note that the order of the columns as named in the query is attribution, phrase. That differs from the order in which they were named when the index was created (phrase, attribution), which illustrates that order does not matter. What matters is that there must be some FULLTEXT index that consists of exactly the columns named.

If you just want to see how many records a search matches, use COUNT(*):

mysql> SELECT COUNT(*) FROM apothegm WHERE MATCH(phrase) AGAINST('time');
+----------+
| COUNT(*) |
+----------+
|    2     |
+----------+

By default, output rows for FULLTEXT searches are ordered by decreasing relevance when you use a MATCH expression in the WHERE clause. Relevance values are non-negative floating point values, with zero indicating "no relevance." To see these values, use a MATCH expression in the column output list:

mysql> SELECT phrase, MATCH(phrase) AGAINST('time') AS relevance
  -> FROM apothegm;
+-----------------------------------------------------+-----------------+
| phrase                                              | relevance       |
+-----------------------------------------------------+-----------------+
| Time as he grows old teaches many lessons           | 1.1976701021194 |
| Mr. Watson, come here. I want you!                  |        0        |
| It is hard for an empty bag to stand upright        |        0        |
| Little strokes fell great oaks                      |        0        |
| Remember that time is money                         | 1.2109839916229 |
| Bell, book, and candle                              |        0        |
| A soft answer turneth away wrath                    |        0        |
| Speak softly and carry a big stick                  |        0        |
| But, soft! what light through yonder window breaks? |        0        |
+-----------------------------------------------------+-----------------+

By default, a search finds records that contain any of the search words, so a query like the following will return records with either "hard" or "soft":

mysql> SELECT * FROM apothegm WHERE MATCH(phrase)
  -> AGAINST('hard soft');
+---------------------+-----------------------------------------------------+
| attribution         | phrase                                              |
+---------------------+-----------------------------------------------------+
| Benjamin Franklin   | It is hard for an empty bag to stand upright        |
| Proverbs 15:1       | A soft answer turneth away wrath                    |
| William Shakespeare | But, soft! what light through yonder window breaks? |
+---------------------+-----------------------------------------------------+

Greater control over multiple-word matching can be obtained as of MySQL 4.0.1, when support was added for boolean mode FULLTEXT searches. This type of search is performed by adding IN BOOLEAN MODE after the search string in the AGAINST() function. Boolean searches have the following characteristics:

• The 50% rule is ignored; searches will find words even if they occur in more than half of the records.

• Results are not sorted by relevance.

• Modifiers can be applied to words in the search string. A leading plus or minus sign requires a word to be present or not present in matching records. A search string of 'bell' matches records that contain "bell," but a search string of '+bell -candle' in boolean mode matches only records that contain "bell" and do not contain "candle."

  mysql> SELECT * FROM apothegm
    -> WHERE MATCH(attribution,phrase)
    -> AGAINST('bell');
  +-----------------------+------------------------------------+
  | attribution           | phrase                             |
  +-----------------------+------------------------------------+
  | Alexander Graham Bell | Mr. Watson, come here. I want you! |
  | Miguel de Cervantes   | Bell, book, and candle             |
  +-----------------------+------------------------------------+
  mysql> SELECT * FROM apothegm
    -> WHERE MATCH(attribution,phrase)
    -> AGAINST('+bell -candle' IN BOOLEAN MODE);
  +-----------------------+------------------------------------+
  | attribution           | phrase                             |
  +-----------------------+------------------------------------+
  | Alexander Graham Bell | Mr. Watson, come here. I want you! |
  +-----------------------+------------------------------------+

• A trailing asterisk acts as a wildcard so that any record containing words beginning with the search word match. For example, 'soft*' matches "soft," "softly," "softness," and so on:

  mysql> SELECT * FROM apothegm WHERE MATCH(phrase)
    -> AGAINST('soft*' IN BOOLEAN MODE);
  +---------------------+-----------------------------------------------------+
  | attribution         | phrase                                              |
  +---------------------+-----------------------------------------------------+
  | Proverbs 15:1       | A soft answer turneth away wrath                    |
  | William Shakespeare | But, soft! what light through yonder window breaks? |
  | Theodore Roosevelt  | Speak softly and carry a big stick                  |
  +---------------------+-----------------------------------------------------+

  However, the wildcard feature cannot be used to match words shorter than the minimum index word length.

  The full set of modifiers is listed in the entry for MATCH in Appendix C, "Operator and Function Reference."

• Stop words are ignored just as for non-boolean searches, even if marked as required. A search for '+Alexander +the +great' will find records containing "Alexander" and "great," but will ignore the stop word "the."

• A phrase search can be performed to require all words to be present in a particular order. Phrase searching requires MySQL 4.0.2. Enclose the search string in double quotes and include punctuation and whitespace as present in the phrase you want to match. In other words, you must specify the exact phrase:

  mysql> SELECT * FROM apothegm
    -> WHERE MATCH(attribution,phrase)
    -> AGAINST('"bell book and candle"' IN BOOLEAN MODE);
  Empty set (0.00 sec)
  mysql> SELECT * FROM apothegm
    -> WHERE MATCH(attribution,phrase)
    -> AGAINST('"bell, book, and candle"' IN BOOLEAN MODE);
  +---------------------+------------------------+
  | attribution         | phrase                 |
  +---------------------+------------------------+
  | Miguel de Cervantes | Bell, book, and candle |
  +---------------------+------------------------+

• It's possible to perform a boolean mode full text search on columns that are not part of a FULLTEXT index, although this will be much slower than using indexed columns.

Prior to MySQL 4, FULLTEXT search parameters can be modified only by making changes to the source code and recompiling the server. MySQL 4 provides several configurable parameters that can be modified by setting server variables. The two that are of most interest are ft_min_word_len and ft_max_word_len, which determine the shortest and longest words that will be indexed. The default values are 4 and 254; words with lengths outside that range are ignored when FULLTEXT indexes are built.

Suppose you want to change the minimum word length from 4 to 3. Do so like this:

1. Start the server with the ft_min_word_len variable set to 3. To ensure that this happens whenever the server starts, it's best to place the setting in an option file such as /etc/my.cnf:

   [mysqld]
   set-variable = ft_min_word_len=3

2. For any existing tables that already have FULLTEXT indexes, you must rebuild those indexes. You can drop and add the indexes, but it's easier just to do the following:

   REPAIR TABLE tbl_name USE_FRM;

3. Any new FULLTEXT indexes that you create after changing the parameter will use the new value automatically.

For more information on option files and setting server variables, see Appendix D.