11.9. Full-Text Search Functions

MATCH (col1,col2,...) AGAINST (expr [search_modifier])

search_modifier:
  {
       IN BOOLEAN MODE
     | IN NATURAL LANGUAGE MODE
     | IN NATURAL LANGUAGE MODE WITH QUERY EXPANSION
     | WITH QUERY EXPANSION
  }

MySQL has support for full-text indexing and searching:

  • A full-text index in MySQL is an index of type FULLTEXT.

  • Full-text indexes can be used only with MyISAM tables, and can be created only for CHAR, VARCHAR, or TEXT columns.

  • A FULLTEXT index definition can be given in the CREATE TABLE statement when a table is created, or added later using ALTER TABLE or CREATE INDEX.

  • For large data sets, it is much faster to load your data into a table that has no FULLTEXT index and then create the index after that, than to load data into a table that has an existing FULLTEXT index.

Full-text searching is performed using MATCH() ... AGAINST syntax. MATCH() takes a comma-separated list that names the columns to be searched. AGAINST takes a string to search for, and an optional modifier that indicates what type of search to perform. The search string must be a literal string, not a variable or a column name. There are three types of full-text searches:

  • A boolean search interprets the search string using the rules of a special query language. The string contains the words to search for. It can also contain operators that specify requirements such that a word must be present or absent in matching rows, or that it should be weighted higher or lower than usual. Common words such as “some” or “then” are stopwords and do not match if present in the search string. The IN BOOLEAN MODE modifier specifies a boolean search. For more information, see Section 11.9.2, “Boolean Full-Text Searches”.

  • A natural language search interprets the search string as a phrase in natural human language (a phrase in free text). There are no special operators. The stopword list applies. In addition, words that are present in 50% or more of the rows are considered common and do not match. Full-text searches are natural language searches if the IN NATURAL LANGUAGE MODE modifier is given or if no modifier is given.

  • A query expansion search is a modification of a natural language search. The search string is used to perform a natural language search. Then words from the most relevant rows returned by the search are added to the search string and the search is done again. The query returns the rows from the second search. The IN NATURAL LANGUAGE MODE WITH QUERY EXPANSION or WITH QUERY EXPANSION modifier specifies a query expansion search. For more information, see Section 11.9.3, “Full-Text Searches with Query Expansion”.

Constraints on full-text searching are listed in Section 11.9.5, “Full-Text Restrictions”.

The myisam_ftdump utility can be used to dump the contents of a full-text index. This may be helpful for debugging full-text queries. See Section 4.6.2, “myisam_ftdump — Display Full-Text Index information”.

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