Dela via

  • Artikel

FREETEXT (Transact-SQL)

Is a predicate used in a WHERE clause to search columns containing character-based data types for values that match the meaning and not just the exact wording of the words in the search condition. When FREETEXT is used, the full-text query engine internally performs the following actions on the freetext_string, assigns each term a weight, and then finds the matches.

  • Separates the string into individual words based on word boundaries (word-breaking).

  • Generates inflectional forms of the words (stemming).

  • Identifies a list of expansions or replacements for the terms based on matches in the thesaurus.

Topic link iconTransact-SQL Syntax Conventions

Syntax

FREETEXT ( { column_name | (column_list) | * } 
          ,'freetext_string' [ , LANGUAGE language_term ] )

Arguments

  • column_name
    Is the name of one or more full-text indexed columns of the table specified in the FROM clause. The columns can be of type char, varchar, nchar, nvarchar, text, ntext, image, xml, varbinary, or varbinary(max).

  • column_list
    Indicates that several columns, separated by a comma, can be specified. column_list must be enclosed in parentheses. Unless language_term is specified, the language of all columns of column_list must be the same.

  • *
    Specifies that all columns that have been registered for full-text searching should be used to search for the given freetext_string. If more than one table is in the FROM clause, * must be qualified by the table name. Unless language_term is specified, the language of all columns of the table must be the same.

  • freetext_string
    Is text to search for in the column_name. Any text, including words, phrases or sentences, can be entered. Matches are generated if any term or the forms of any term is found in the full-text index.

    Unlike in the CONTAINS and CONTAINSTABLE search condition where AND is a keyword, when used in freetext_string the word 'and' is considered a noise word, or stopword, and will be discarded.

    Use of WEIGHT, FORMSOF, wildcards, NEAR and other syntax is not allowed. freetext_string is wordbroken, stemmed, and passed through the thesaurus.

    freetext_string is nvarchar. An implicit conversion occurs when another character data type is used as input. In the following example, the @SearchWord variable, which is defined as varchar(30), causes an implicit conversion in the FREETEXT predicate.

    USE AdventureWorks2008R2;
GO
DECLARE @SearchWord varchar(30)
SET @SearchWord ='performance'
SELECT Description 
FROM Production.ProductDescription 
WHERE FREETEXT(Description, @SearchWord);

    Because "parameter sniffing" does not work across conversion, use nvarchar for better performance. In the example, declare @SearchWord as nvarchar(30).

    USE AdventureWorks2008R2;
GO
DECLARE @SearchWord nvarchar(30)
SET @SearchWord = N'performance'
SELECT Description 
FROM Production.ProductDescription 
WHERE FREETEXT(Description, @SearchWord);

    You can also use the OPTIMIZE FOR query hint for cases in which a non-optimal plan is generated.

  • LANGUAGE language_term
    Is the language whose resources will be used for word breaking, stemming, and thesaurus and stopword removal as part of the query. This parameter is optional and can be specified as a string, integer, or hexadecimal value corresponding to the locale identifier (LCID) of a language. If language_term is specified, the language it represents will be applied to all elements of the search condition. If no value is specified, the column full-text language is used.

    If documents of different languages are stored together as binary large objects (BLOBs) in a single column, the locale identifier (LCID) of a given document determines what language is used to index its content. When querying such a column, specifying LANGUAGElanguage_term can increase the probability of a good match.

    When specified as a string, language_term corresponds to the alias column value in he sys.syslanguages (Transact-SQL) compatibility view. The string must be enclosed in single quotation marks, as in 'language_term'. When specified as an integer, language_term is the actual LCID that identifies the language. When specified as a hexadecimal value, language_term is 0x followed by the hexadecimal value of the LCID. The hexadecimal value must not exceed eight digits, including leading zeros.

    If the value is in double-byte character set (DBCS) format, Microsoft SQL Server will convert it to Unicode.

    If the language specified is not valid or there are no resources installed that correspond to that language, Microsoft SQL Server returns an error. To use the neutral language resources, specify 0x0 as language_term.

Remarks

Full-text predicates and functions work on a single table, which is implied in the FROM predicate. To search on multiple tables, use a joined table in your FROM clause to search on a result set that is the product of two or more tables.

Full-text queries using FREETEXT are less precise than those full-text queries using CONTAINS. The SQL Server full-text search engine identifies important words and phrases. No special meaning is given to any of the reserved keywords or wildcard characters that typically have meaning when specified in the <contains_search_condition> parameter of the CONTAINS predicate.

FREETEXT is not recognized as a keyword if the compatibility level is less than 70. For more information, see sp_dbcmptlevel (Transact-SQL).

Full-text predicates are not allowed in the OUTPUT clause when the database compatibility level is set to 100.

Comparison of LIKE with Full-Text Search

In contrast to full-text search, the LIKE Transact-SQL predicate works on character patterns only. Also, you cannot use the LIKE predicate to query formatted binary data. Furthermore, a LIKE query against a large amount of unstructured text data is much slower than an equivalent full-text query against the same data. A LIKE query against millions of rows of text data can take minutes to return; whereas a full-text query can take only seconds or less against the same data, depending on the number of rows that are returned.

Examples

A. Using FREETEXT to search for words containing specified character values

The following example searches for all documents containing the words related to "vital", "safety", and "components".

USE AdventureWorks2008R2;
GO
SELECT Title
FROM Production.Document
WHERE FREETEXT (Document, 'vital safety components' );
GO

B. Using FREETEXT with variables

The following example uses a variable instead of a specific search term.

USE AdventureWorks2008R2;
GO
DECLARE @SearchWord nvarchar(30);
SET @SearchWord = N'high-performance';
SELECT Description 
FROM Production.ProductDescription 
WHERE FREETEXT(Description, @SearchWord);
GO