FREETEXT (Transact-SQL)

Artykuł
02/13/2015

Is a predicate used in the Transact-SQL WHERE clause of a Transact-SQL SELECT statement to perform a SQL Server full-text search on full-text indexed columns containing character-based data types. This predicate searches 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.

[!UWAGA]

For information about the forms of full-text searches that are supported by SQL Server, see Query with Full-Text Search.

Ikona łącza do tematu Transact-SQL Syntax Conventions

Składnia

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 AdventureWorks2012;
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 AdventureWorks2012;
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 nonoptimal 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 LANGUAGE language_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.

General 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.

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

[!UWAGA]

The FREETEXTTABLE function is useful for the same kinds of matches as the FREETEXT predicate. You can reference this function like a regular table name in the FROM clause of a SELECT statement. For more information, see FREETEXTTABLE (Transact-SQL).

Querying Remote Servers

You can use a four-part name in the CONTAINS or FREETEXT predicate to query full-text indexed columns of the target tables on a linked server. To prepare a remote server to receive full-text queries, create a full-text index on the target tables and columns on the remote server and then add the remote server as a linked server.

Comparison of LIKE to 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, components.

USE AdventureWorks2012;
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 AdventureWorks2012;
GO
DECLARE @SearchWord nvarchar(30);
SET @SearchWord = N'high-performance';
SELECT Description 
FROM Production.ProductDescription 
WHERE FREETEXT(Description, @SearchWord);
GO

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