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Pattern Value Arguments

  • Článek

Some arguments in the catalog functions, such as the TableName argument in SQLTables, accept search patterns. These arguments accept search patterns if the SQL_ATTR_METADATA_ID statement attribute is set to SQL_FALSE; they are identifier arguments that do not accept a search pattern if this attribute is set to SQL_TRUE.

The search pattern characters are:

  • An underscore (_), which represents any single character.

  • A percent sign (%), which represents any sequence of zero or more characters.

  • An escape character, which is driver-specific and is used to include underscores, percent signs, and the escape character as literals. If the escape character precedes a non-special character, the escape character has no special meaning. If the escape character precedes a special character, it escapes the special character. For example, "\a" would be treated as two characters, "\" and "a", but "\%" would be treated as the non-special single character "%".

The escape character is retrieved with the SQL_SEARCH_PATTERN_ESCAPE option in SQLGetInfo. It must precede any underscore, percent sign, or escape character in an argument that accepts search patterns to include that character as a literal. Examples are shown in the following table.

Search pattern Description
%A% All identifiers containing the letter A
ABC_ All four character identifiers starting with ABC
ABC\_ The identifier ABC_, assuming the escape character is a backslash (\)
\\% All identifiers starting with a backslash (\), assuming the escape character is a backslash

Special care must be taken to escape search pattern characters in arguments that accept search patterns. This is particularly true for the underscore character, which is commonly used in identifiers. A common mistake in applications is to retrieve a value from one catalog function and pass that value to a search pattern argument in another catalog function. For example, suppose an application retrieves the table name MY_TABLE from the result set for SQLTables and passes this to SQLColumns to retrieve a list of columns in MY_TABLE. Instead of getting the columns for MY_TABLE, the application will get the columns for all the tables that match the search pattern MY_TABLE, such as MY_TABLE, MY1TABLE, MY2TABLE, and so on.

Note

ODBC 2.x drivers do not support search patterns in the CatalogName argument in SQLTables. ODBC 3*.x* drivers accept search patterns in this argument if the SQL_ATTR_ ODBC_VERSION environment attribute is set to SQL_OV_ODBC3; they do not accept search patterns in this argument if it is set to SQL_OV_ODBC2.

Passing a null pointer to a search pattern argument does not constrain the search for that argument; that is, a null pointer and the search pattern % (any characters) are equivalent. However, a zero-length search pattern - that is, a valid pointer to a string of length zero - matches only the empty string ("").