(mysql.info) string-syntax
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9.1.1 Strings
-------------
A string is a sequence of bytes or characters, enclosed within either
single quote (‘`''’) or double quote (‘`"'’) characters.
Examples:
'a string'
"another string"
If the `ANSI_QUOTES' SQL mode is enabled, string literals can be quoted
only within single quotes. A string quoted within double quotes is
interpreted as an identifier.
A binary string is a string of bytes that has no character set or
collation. A non-binary string is a string of characters that has a
character set and collation. For both types of strings, comparisons are
based on the numeric values of the string unit. For binary strings,
the unit is the byte. For non-binary strings the unit is the character
and some character sets allow multi-byte characters.
String literals may have an optional character set introducer and
`COLLATE' clause:
[_CHARSET_NAME]'STRING' [COLLATE COLLATION_NAME]
Examples:
SELECT _latin1'STRING';
SELECT _latin1'STRING' COLLATE latin1_danish_ci;
For more information about these forms of string syntax, see
charset-literal.
Within a string, certain sequences have special meaning. Each of these
sequences begins with a backslash (‘`\'’), known as the _escape
character_. MySQL recognizes the following escape sequences:
`\0' An ASCII 0 (`NUL') character.
`\'' A single quote (‘`''’) character.
`\"' A double quote (‘`"'’) character.
`\b' A backspace character.
`\n' A newline (linefeed) character.
`\r' A carriage return character.
`\t' A tab character.
`\Z' ASCII 26 (Control-Z). See note following the table.
`\\' A backslash (‘`\'’) character.
`\%' A ‘`%'’ character. See note following the table.
`\_' A ‘`_'’ character. See note following the table.
These sequences are case sensitive. For example, ‘`\b'’ is
interpreted as a backspace, but ‘`\B'’ is interpreted as ‘`B'’.
The ASCII 26 character can be encoded as ‘`\Z'’ to enable you to
work around the problem that ASCII 26 stands for END-OF-FILE on Windows.
ASCII 26 within a file causes problems if you try to use `mysql DB_NAME
< FILE_NAME'.
The ‘`\%'’ and ‘`\_'’ sequences are used to search for literal
instances of ‘`%'’ and ‘`_'’ in pattern-matching contexts where
they would otherwise be interpreted as wildcard characters. See the
description of the `LIKE' operator in
string-comparison-functions. If you use ‘`\%'’ or ‘`\_'’ in
non-pattern-matching contexts, they evaluate to the strings ‘`\%'’
and ‘`\_'’, not to ‘`%'’ and ‘`_'’.
For all other escape sequences, backslash is ignored. That is, the
escaped character is interpreted as if it was not escaped. For
example, ‘`\x'’ is just ‘`x'’.
There are several ways to include quote characters within a string:
* A ‘`''’ inside a string quoted with ‘`''’ may be written as
‘`'''’.
* A ‘`"'’ inside a string quoted with ‘`"'’ may be written as
‘`""'’.
* Precede the quote character by an escape character (‘`\'’).
* A ‘`''’ inside a string quoted with ‘`"'’ needs no special
treatment and need not be doubled or escaped. In the same way,
‘`"'’ inside a string quoted with ‘`''’ needs no special
treatment.
The following `SELECT' statements demonstrate how quoting and escaping
work:
mysql> SELECT 'hello', '"hello"', '""hello""', 'hel''lo', '\'hello';
+-------+---------+-----------+--------+--------+
| hello | "hello" | ""hello"" | hel'lo | 'hello |
+-------+---------+-----------+--------+--------+
mysql> SELECT "hello", "'hello'", "''hello''", "hel""lo", "\"hello";
+-------+---------+-----------+--------+--------+
| hello | 'hello' | ''hello'' | hel"lo | "hello |
+-------+---------+-----------+--------+--------+
mysql> SELECT 'This\nIs\nFour\nLines';
+--------------------+
| This
Is
Four
Lines |
+--------------------+
mysql> SELECT 'disappearing\ backslash';
+------------------------+
| disappearing backslash |
+------------------------+
If you want to insert binary data into a string column (such as a
`BLOB' column), the following characters must be represented by escape
sequences:
`NUL' `NUL' byte (ASCII 0). Represent this character by ‘`\0'’ (a
backslash followed by an ASCII ‘`0'’ character).
`\' Backslash (ASCII 92). Represent this character by ‘`\\'’.
`'' Single quote (ASCII 39). Represent this character by ‘`\''’.
`"' Double quote (ASCII 34). Represent this character by ‘`\"'’.
When writing application programs, any string that might contain any of
these special characters must be properly escaped before the string is
used as a data value in an SQL statement that is sent to the MySQL
server. You can do this in two ways:
* Process the string with a function that escapes the special
characters. In a C program, you can use the
`mysql_real_escape_string()' C API function to escape characters.
See mysql-real-escape-string. The Perl DBI interface
provides a `quote' method to convert special characters to the
proper escape sequences. See perl. Other language
interfaces may provide a similar capability.
* As an alternative to explicitly escaping special characters, many
MySQL APIs provide a placeholder capability that enables you to
insert special markers into a statement string, and then bind data
values to them when you issue the statement. In this case, the API
takes care of escaping special characters in the values for you.
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