Chapter聽9.聽Language Structure - MySQL 4.1参考手册英文版

A string is a sequence of bytes or characters, enclosed within either single quote (鈥�'鈥�) or double quote (鈥�"鈥�) characters. Examples:

'a string'
"another string"

If the ANSI_QUOTES SQL mode is enabled, string literals can be quoted only within single quotes because a string quoted within double quotes is interpreted as an identifier.

As of MySQL 4.1.1, 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. Character value ordering is a function of the string collation.

Also as of MySQL 4.1.1, 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 Section聽10.3.5, 鈥淐haracter String Literal Character Set and Collation鈥�.

Within a string, certain sequences have special meaning. Each of these sequences begins with a backslash (鈥�\鈥�), known as the escape character. MySQL recognizes the following escape sequences:

For all other escape sequences, backslash is ignored. That is, the escaped character is interpreted as if it was not escaped. For example, 鈥�\x鈥� is just 鈥�x鈥�.

These sequences are case sensitive. For example, 鈥�\b鈥� is interpreted as a backspace, but 鈥�\B鈥� is interpreted as 鈥�B鈥�.

The ASCII 26 character can be encoded as 鈥�\Z鈥� 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.

Escape processing is done according to the character set indicated by the character_set_connection system variable. This is true even for strings that are preceded by an introducer that indicates a different character set, as discussed in Section聽10.3.5, 鈥淐haracter String Literal Character Set and Collation鈥�.

The 鈥�\%鈥� and 鈥�\_鈥� sequences are used to search for literal instances of 鈥�%鈥� and 鈥�_鈥� in pattern-matching contexts where they would otherwise be interpreted as wildcard characters. See the description of the LIKE operator in Section聽12.4.1, 鈥淪tring Comparison Functions鈥�. If you use 鈥�\%鈥� or 鈥�\_鈥� in non-pattern-matching contexts, they evaluate to the strings 鈥�\%鈥� and 鈥�\_鈥�, not to 鈥�%鈥� and 鈥�_鈥�.

There are several ways to include quote characters within a string:

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:

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:

Integers are represented as a sequence of digits. Floats use 鈥�.鈥� as a decimal separator. Either type of number may be preceded by 鈥�-鈥� or 鈥�+鈥� to indicate a negative or positive value, respectively

Examples of valid integers:

1221
0
-32

Examples of valid floating-point numbers:

294.42
-32032.6809e+10
148.00

An integer may be used in a floating-point context; it is interpreted as the equivalent floating-point number.

MySQL supports hexadecimal values. In numeric contexts, these act like integers (64-bit precision). In string contexts, these act like binary strings, where each pair of hex digits is converted to a character:

mysql> SELECT x'4D7953514C';
        -> 'MySQL'
mysql> SELECT 0xa+0;
        -> 10
mysql> SELECT 0x5061756c;
        -> 'Paul'

In MySQL 4.1 (and in MySQL 4.0 when using the --new option), the default type of a hexadecimal value is a string. If you want to ensure that the value is treated as a number, you can use CAST(... AS UNSIGNED):

mysql> SELECT 0x41, CAST(0x41 AS UNSIGNED);
        -> 'A', 65

The x'hexstring' syntax is new in 4.0 and is based on standard SQL. The 0x syntax is based on ODBC. Hexadecimal strings are often used by ODBC to supply values for BLOB columns.

Beginning with MySQL 4.0.1, you can convert a string or a number to a string in hexadecimal format with the HEX() function:

mysql> SELECT HEX('cat');
        -> '636174'
mysql> SELECT 0x636174;
        -> 'cat'

Beginning with MySQL 4.1, The constants TRUE and FALSE evaluate to 1 and 0, respectively. The constant names can be written in any lettercase.

mysql> SELECT TRUE, true, FALSE, false;
        -> 1, 1, 0, 0

The NULL value means 鈥�no data.鈥� NULL can be written in any lettercase.

Be aware that the NULL value is different from values such as 0 for numeric types or the empty string for string types. See Section聽A.1.5.3, 鈥淧roblems with NULL Values鈥�.

For text file import or export operations performed with LOAD DATA INFILE or SELECT ... INTO OUTFILE, NULL is represented by the \N sequence. See Section聽13.2.5, 鈥�LOAD DATA INFILE Syntax鈥�.

MySQL allows names that consist of a single identifier or multiple identifiers. The components of a multiple-part name should be separated by period (鈥�.鈥�) characters. The initial parts of a multiple-part name act as qualifiers that affect the context within which the final identifier is interpreted.

In MySQL you can refer to a column using any of the following forms:

If any components of a multiple-part name require quoting, quote them individually rather than quoting the name as a whole. For example, write `my-table`.`my-column`, not `my-table.my-column`.

You need not specify a tbl_name or db_name.tbl_name prefix for a column reference in a statement unless the reference would be ambiguous. Suppose that tables t1 and t2 each contain a column c, and you retrieve c in a SELECT statement that uses both t1 and t2. In this case, c is ambiguous because it is not unique among the tables used in the statement. You must qualify it with a table name as t1.c or t2.c to indicate which table you mean. Similarly, to retrieve from a table t in database db1 and from a table t in database db2 in the same statement, you must refer to columns in those tables as db1.t.col_name and db2.t.col_name.

A word that follows a period in a qualified name must be an identifier, so it is not necessary to quote it, even if it is a reserved word.

The syntax .tbl_name means the table tbl_name in the default database. This syntax is accepted for ODBC compatibility because some ODBC programs prefix table names with a 鈥�.鈥� character.

In MySQL, databases correspond to directories within the data directory. Each table within a database corresponds to at least one file within the database directory (and possibly more, depending on the storage engine). Consequently, the case sensitivity of the underlying operating system determines the case sensitivity of database and table names. This means database and table names are not case sensitive in Windows, and case sensitive in most varieties of Unix. One notable exception is Mac OS X, which is Unix-based but uses a default filesystem type (HFS+) that is not case sensitive. However, Mac OS X also supports UFS volumes, which are case sensitive just as on any Unix. See Section聽1.9.4, 鈥淢ySQL Extensions to Standard SQL鈥�. The lower_case_table_names system variable also affects how the server handles identifier case sensitivity, as described later in this section.

MySQL Enterprise lower_case_table_names is just one of the system variables monitored by the MySQL Network Monitoring and Advisory Service. For information about subscribing to this service see, http://www.mysql.com/products/enterprise/advisors.html.

Note: Although database and table names are not case sensitive on some platforms, you should not refer to a given database or table using different cases within the same statement. The following statement would not work because it refers to a table both as my_table and as MY_TABLE:

mysql> SELECT * FROM my_table WHERE MY_TABLE.col=1;

Column, index, stored routine, and trigger names are not case sensitive on any platform, nor are column aliases.

Table aliases are case sensitive before MySQL 4.1.1. The following query would not work because it refers to the alias both as a and as A:

mysql> SELECT col_name FROM tbl_name AS a
    -> WHERE a.col_name = 1 OR A.col_name = 2;

If you have trouble remembering the allowable lettercase for database and table names, it is best to adopt a consistent convention, such as always creating and referring to databases and tables using lowercase names. This convention is recommended for maximum portability and ease of use.

How table and database names are stored on disk and used in MySQL is affected by the lower_case_table_names system variable, which you can set when starting mysqld. lower_case_table_names can take the values shown in the following table. On Unix, the default value of lower_case_table_names is 0. On Windows, the default value is 1. On Mac OS X, the default is 1 before MySQL 4.0.18 and 2 as of 4.0.18.

If you are using MySQL on only one platform, you don't normally have to change the lower_case_table_names variable. However, you may encounter difficulties if you want to transfer tables between platforms that differ in filesystem case sensitivity. For example, on Unix, you can have two different tables named my_table and MY_TABLE, but on Windows those names are considered identical. To avoid data transfer problems stemming from lettercase of database or table names, you have two options:

Note that if you plan to set the lower_case_table_names system variable to 1 on Unix, you must first convert your old database and table names to lowercase before restarting mysqld with the new variable setting.

MySQL 4.1 supports built-in (native) functions and user-defined functions (UDFs). This section describes how the server recognizes whether the name of a built-in function is used as a function call or as an identifier, and how the server determines which function to use in cases when functions of different types exist with a given name.

Built-In Function Name Parsing

The parser uses default rules for parsing names of built-in functions. These rules can be changed by enabling the IGNORE_SPACE SQL mode.

When the parser encounters a word that is the name of a built-in function, it must determine whether the name signifies a function call or is instead a non-expression reference to an identifier such as a table or column name. For example, in the following statements, the first reference to count is a function call, whereas the second reference is a table name:

SELECT COUNT(*) FROM mytable;
CREATE TABLE count (i INT);

The parser should recognize the name of a built-in function as indicating a function call only when parsing what is expected to be an expression. That is, in non-expression context, function names are permitted as identifiers.

However, some built-in functions have special parsing or implementation considerations, so the parser uses the following rules by default to distinguish whether their names are being used as function calls or as identifiers in non-expression context:

The requirement that function calls be written with no whitespace between the name and the parenthesis applies only to the built-in functions that have special considerations. COUNT is one such name. The exact list of function names for which following whitespace determines their interpretation are those listed in the sql_functions[] array of the sql/lex.h source file. Before MySQL 5.1, they are rather numerous (about 200), so you may find it easiest to treat the no-whitespace requirement as applying to all function calls. In MySQL 5.1, parser improvements reduce to about 30 the number of affected function names.

For functions not listed in the sql_functions[]) array, whitespace does not matter. They are interpreted as function calls only when used in expression context and may be used freely as identifiers otherwise. ASCII is one such name. However, for these non-affected function names, interpretation may vary in expression context: func_name () is interpreted as a built-in function if there is one; if not, func_name () is interpreted as a user-defined function if one exists with that name.

The IGNORE_SPACE SQL mode can be used to modify how the parser treats function names that are whitespace-sensitive:

To enable the IGNORE_SPACE SQL mode, use this statement:

SET sql_mode = 'IGNORE_SPACE';

IGNORE_SPACE is also enabled by certain other composite modes such as ANSI that include it in their value:

SET sql_mode = 'ANSI';

Check Section聽5.2.6, 鈥淪QL Modes鈥�, to see which composite modes enable IGNORE_SPACE.

To minimize the dependency of SQL code on the IGNORE_SPACE setting, use these guidelines:

Function Name Resolution

The server resolves references to function names for function creation and invocation as follows: A UDF can be created with the same name as a built-in function but the UDF cannot be invoked because the parser resolves invocations of the function to refer to the built-in function. For example, if you create a UDF named ABS, references to ABS() invoke the built-in function.

The preceding function name resolution rules have implications for upgrading to versions of MySQL that implement new built-in functions. If you have already created a user-defined function with a given name and upgrade MySQL to a version that implements a new built-in function with the same name, the UDF becomes inaccessible. To correct this, use DROP FUNCTION to drop the UDF, and then use CREATE FUNCTION to re-create the UDF with a different non-conflicting name.