Chapter 9. Language Structure
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## Chapter 9. Language Structure

This chapter discusses the rules for writing the following elements of SQL statements when using MySQL:

- Literal values such as strings and numbers

- Identifiers such as database, table, and column names

- Reserved words

- User-defined and system variables

- Comments

## 9.1. Literal Values

This section describes how to write literal values in MySQL. These include strings, numbers, hexadecimal values, boolean values, and `NULL`. The section also covers the various nuances and “gotchas” that you may run into when dealing with these basic types in MySQL.

### 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 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 10.3.5, “Character String Literal Character Set and Collation”.

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.

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`’.

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``.

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 10.3.5, “Character String Literal Character Set and Collation”.

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 Section 12.3.1, “String Comparison Functions”. If you use ‘`\%`’ or ‘`\_`’ in non-pattern-matching contexts, they evaluate to the strings ‘`\%`’ and ‘`\_`’, not to ‘`%`’ and ‘`_`’.

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 Section 17.2.3.51, “`mysql_real_escape_string()`”. The Perl DBI interface provides a `quote` method to convert special characters to the proper escape sequences. See Section 17.4, “MySQL Perl API”. 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.

### 9.1.2. Numbers

Integers are represented as a sequence of digits. Floats use ‘`.`’ as a decimal separator. Either type of number may be preceded by ‘`-`’ or ‘`+`’ 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.

### 9.1.3. Hexadecimal Values

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'

### 9.1.4. Boolean Values

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

### 9.1.5. `NULL` Values

The `NULL` value means “no data.” `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 A.5.3, “Problems with `NULL` Values”.

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 13.2.5, “`LOAD DATA INFILE` Syntax”.

## 9.2. Database, Table, Index, Column, and Alias Names

Database, table, index, column, and alias names are identifiers. This section describes the allowable syntax for identifiers in MySQL.

The following table describes the maximum length for each type of identifier.

Identifier | Maximum Length (bytes)
Database | 64
Table | 64
Column | 64
Index | 64
Alias | 255

There are some restrictions on the characters that may appear in identifiers:

- No identifier can contain ASCII 0 (`0x00`) or a byte with a value of 255.

- Before MySQL 4.1, identifier quote characters should not be used in identifiers. As of 4.1, the use of identifier quote characters in identifiers is permitted, although it is best to avoid doing so if possible.

- Database, table, and column names should not end with space characters.

- Database names cannot contain ‘`/`’, ‘`\`’, ‘`.`’, or characters that are not allowed in a directory name.

- Table names cannot contain ‘`/`’, ‘`\`’, ‘`.`’, or characters that are not allowed in a filename.

- The length of the identifier is in bytes, not characters. If you use multi-byte characters in your identifier names, then the maximum length will depend on the byte count of all the characters used.

Beginning with MySQL 4.1, identifiers are stored using Unicode (UTF-8). This applies to identifiers in table definitions that stored in `.frm` files and to identifiers stored in the grant tables in the `mysql` database. Although Unicode identifiers can include multi-byte characters, note that the maximum lengths shown in the table are byte counts until MySQL 4.1.5; until that version, if an identifier does contain multi-byte characters, the number of characters allowed in the identifier is less than the value shown in the table.

An identifier may be quoted or unquoted. If an identifier is a reserved word or contains special characters, you must quote it whenever you refer to it. (Exception: A word that follows a period in a qualified name must be an identifier, so it need not be quoted even if it is reserved.) For a list of reserved words, see Section 9.3, “Reserved Words”. Special characters are those outside the set of alphanumeric characters from the current character set, ‘`_`’, and ‘`$`’.

The identifier quote character is the backtick (‘```’):

    mysql> SELECT * FROM `select` WHERE `select`.id > 100;

If the `ANSI_QUOTES` SQL mode is enabled, it is also allowable to quote identifiers within double quotes:

    mysql> CREATE TABLE "test" (col INT);
    ERROR 1064: You have an error in your SQL syntax. (...)
    mysql> SET sql_mode='ANSI_QUOTES';
    mysql> CREATE TABLE "test" (col INT);
    Query OK, 0 rows affected (0.00 sec)

Note: Because the `ANSI_QUOTES` mode causes the server to interpret double-quoted strings as identifiers, string literals must be enclosed within single quotes when this mode is enabled. They cannot be enclosed within double quotes.

The server SQL mode is controlled as described in Section 5.2.5, “SQL Modes”.

As of MySQL 4.1, identifier quote characters can be included within an identifier if you quote the identifier. If the character to be included within the identifier is the same as that used to quote the identifier itself, double the character. The following statement creates a table named `a`b` that contains a column named `c"d`:

    mysql> CREATE TABLE `a``b` (`c"d` INT);

Identifier quoting was introduced in MySQL 3.23.6 to allow use of identifiers that are reserved words or that contain special characters. Before 3.23.6, you cannot use identifiers that require quotes, so the rules for legal identifiers are more restrictive:

- A name may consist of alphanumeric characters from the current character set, ‘`_`’, and ‘`$`’. The default character set is cp1252 (Latin1). This may be changed with the `--default-character-set` option to mysqld. See Section 5.10.1, “The Character Set Used for Data and Sorting”.

- A name may start with any character that is legal in a name. In particular, a name may start with a digit; this differs from many other database systems! However, an unquoted name cannot consist only of digits.

- You cannot use the ‘`.`’ character in names because it is used to extend the format by which you can refer to columns (see Section 9.2.1, “Identifier Qualifiers”).

Identifiers may begin with a digit but unless quoted may not consist solely of digits.

It is recommended that you do not use names of the form ``M`e` or ``M`e`N``, where `M` and `N` are integers. For example, avoid using `1e` or `2e2` as identifiers, because an expression such as `1e+3` is ambiguous. Depending on context, it might be interpreted as the expression `1e + 3` or as the number `1e+3`.

Be careful when using `MD5()` to produce table names because it can produce names in illegal or ambiguous formats such as those just described.

### 9.2.1. Identifier Qualifiers

MySQL allows names that consist of a single identifier or multiple identifiers. The components of a multiple-part name should be separated by period (‘`.`’) 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:

Column Reference | Meaning
`col_name` | The column `col_name` from whichever table used in the statement contains a column of that name.
`tbl_name.col_name` | The column `col_name` from table `tbl_name` of the default database.
`db_name.tbl_name.col_name` | The column `col_name` from table `tbl_name` of the database `db_name`. This syntax is unavailable before MySQL 3.22.

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 ‘`.`’ character.

### 9.2.2. Identifier Case Sensitivity

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 1.9.4, “MySQL Extensions to Standard SQL”. The `lower_case_table_names` system variable also affects how the server handles identifier case sensitivity, as described later in this section.

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.

Value | Meaning
`0` | Table and database names are stored on disk using the lettercase specified in the `CREATE TABLE` or `CREATE DATABASE` statement. Name comparisons are case sensitive. Note that if you force this variable to 0 with `--lower-case-table-names=0` on a case-insensitive filesystem and access `MyISAM` tablenames using different lettercases, this may lead to index corruption.
`1` | Table names are stored in lowercase on disk and name comparisons are not case sensitive. MySQL converts all table names to lowercase on storage and lookup. This behavior also applies to database names as of MySQL 4.0.2, and to table aliases as of 4.1.1.

`2`

Table and database names are stored on disk using the lettercase specified in the `CREATE TABLE` or `CREATE DATABASE` statement, but MySQL converts them to lowercase on lookup. Name comparisons are not case sensitive. Note: This works only on filesystems that are not case sensitive! `InnoDB` table names are stored in lowercase, as for `lower_case_table_names=1`. Setting `lower_case_table_names` to `2` can be done as of MySQL 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:

- Use `lower_case_table_names=1` on all systems. The main disadvantage with this is that when you use `SHOW TABLES` or `SHOW DATABASES`, you don't see the names in their original lettercase.

- Use `lower_case_table_names=0` on Unix and `lower_case_table_names=2` on Windows. This preserves the lettercase of database and table names. The disadvantage of this is that you must ensure that your statements always refer to your database and table names with the correct lettercase on Windows. If you transfer your statements to Unix, where lettercase is significant, they do not work if the lettercase is incorrect.

  Exception: If you are using `InnoDB` tables, you should set `lower_case_table_names` to 1 on all platforms to force names to be converted to lowercase.

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.

### 9.2.3. Function Name Parsing and Resolution

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:

- To use the name as a function call in an expression, there must be no whitespace between the name and the following ‘`(`’ parenthesis character.

- Conversely, to use the function name as an identifier, it must not be followed immediately by a parenthesis.

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:

- With `IGNORE_SPACE` disabled, the parser interprets the name as a function call when there is no whitespace between the name and the following parenthesis. This occurs even when the function name is used in non-expression context:

      mysql> CREATE TABLE count(i INT);
      ERROR 1064 (42000): You have an error in your SQL syntax ...
      near 'count(i INT)'

  To eliminate the error and cause the name to be treated as an identifier, either use whitespace following the name or write it as a quoted identifier (or both):

      CREATE TABLE count (i INT);
      CREATE TABLE `count`(i INT);
      CREATE TABLE `count` (i INT);

- With `IGNORE_SPACE` enabled, the parser loosens the requirement that there be no whitespace between the function name and the following parenthesis. This provides more flexibility in writing function calls. For example, either of the following function calls are legal:

      SELECT COUNT(*) FROM mytable;
      SELECT COUNT (*) FROM mytable;

  However, enabling `IGNORE_SPACE` also has the side effect that the parser treats the affected function names as reserved words (see Section 9.3, “Reserved Words”). This means that a space following the name no longer signifies its use as an identifier. The name can be used in function calls with or without following whitespace, but causes a syntax error in non-expression context unless it is quoted. For example, with `IGNORE_SPACE` enabled, both of the following statements fail with a syntax error because the parser interprets `count` as a reserved word:

      CREATE TABLE count(i INT);
      CREATE TABLE count (i INT);

  To use the function name in non-expression context, write it as a quoted identifier:

      CREATE TABLE `count`(i INT);
      CREATE TABLE `count` (i INT);

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 5.2.5, “SQL Modes”, to see which composite modes enable `IGNORE_SPACE`.

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

- Avoid creating UDFs that have the same name as a built-in function.

- Avoid using function names in non-expression context. For example, these statements use `count` (one of the affected function names affected by `IGNORE_SPACE`), so they fail with or without whitespace following the name if `IGNORE_SPACE` is enabled:

      CREATE TABLE count(i INT);
      CREATE TABLE count (i INT);

  If you must use a function name in non-expression context, write it as a quoted identifier:

      CREATE TABLE `count`(i INT);
      CREATE TABLE `count` (i INT);

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.

## 9.3. Reserved Words

Certain words such as `SELECT`, `DELETE`, or `BIGINT` are reserved and require special treatment for use as identifiers such as table and column names. This may also be true for the names of built-in functions.

Reserved words are permitted as identifiers if you quote them as described in Section 9.2, “Database, Table, Index, Column, and Alias Names”:

    mysql> CREATE TABLE interval (begin INT, end INT);
    ERROR 1064 (42000): You have an error in your SQL syntax ...
    near 'interval (begin INT, end INT)'

    mysql> CREATE TABLE `interval` (begin INT, end INT);
    Query OK, 0 rows affected (0.01 sec)

Exception: A word that follows a period in a qualified name must be an identifier, so it need not be quoted even if it is reserved:

    mysql> CREATE TABLE mydb.interval (begin INT, end INT);
    Query OK, 0 rows affected (0.01 sec)

Names of built-in functions are permitted as identifiers but may require care to be used as such. For example, `COUNT` is acceptable as a column name. However, by default, no whitespace is allowed in function invocations between the function name and the following ‘`(`’ character. This requirement enables the parser to distinguish whether the name is used in a function call or in non-function context. For further detail on recognition of function names, see Section 9.2.3, “Function Name Parsing and Resolution”.

The words in the following table are explicitly reserved in MySQL 4.1. At some point, you might upgrade to a higher version, so it's a good idea to have a look at future reserved words, too. You can find these in the manuals that cover higher versions of MySQL. Most of the words in the table are forbidden by standard SQL as column or table names (for example, `GROUP`). A few are reserved because MySQL needs them and uses a yacc parser. A reserved word can be used as an identifier if you quote it.

`ADD` | `ALL` | `ALTER`
`ANALYZE` | `AND` | `AS`
`ASC` | `BEFORE` | `BETWEEN`
`BIGINT` | `BINARY` | `BLOB`
`BOTH` | `BY` | `CASCADE`
`CASE` | `CHANGE` | `CHAR`
`CHARACTER` | `CHECK` | `COLLATE`
`COLUMN` | `COLUMNS` | `CONSTRAINT`
`CONVERT` | `CREATE` | `CROSS`
`CURRENT_DATE` | `CURRENT_TIME` | `CURRENT_TIMESTAMP`
`CURRENT_USER` | `DATABASE` | `DATABASES`
`DAY_HOUR` | `DAY_MICROSECOND` | `DAY_MINUTE`
`DAY_SECOND` | `DEC` | `DECIMAL`
`DEFAULT` | `DELAYED` | `DELETE`
`DESC` | `DESCRIBE` | `DISTINCT`
`DISTINCTROW` | `DIV` | `DOUBLE`
`DROP` | `DUAL` | `ELSE`
`ENCLOSED` | `ESCAPED` | `EXISTS`
`EXPLAIN` | `FALSE` | `FIELDS`
`FLOAT` | `FLOAT4` | `FLOAT8`
`FOR` | `FORCE` | `FOREIGN`
`FROM` | `FULLTEXT` | `GRANT`
`GROUP` | `HAVING` | `HIGH_PRIORITY`
`HOUR_MICROSECOND` | `HOUR_MINUTE` | `HOUR_SECOND`
`IF` | `IGNORE` | `IN`
`INDEX` | `INFILE` | `INNER`
`INSERT` | `INT` | `INT1`
`INT2` | `INT3` | `INT4`
`INT8` | `INTEGER` | `INTERVAL`
`INTO` | `IS` | `JOIN`
`KEY` | `KEYS` | `KILL`
`LEADING` | `LEFT` | `LIKE`
`LIMIT` | `LINES` | `LOAD`
`LOCALTIME` | `LOCALTIMESTAMP` | `LOCK`
`LONG` | `LONGBLOB` | `LONGTEXT`
`LOW_PRIORITY` | `MATCH` | `MEDIUMBLOB`
`MEDIUMINT` | `MEDIUMTEXT` | `MIDDLEINT`
`MINUTE_MICROSECOND` | `MINUTE_SECOND` | `MOD`
`NATURAL` | `NOT` | `NO_WRITE_TO_BINLOG`
`NULL` | `NUMERIC` | `ON`
`OPTIMIZE` | `OPTION` | `OPTIONALLY`
`OR` | `ORDER` | `OUTER`
`OUTFILE` | `PRECISION` | `PRIMARY`
`PRIVILEGES` | `PROCEDURE` | `PURGE`
`RAID0` | `READ` | `REAL`
`REFERENCES` | `REGEXP` | `RENAME`
`REPLACE` | `REQUIRE` | `RESTRICT`
`REVOKE` | `RIGHT` | `RLIKE`
`SECOND_MICROSECOND` | `SELECT` | `SEPARATOR`
`SET` | `SHOW` | `SMALLINT`
`SONAME` | `SPATIAL` | `SQL_BIG_RESULT`
`SQL_CALC_FOUND_ROWS` | `SQL_SMALL_RESULT` | `SSL`
`STARTING` | `STRAIGHT_JOIN` | `TABLE`
`TABLES` | `TERMINATED` | `THEN`
`TINYBLOB` | `TINYINT` | `TINYTEXT`
`TO` | `TRAILING` | `TRUE`
`UNION` | `UNIQUE` | `UNLOCK`
`UNSIGNED` | `UPDATE` | `USAGE`
`USE` | `USING` | `UTC_DATE`
`UTC_TIME` | `UTC_TIMESTAMP` | `VALUES`
`VARBINARY` | `VARCHAR` | `VARCHARACTER`
`VARYING` | `WHEN` | `WHERE`
`WITH` | `WRITE` | `X509`
`XOR` | `YEAR_MONTH` | `ZEROFILL`

The following are new reserved words in MySQL 4.0:

`CHECK` | `FORCE` | `LOCALTIME`
`LOCALTIMESTAMP` | `REQUIRE` | `SQL_CALC_FOUND_ROWS`
`SSL` | `X509` | `XOR`

The following are new reserved words in MySQL 4.1:

`BEFORE` | `COLLATE` | `CONVERT`
`CURRENT_USER` | `DAY_MICROSECOND` | `DIV`
`DUAL` | `FALSE` | `HOUR_MICROSECOND`
`MINUTE_MICROSECOND` | `MOD` | `NO_WRITE_TO_BINLOG`
`SECOND_MICROSECOND` | `SEPARATOR` | `SPATIAL`
`TRUE` | `UTC_DATE` | `UTC_TIME`
`UTC_TIMESTAMP` | `VARCHARACTER` |

MySQL allows some keywords to be used as unquoted identifiers because many people previously used them. Examples are those in the following list:

- `ACTION`

- `BIT`

- `DATE`

- `ENUM`

- `NO`

- `TEXT`

- `TIME`

- `TIMESTAMP`

## 9.4. User-Defined Variables

MySQL supports user variables as of version 3.23.6. You can store a value in a user-defined variable and then refer to it later. This enables you to pass values from one statement to another. User-defined variables are connection-specific. That is, a user variable defined by one client cannot be seen or used by other clients. All variables for a given client connection are automatically freed when that client exits.

User variables are written as `@`var_name``, where the variable name `var_name` may consist of alphanumeric characters from the current character set, ‘`.`’, ‘`_`’, and ‘`$`’. The default character set is `latin1` (cp1252 West European). This may be changed with the `--default-character-set` option to mysqld. See Section 5.10.1, “The Character Set Used for Data and Sorting”. Section 5.10.1, “The Character Set Used for Data and Sorting”. A user variable name can contain other characters if you quote it as a string or identifier (for example, `@'my-var'`, `@"my-var"`, or `@`my-var``).

Note: User variable names are case sensitive prior to MySQL 5.0.

One way to set a user variable is by issuing a `SET` statement:

    SET @var_name = expr [, @var_name = expr] ...

For `SET`, either `=` or `:=` can be used as the assignment operator. The `expr` assigned to each variable can evaluate to an integer, real, string, or `NULL` value. However, if the value of the variable is selected in a result set, it is returned to the client as a string.

You can also assign a value to a user variable in statements other than `SET`. In this case, the assignment operator must be `:=` and not `=` because `=` is treated as a comparison operator in non-`SET` statements:

    mysql> SET @t1=0, @t2=0, @t3=0;
    mysql> SELECT @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3;
    +----------------------+------+------+------+
    | @t1:=(@t2:=1)+@t3:=4 | @t1  | @t2  | @t3  |
    +----------------------+------+------+------+
    |                    5 |    5 |    1 |    4 |
    +----------------------+------+------+------+

User variables may be used in contexts where expressions are allowed. This does not currently include contexts that explicitly require a literal value, such as in the `LIMIT` clause of a `SELECT` statement, or the `IGNORE `N` LINES` clause of a `LOAD DATA` statement.

Beginning with MySQL 4.1.1, if a user variable is assigned a string value, it has the same character set and collation as the string. The coercibility of user variables is “implicit” as of MySQL 4.1.11 and 5.0.3. (This is the same coercibility as table column values.)

Note: In a `SELECT` statement, each expression is evaluated only when sent to the client. This means that in a `HAVING`, `GROUP BY`, or `ORDER BY` clause, you cannot refer to an expression that involves variables that are set in the `SELECT` list. For example, the following statement does not work as expected:

    mysql> SELECT (@aa:=id) AS a, (@aa+3) AS b FROM tbl_name HAVING b=5;

The reference to `b` in the `HAVING` clause refers to an alias for an expression in the `SELECT` list that uses `@aa`. This does not work as expected: `@aa` contains the value of `id` from the previous selected row, not from the current row.

The order of evaluation for user variables is undefined and may change based on the elements contained within a given query. In `SELECT @a, @a := @a+1 ...`, you might think that MySQL will evaluate `@a` first and then do an assignment second, but changing the query (for example, by adding a `GROUP BY`, `HAVING`, or `ORDER BY` clause) may change the order of evaluation.

The general rule is never to assign a value to a user variable in one part of a statement and use the same variable in some other part of the same statement. You might get the results you expect, but this is not guaranteed.

Another issue with setting a variable and using it in the same statement is that the default result type of a variable is based on the type of the variable at the start of the statement. The following example illustrates this:

    mysql> SET @a='test';
    mysql> SELECT @a,(@a:=20) FROM tbl_name;

For this `SELECT` statement, MySQL reports to the client that column one is a string and converts all accesses of `@a` to strings, even though @a is set to a number for the second row. After the `SELECT` statement executes, `@a` is regarded as a number for the next statement.

To avoid problems with this behavior, either do not set and use the same variable within a single statement, or else set the variable to `0`, `0.0`, or `''` to define its type before you use it.

If you refer to a variable that has not been initialized, it has a value of `NULL` and a type of string.

## 9.5. Comment Syntax

MySQL Server supports three comment styles:

- From a ‘`#`’ character to the end of the line.

- From a ‘`--` ’ sequence to the end of the line. This style is supported as of MySQL 3.23.3. In MySQL, the <squo;`--` ’ (double-dash) comment style requires the second dash to be followed by at least one whitespace or control character (such as a space, tab, newline, and so on). This syntax differs slightly from standard SQL comment syntax, as discussed in Section 1.9.5.7, “'`--`' as the Start of a Comment”.

- From a `/*` sequence to the following `*/` sequence, as in the C programming language. This syntax allows a comment to extend over multiple lines because the beginning and closing sequences need not be on the same line.

The following example demonstrates all three comment styles:

    mysql> SELECT 1+1;     # This comment continues to the end of line
    mysql> SELECT 1+1;     -- This comment continues to the end of line
    mysql> SELECT 1 /* this is an in-line comment */ + 1;
    mysql> SELECT 1+
    /*
    this is a
    multiple-line comment
    */
    1;

MySQL Server supports some variants of C-style comments. These enable you to write code that includes MySQL extensions, but is still portable, by using comments of the following form:

    /*! MySQL-specific code */

In this case, MySQL Server parses and executes the code within the comment as it would any other SQL statement, but other SQL servers will ignore the extensions. For example, MySQL Server recognizes the `STRAIGHT_JOIN` keyword in the following statement, but other servers will not:

    SELECT /*! STRAIGHT_JOIN */ col1 FROM table1,table2 WHERE ...

If you add a version number after the ‘`!`’ character, the syntax within the comment is executed only if the MySQL version is greater than or equal to the specified version number. The `TEMPORARY` keyword in the following comment is executed only by servers from MySQL 3.23.02 or higher:

    CREATE /*!32302 TEMPORARY */ TABLE t (a INT);

The comment syntax just described applies to how the mysqld server parses SQL statements. The mysql client program also performs some parsing of statements before sending them to the server. (It does this to determine statement boundaries within a multiple-statement input line.) However, there are some limitations on the way that mysql parses `/* ... */` comments:

- A semicolon within the comment is taken to indicate the end of the current SQL statement and anything following it to indicate the beginning of the next statement. This problem was fixed in MySQL 4.0.13.

- A single quote, double quote, or backtick character is taken to indicate the beginning of a quoted string or identifier, even within a comment. If the quote is not matched by a second quote within the comment, the parser doesn't realize the comment has ended. If you are running mysql interactively, you can tell that it has gotten confused like this because the prompt changes from `mysql>` to `'>`, `">`, or ``>`. This problem was fixed in MySQL 4.1.1.

For affected versions of MySQL, these limitations apply both when you run mysql interactively and when you put commands in a file and use mysql in batch mode to process the file with mysql < `file_name`.

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