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User-Defined Variables
user guide

User-Defined Variables

The format of the user-defined variables is @var_name. @var_name consists of alphanumeric characters, _, and $. The user-defined variables are case-insensitive.

The user-defined variables are session specific, which means a user variable defined by one client cannot be seen or used by other clients. You can use the SET statement to set a user variable:

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

or

SET @var_name := expr

For SET, you can use = or := as the assignment operator.

For example:

mysql> SET @a1=1, @a2=2, @a3:=4;
mysql> SELECT @a1, @a2, @t3, @a4 := @a1+@a2+@a3;
+------+------+------+--------------------+
| @a1 | @a2 | @a3 | @a4 := @a1+@a2+@a3 |
+------+------+------+--------------------+
| 1 | 2 | 4 | 7 |
+------+------+------+--------------------+

Hexadecimal or bit values assigned to user variables are treated as binary strings in TiDB. To assign a hexadecimal or bit value as a number, use it in numeric context. For example, add 0 or use CAST(... AS UNSIGNED):

mysql> SELECT @v1, @v2, @v3;
+------+------+------+
| @v1 | @v2 | @v3 |
+------+------+------+
| A | 65 | 65 |
+------+------+------+
1 row in set (0.00 sec)

mysql> SET @v1 = b'1000001';
Query OK, 0 rows affected (0.00 sec)

mysql> SET @v2 = b'1000001'+0;
Query OK, 0 rows affected (0.00 sec)

mysql> SET @v3 = CAST(b'1000001' AS UNSIGNED);
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @v1, @v2, @v3;
+------+------+------+
| @v1 | @v2 | @v3 |
+------+------+------+
| A | 65 | 65 |
+------+------+------+
1 row in set (0.00 sec)

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

mysql> select @not_exist;
+------------+
| @not_exist |
+------------+
| NULL |
+------------+
1 row in set (0.00 sec)

The user-defined variables cannot be used as an identifier in the SQL statement. For example:

mysql> select * from t;
+------+
| a |
+------+
| 1 |
+------+
1 row in set (0.00 sec)

mysql> SET @col = "a";
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @col FROM t;
+------+
| @col |
+------+
| a |
+------+
1 row in set (0.00 sec)

mysql> SELECT `@col` FROM t;
ERROR 1054 (42S22): Unknown column '@col' in 'field list'

mysql> SET @col = "`a`";
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @col FROM t;
+------+
| @col |
+------+
| `a` |
+------+
1 row in set (0.01 sec)

An exception is that when you are constructing a string for use as a prepared statement to execute later:

mysql> PREPARE stmt FROM "SELECT @c FROM t";
Query OK, 0 rows affected (0.00 sec)

mysql> EXECUTE stmt;
+------+
| @c |
+------+
| a |
+------+
1 row in set (0.01 sec)

mysql> DEALLOCATE PREPARE stmt;
Query OK, 0 rows affected (0.00 sec)

For more information, see User-Defined Variables in MySQL.

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