User-Defined Variables

This document describes the concept of user-defined variables in TiDB and the methods to set and read the user-defined variables.

The format of the user-defined variables is @var_name. The characters that compose var_name can be any characters that can compose an identifier, including the numbers 0-9, the letters a-zA-Z, the underscore _, the dollar sign $, and the UTF-8 characters. In addition, it also includes the English period .. The user-defined variables are case-insensitive.

The user-defined variables are session-specific, which means a user variable defined by one client connection cannot be seen or used by other client connections.

Set the user-defined variables

You can use the SET statement to set a user-defined variable, and the syntax is SET @var_name = expr [, @var_name = expr] ...;. For example:

SET @favorite_db = 'TiDB';

SET @a = 'a', @b = 'b', @c = 'c';

For the assignment operator, you can also use :=. For example:

SET @favorite_db := 'TiDB';

The content to the right of the assignment operator can be any valid expression. For example:

SET @c = @a + @b;

set @c = b'1000001' + b'1000001';

Read the user-defined variables

To read a user-defined variable, you can use the SELECT statement to query:

SELECT @a1, @a2, @a3

+------+------+------+
| @a1  | @a2  | @a3  |
+------+------+------+
|    1 |    2 |    4 |
+------+------+------+

You can also assign values in the SELECT statement:

SELECT @a1, @a2, @a3, @a4 := @a1+@a2+@a3;

+------+------+------+--------------------+
| @a1  | @a2  | @a3  | @a4 := @a1+@a2+@a3 |
+------+------+------+--------------------+
|    1 |    2 |    4 |                  7 |
+------+------+------+--------------------+

Before the variable @a4 is modified or the connection is closed, its value is always 7.

If a hexadecimal literal or binary literal is used when setting the user-defined variable, TiDB will treat it as a binary string. If you want to set it to a number, you can manually add the CAST conversion, or use the numeric operator in the expression:

SET @v1 = b'1000001';
SET @v2 = b'1000001'+0;
SET @v3 = CAST(b'1000001' AS UNSIGNED);

SELECT @v1, @v2, @v3;

+------+------+------+
| @v1  | @v2  | @v3  |
+------+------+------+
| A    | 65   | 65   |
+------+------+------+

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

SELECT @not_exist;

+------------+
| @not_exist |
+------------+
| NULL       |
+------------+

In addition to using the SELECT statement to read the user-defined variables, another common usage is the PREPARE statement. For example:

SET @s = 'SELECT SQRT(POW(?,2) + POW(?,2)) AS hypotenuse';
PREPARE stmt FROM @s;
SET @a = 6;
SET @b = 8;
EXECUTE stmt USING @a, @b;

+------------+
| hypotenuse |
+------------+
|         10 |
+------------+

The contents of the user-defined variables are not recognized as identifiers in the SQL statements. For example:

SELECT * from t;

+---+
| a |
+---+
| 1 |
+---+

SET @col = "`a`";
SELECT @col FROM t;

+------+
| @col |
+------+
| `a`  |
+------+

MySQL compatibility

Except for SELECT ... INTO <variable>, the syntax supported in MySQL and TiDB is identical.

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