| title | summary | aliases | ||
|---|---|---|---|---|
User-Defined Variables |
Learn how to use 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.
Warning:
User-defined variables are still an experimental feature. It is NOT recommended that you use them in the production environment.
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.
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';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` |
+------+
Except for SELECT ... INTO <variable>, the syntax supported in MySQL and TiDB is identical.
For more information, see User-Defined Variables in MySQL.