SET

SET @my_var = <expr> [, @my_var_b = <expr_b>, @my_var_c = ...]

Setting the variable's value via a basic expression:

SET @a = 5 + 5;

SELECT @a;
****
+------+
| @a   |
+------+
|   10 |
+------+
1 row in set (0.054 sec)

Setting the value referencing another user defined variable:

SET @a = 5 + 5;

SET @b = @a * @a;

SELECT @b;
****
+------+
| @b   |
+------+
|  100 |
+------+
1 row in set (0.051 sec)

Setting the value as the result of a SQL statement:

CREATE TABLE set_example (a int, b int);
INSERT INTO set_example VALUES(1,2),(3,4),(5,6),(7,8);

SELECT * FROM set_example;
****
+------+------+
| a    | b    |
+------+------+
|    1 |    2 |
|    3 |    4 |
|    5 |    6 |
|    7 |    8 |
+------+------+
4 rows in set (0.001 sec)

SET @a = (SELECT COUNT(*) FROM set_example);

SELECT @a;
****
+------+
| @a   |
+------+
|    4 |
+------+
1 row in set (0.000 sec)

SET @a = (SELECT b FROM set_example WHERE a = 7);

SELECT @a;
****
+------+
| @a   |
+------+
|    8 |
+------+
1 row in set (0.000 sec)

SET @a = (SELECT b,a FROM set_example WHERE a = 7);
ERROR 1241 (21000): Operand should contain 1 column(s)

SET @a = (SELECT b FROM set_example);
ERROR 1242 (21000): Subquery returns more than 1 row

User-defined variables on the right-hand side of assignments are bound before the SET statement is run.

SET @x = 1;

SET @y = 2;

SET @x = @y, @y = @x;

SELECT @x, @y;
****
+----+----+
| @x | @y |
+----+----+
| 2  | 1  |
+----+----+

Other than that, the assignments in the list are run left-to-right.

SET @x = 1, @x = 2;

SELECT @x;
****
+----+
| @x |
+----+
| 2  |
+----+

User defined variables and functions:

SET @a = 8;

DELIMITER //
USE db //

CREATE OR REPLACE FUNCTION foo (a INT) RETURNS INT AS BEGIN RETURN a * a; END //

DELIMITER ;

SET @c = foo(@a);

SELECT @a, @c;
****
+------+------+
| @a   | @c   |
+------+------+
|    8 |    64|
+------+------+


SET @d = (SELECT @a) * 2;
Query OK, 0 rows affected (0.098 sec)

SELECT @d;
+------+
| @d   |
+------+
|   16 |
+------+
1 row in set (0.055 sec)

SET @d = (SELECT @a) * 2, @e = foo(10), @f = @a + @b + @@SESSION.net_read_timeout;
Query OK, 0 rows affected (0.062 sec)

SELECT @d, @e, @f;
+------+------+------+
| @d   | @e   | @f   |
+------+------+------+
|   16 |  100 | 3708 |
+------+------+------+
1 row in set (0.055 sec)

SELECT @@SESSION.net_read_timeout;
+----------------------------+
| @@SESSION.net_read_timeout |
+----------------------------+
|                       3600 |
+----------------------------+
1 row in set (0.075 sec)

SET @g := @a + @b + @@SESSION.net_read_timeout;
Query OK, 0 rows affected (0.081 sec)

SELECT @g;
+------+
| @g   |
+------+
| 3708 |
+------+
1 row in set (0.069 sec)

Using strings and objects:

SET @t = FROM_UNIXTIME(1234567890), @s = 'abc', @fp = 3.14;

SELECT @t, @s, @fp;
****
+---------------------+-----+------+
| @t                  | @s  | @fp  |
+---------------------+-----+------+
| 2009-02-14 01:31:30 | abc | 3.14 |
+---------------------+-----+------+
1 row in set (0.053 sec)

SET @str = CONCAT(@s, @@GLOBAL.sql_mode);

SELECT @str;
****
+-----------------------------------------------------+
| @str                                                |
+-----------------------------------------------------+
| abcANSI_QUOTES,ONLY_FULL_GROUP_BY,STRICT_ALL_TABLES |
+-----------------------------------------------------+
1 row in set (0.059 sec)