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用户自定义变量
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用户自定义变量

用户自定义变量格式为 @var_namevar_name 目前只支持字母,数字,_$组成。用户自定义变量是大小写不敏感的。

用户自定义变量是跟 session 绑定的,也就是说只有当前连接可以看见设置的用户变量,其他客户端连接无法查看到。

SET 语句可以设置用户自定义变量:

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

对于 SET 语句,赋值操作符可以是 = 也可以是 :=

例:

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 |
+------+------+------+--------------------+

如果设置用户变量用了 HEX 或者 BIT 值,TiDB会把它当成二进制字符串。如果你要将其设置成数字,那么需要手动加上 CAST转换: 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)

如果获取一个没有设置过的变量,会返回一个 NULL:

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

用户自定义变量不能直接在 SQL 语句中被当成 identifier,例:

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)

但是有一个例外是如果你在 PREPARE 语句中使用它,是可以的:

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)

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