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MySQL中=与:=有什么区别？

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mysql> set @num := 1;
Query OK, 0 rows affected (0.00 sec)

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

mysql> set @num = 0;
Query OK, 0 rows affected (0.00 sec)

mysql> select @num;
+------+
| @num |
+------+
|    0 |
+------+
1 row in set (0.00 sec)

好像效果没有不同？

 

1 个答复

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对于SET，=或:=都是一样的，表示赋值。

但是在没有SET的情况下，=检测两个值是否相等，而:=仍表示赋值。

例：

mysql> SET @t1=1, @t2=2, @t3:=4;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @t1, @t2, @t3, @t4 := @t1+@t2+@t3;
+------+------+------+--------------------+
| @t1  | @t2  | @t3  | @t4 := @t1+@t2+@t3 |
+------+------+------+--------------------+
|    1 |    2 |    4 |                  7 |
+------+------+------+--------------------+
1 row in set (0.00 sec)

mysql> SELECT @t1, @t2, @t3, @t4 = @t1+@t2+@t3;
+------+------+------+-------------------+
| @t1  | @t2  | @t3  | @t4 = @t1+@t2+@t3 |
+------+------+------+-------------------+
|    1 |    2 |    4 |                 1 |
+------+------+------+-------------------+
1 row in set (0.00 sec)

mysql> select @t4;
+------+
| @t4  |
+------+
|    7 |
+------+
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