[英]What's the difference between := and = in MySQL?
mysql> set @num := 1;
Query OK, 0 rows affected (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)
Seems both works. 似乎两个都有效。
In short, when using SET
they both act as assignment operators, but in any non-set statements, :=
is for assingment and =
checks for equality . 简而言之,当使用SET
它们都充当赋值运算符,但在任何非set语句中, :=
表示assingment, =
检查是否相等 。
For SET, either = or := can be used as the assignment operator. 对于SET,可以使用=或:=作为赋值运算符。
You can also assign a value to a user variable in statements other than SET. 您还可以在SET以外的语句中为用户变量赋值。 In this case, the assignment operator must be := and not = because = is treated as a comparison operator in non-SET statements 在这种情况下,赋值运算符必须是:=而不是=因为=在非SET语句中被视为比较运算符
mysql> SET @t1=1, @t2=2, @t3:=4;
mysql> SELECT @t1, @t2, @t3, @t4 := @t1+@t2+@t3;
+------+------+------+--------------------+
| @t1 | @t2 | @t3 | @t4 := @t1+@t2+@t3 |
+------+------+------+--------------------+
| 1 | 2 | 4 | 7 |
+------+------+------+--------------------+
Taken from MySQL 8.4 User Defined Variables 取自MySQL 8.4用户定义的变量
Both are assignment operator. 两者都是赋值运算符。
:= was created to be clearer since = is also an equivalence operator in SQL. :=创建为更清晰,因为=也是SQL中的等价运算符。 WHERE x = 1;
(most computer languages it is ==
) (大多数计算机语言是==
)
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