MySQL 中 = 和 := 有什么区别？

Question

What is the difference in between

set test_var = 20;

and

set test_var:=20;

as they both seem to assign the value ?

Answer 1

Both of them are assignment operators but one thing I can find their differences is that = can be used to perform boolean operation while := cannot.

valid : SUM(val = 0)
Invalid: SUM(val := 0)

FROM User-Defined Variables

One more thing, You can also assign a value to a user variable in statements other than SET. In this case, the assignment operator must be := and not = because the latter is treated as the comparison operator = in non-SET statements.

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

Answer 2

It's more or less Syntactic sugar.

Take a look here

Most important difference is

Unlike =, the := operator is never interpreted as a comparison operator. This means you can use := in any valid SQL statement (not just in SET statements) to assign a value to a variable.

Answer 3

You can only use := for assignment - never for comparison. It's just a bit of syntactic sugar, it doesn't really change the functionality at all. You'll see it a lot in generated SQL from code.

Answer 4

Using := in a SET and SELECT statement is interpreted as assignment.

Using = in a SET statement is interpreted as assignment.

However, Using = in a SELECT statement is interpreted as a bool operator (comparison).

So if you mean assignment, it is always safe to use :=