问题
What is the difference in between
set test_var = 20;
and
set test_var:=20;
as they both seem to assign the value ?
回答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 |
+------+------+------+--------------------+
回答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.
回答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.
来源：https://stackoverflow.com/questions/15614169/what-is-the-difference-between-and-in-mysql