MYSQL, assign and read a variable in same statement

Question

LINK: https://dev.mysql.com/doc/refman/5.7/en/user-variables.html

I was reading mysql documentation and I got really confused about that general rule!

Documentation:

As a general rule, OTHER THAN IN SET statements, you should never assign a value to a user variable and read the value within the same statement.

Ok, so does it means that I can use SET statement to assign AND READ a user variable? right? BUT when I try it...

SET @a:=1, @b:=@a+1;
SELECT @a,@b; # @a = 1, @b = NULL

Looks like the SET statement have the same problem of any other statement for assign and read user variables in the same statement.

SET @a:=1;
SET @b:=@a+1;
SELECT @a,@b; # @a=1 , @b=2

So, what am I missing here?

Barmar · Accepted Answer · 2017-03-25 00:15:29Z

1

I think the documentation is simply wrong. As you've shown, there's not really any difference between SET and SELECT in this regard.

edited Mar 25, 2017 at 0:15

answered Mar 25, 2017 at 0:07

Barmar

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3 Comments

J.L. Yokata Over a year ago

and this is not ok either: SET @a:=1, @b:=@a+1; so what exacly is the difference between a SET statement and SELECT? that part: "OTHER THAN IN SET statements" makes no sense

Barmar Over a year ago

I know, that's in your question.

J.L. Yokata Over a year ago

Oh okay. Thanks for replying. :P

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