Setting user variables within expressions is deprecated and will be
removed in a future release. Please set variables in separate statements
instead.
to
Setting user variables within expressions is deprecated and will be
emoved in a future release. Consider alternatives:
'SET variable=expression, ...', or 'SELECT expression(s) INTO variables(s)'.
The advantage of the new message is in this situation:
select @a:=<expr1>, @b:=<expr2> from complex_query ;
"Separate statements", as in the original warning, implicitely
suggests to use SET, and thus a subquery; as SET only supports scalar
subqueries it yields:
SET @a:=(SELECT <expr1> from complex_query),
    @a:=(SELECT <expr2> from complex_query);
which is inefficient (runs complex_query twice); a better and simpler
rewrite is:
SELECT <expr1>, <expr2> INTO @a,@b from complex_query,
as suggested by the new message.