Question

With LEFT JOIN

SET @foo:=0;
SELECT @foo 
FROM test t1 
  LEFT JOIN test t2 ON t2.id=t1.id AND (@foo:=@foo+1);

prints 0,0,0,0,0,0,0…

but with JOIN

SET @foo:=0;
SELECT @foo 
FROM test t1 
   JOIN test t2 ON t2.id=t1.id AND (@foo:=@foo+1);

prints 1,2,3,4,5,6…

Anybody knows? Thanks!

UPD. This queries produce the same rows except @foo value

Answer 1

Assignments for session variables don’t work inside ON clause.

The reason you see @foo incremented for INNER JOIN is that optimizer runs your query as:

SET @foo:=0;
SELECT @foo 
FROM test t1 
   JOIN test t2 
   WHERE t2.id=t1.id AND (@foo:=@foo+1);

Using EXPLAIN SELECT... you should see using where in extra column.

You can also use EXPLAIN EXTENDED SELECT... and then SHOW WARNINGS to see how exactly optimizer runs your query.

In your case the LEFT JOIN query is run exactly as it is written, as for this type of join you can get different results for the same condition in ON and WHERE.

If you want to increment @foo for LEFT JOIN use:

SET @foo:=0;
SELECT @foo 
FROM test t1 
  LEFT JOIN test t2 ON t2.id=t1.id
  WHERE @foo:=@foo+1