the SQL designer do not recognize LEFT JOIN, but worse it screw up the previously written SQL query.
In the syntax part of the designer, write :
SELECT a.c, a.d, b.e, b.f FROM tbl_a a LEFT JOIN tbl_b b ON b.a = a.a
go in the designer tab and choose to apply the changes. now go back to the syntax tab : the query has been rewritten to :
SELECT a."c", a."d", b."e", b."f" FROM "tbl_a" b INNER JOIN "a" a ON b."a" = a."a"
-> LEFT JOIN became an INNER JOIN, no more tbl_b in the query, b is not the good alias ....