CREATE TABLE t1(a INT, b INT);
CREATE TABLE t2(c INT, d INT);
CREATE TABLE t3(e TEXT, f TEXT);

INSERT INTO t1 VALUES('x',1);
INSERT INTO t2 VALUES(1,2);

SELECT *
FROM t1, t2
LEFT JOIN t3 ON (t2.d=1)
WHERE t2.c = +t1.a;

SELECT *
FROM t1
LEFT JOIN t2 ON (t2.c=+t1.a)
LEFT JOIN t3 ON (t2.d IS NULL);