SELECT tab1.* FROM Table1 tab1 LEFT JOIN Table2 tab2 ON tab1.someId = tab2.someID and tab2.condition = 1 WHERE (tab1.condition=0 AND tab1.condition is NULL) OR tab2.someID IS NOT NULL
SELECT tab1.* FROM Table1 tab1 LEFT JOIN Table2 tab2 ON tab1.someId = tab2.someID and tab2.condition = 1 WHERE (tab1.condition=0 AND tab1.condition is NULL) OR tab2.someID IS NOT NULL