Suppose I have two tables with a foreign key relationship. How can I find the rows in the "parent" table for which there is no corresponding "child" row?
create table a(x number primary key); create table b(x number); alter table b add constraint b_fk foreign key(x) references a(x) enable; insert into a values(1); insert into b values(1); insert into a values(2); commit; delete from a where x = 2; -- will succeed delete from a where x = 1; -- will fail, because there is a child row. select ??? as "rows with no children" from a; rows with no children --------------------- 2