So I found it surprising that it allows you to drop a table which is being referred to in a foreign key. DB2 doesn't even warn you about the fact that the child table(s) have lost a potentially important constraint. That's evil.
I know of no other DBMS which lets you do drop a parent table: PostgreSQL refuses it (unless you add a CASCADE option to the DROP statement), MSSQL refuses it. Not even MySQL lets you do it.
As always, MySQL has little surprises:
CREATE TABLE child (
child_id INT NOT NULL,
parent_id INT NOT NULL,
whatever VARCHAR(50) NOT NULL,
CONSTRAINT child_fk FOREIGN KEY (parent_id) REFERENCES parent
ERROR 1005 (HY000): Can't create table './test/child.frm' (errno: 150)
So what is the reason behind that confusing error message (which also qualifies as evil)? -- "REFERENCES parent" must be explicit: "REFERENCES parent(parent_id)"...