Consider the following relations

A$($aid: integer, a$1$: integer, a$2$: integer, cid: integer$)$

B$($bid: integer, b$1$: integer, b$2$: integer,$)$

C$($cid: integer, c$1$: integer, c$2$: integer, bid: integer$)$

D$($did: integer, d$1$: integer, d$2$: integer, aid: integer$)$

Consider the SQL query:

SELECT A$.$a$1,$ B$.$b$2$

FROM A$,$ B$,$ C$,$ D

WHERE A$.$cid $=$ C$.$cid AND C$.$bid $=$ B$.$bid AND A$.$aid $=$ D$.$aid