\[O \Join C\]
\[\delta_{aid = 'a001'}(O \Join C)\]
\[C - \pi_{cid, cname, city, discnt}(\delta_{aid = 'a001'}(O \Join C))\]
\[\pi_{cname}(C - \pi_{cid, cname, city, discnt}(\delta_{aid = 'a001'}(O \Join C))\]
\[\pi_{cid}(\delta_{city = '南京'}(C))\]
\[\pi_{cid,aid}(O) \div \pi_{cid}(\delta_{city = '南京'}(C))\]
\[\pi_{aid}(\pi_{cid,aid}(O) \div \pi_{cid}(\delta_{city = '南京'}(C)))\]
\[\pi_{O.pid,O.oid}(\delta_{O.pid = S.pid \wedge O.dols < S.dols}(O \times S))\]
\[\pi_{O.pid,O.oid}(O) - \pi_{O.pid,O.oid}(\delta_{O.pid = S.pid \wedge O.dols < S.dols}(O \times S))\]
BINISM
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