sql null造句

SQL null is a state, not a value.
Well-known examples of inexpressible queries include simple SQL nulls and the three-valued logic they entail; the logical treatment of nulls remains mired in controversy . ( For recent work extending Codd's theorem in this direction see the 2012 paper of Franconi and Tessaris . ) Additionally, SQL allows duplicate rows ( has multiset semantics . ) Nevertheless, relational completeness constitutes an important yardstick by which the expressive power of query languages can be compared.
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