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CHECK Constraints in SQL
A CHECK constraint is a table constraint defined using the key word CHECK, as already illustrated in several examples in this chapter. In particular, a CHECK constraint can be used to express a constraint such as the one shown in Example, referred to in the theory book as a tuple constraint (so one might call it a row constraint in SQL). This is clearly the way most SQL users would prefer to express such a constraint.
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Using SET TRANSACTION You use the SET TRANSACTION statement to begin the read-only or read-write transaction, start an isolation level, or assign your present transaction to a
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Relational Operators The relational operators permit you to compare randomly complex expressions. The list below provides the meaning of each operator:
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