The Basic Predicate
A basic predicate compares a value with one and only one other value, and has the syntax:
The comparison operators, comp-operator, are described in Comparison and Relational Operators.
The expressions on either side of the comparison operator must have compatible data types, see Comparisons.
Within the context of a basic predicate, a select-specification must result in either an empty set or a single value.
The result of the predicate is unknown if either of the expressions used evaluates to NULL, or if the select-specification used results in an empty set.
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