A query expression is an expression used to represent a table of values in an SQL statement. The operands in a query expression are tables represented by query specifications. A query expression can include one or more query specifications. Each query specification is linked to the next by the UNION operator. The data lengths of the unioned columns must be identical.
The table resulting from the evaluation of a query expression is derived from the concatenation of the rows in the tables defined by the operands.
Note: For more information about other types of expressions, see Values and Value Expressions.
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