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7.20.6 Row-wise Comparison

row_constructor operator (subquery)

The left-hand side is a row constructor, as described in section 2.2.12 Row Constructors. The right-hand side is a parenthesized subquery, which must return exactly as many columns as there are expressions in the left-hand row. Furthermore, the subquery cannot return more than one row. (If it returns zero rows, the result is taken to be null.) The left-hand side is evaluated and compared row-wise to the single subquery result row.

See section 7.21.5 Row-wise Comparison for details about the meaning of a row-wise comparison.