Getting Started with PostgreSQL Set Operators

Getting Started with PostgreSQL Set Operators

Getting Started with PostgreSQL Set Operators. Postgres offers set operators that make it easy to query and filter the results of searches from your database. Set operators are used to join the results of two or more SELECT statements. These operators are UNION, UNION ALL, INTERSECT, and EXCEPT—each can be used to construct queries across mutliple tables and filter for the specific data that you need.

Postgres offers set operators that make it easy to query and filter the results of searches from your database. Set operators are used to join the results of two or more SELECT statements. These operators are UNION, UNION ALL, INTERSECT, and EXCEPT—each can be used to construct queries across mutliple tables and filter for the specific data that you need.

To return the combined results of two SELECT statements, we use the UNION set operator. This operator removes all the duplicates from the queried results—only listing one row for each duplicated result. To examine this behavior, the UNION ALL set operator can be used, as it retains duplicates in the final result. The INTERSECT set operator only lists records that are shared by both SELECT queries, and conversely, the EXCEPT set operator removes the results from the second SELECT query. Thus, the INTERSECT and EXCEPT set operators are used to produce unduplicated results.

All set operators initially share the same degree of precedence (except for INTERSECT, which we'll discuss later). Because parentheses are prioritized above dangling operators, they can cause the order to differ.

Note — All SELECT statements that are used with a SET operator must select the same number of columns. The columns used in the display are obtained from the first query.

The column-list data types must be implicitly convertible by Postgres. Postgres does not perform implicit type conversion if the corresponding columns in the queries belong to different data types. If a column in the first query of type INT, and the corresponding column in the second query is of type CHAR, Postgres will not perform an implicit conversion—instead, it will raise a type error.

To sort the result set, positional ordering is used. Set operators do not allow individual result set ordering. At the end of the query, ORDER BY can only appear once.

The order of the queries in UNION and INTERSECT operators is not important, and doesn't change the final result - UNION and INTERSECT operators are commutative.

UNION ALL has better performance compared to the UNION operator because resources are not used in filtering duplicates and sorting the results.

  • It is possible to use set operators as part of subqueries.
  • It is not possible to use set operators in SELECT statements containing TABLE collection expressions.

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