PostgreSQL: UNION ALL Operator

In this PostgreSQL post explains how to use the PostgreSQL UNION ALL operator with syntax and examples.

Description

The PostgreSQL UNION ALL operator is used to combine the result sets of 2 or more SELECT statements. It returns all rows from the query and it does not remove duplicate rows between the various SELECT statements.

Each SELECT statement within the PostgreSQL UNION ALL operator must have the same number of fields in the result sets with similar data types.

Syntax

The syntax for the UNION ALL operator in PostgreSQL is:

SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions]
UNION ALL
SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions];

Parameters or Arguments

expression1, expression2, ... expression_n

The columns or calculations that you wish to retrieve.

tables

The tables that you wish to retrieve records from. There must be at least one table listed in the FROM clause.

WHERE conditions

Optional. The conditions that must be met for the records to be selected.

Note

• There must be same number of expressions in both SELECT statements.
• The column names from the first SELECT statement are used as the column names for the result set.

Example - Return single field

The following is an example of the PostgreSQL UNION ALL operator that returns one field from multiple SELECT statements (and both fields have the same data type):

SELECT category_id
FROM products
UNION ALL
SELECT category_id
FROM categories;

This PostgreSQL UNION ALL operator would return a category_id multiple times in your result set if the category_id appeared in both the products and categories table. The PostgreSQL UNION ALL operator does not remove duplicates. If you wish to remove duplicates, try using the PostgreSQL UNION operator.

Example - Using ORDER BY

The PostgreSQL UNION ALL operator can use the ORDER BY clause to order the results of the operator.

For example:

SELECT product_id, product_name
FROM products
WHERE product_name LIKE 'S%'
UNION
SELECT category_id, category_name
FROM categories
WHERE category_id  99
ORDER BY 2;

In this PostgreSQL UNION ALL operator, since the column names are different between the two SELECT statements, it is more advantageous to reference the columns in the ORDER BY clause by their position in the result set. In this example, we've sorted the results by product_name / category_name in ascending order, as denoted by the ORDER BY 2.

The product_name / category_name fields are in position #2 in the result set.