SQLite: UNION ALL Operator

This SQLite post explains how to use the SQLite UNION ALL operator with syntax and examples.

Description

The SQLite 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 SQLite 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 SQLite 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 SQLite UNION ALL operator that returns one field from multiple SELECT statements (and both fields have the same data type):

SELECT department_id
FROM departments
UNION ALL
SELECT department_id
FROM employees;

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

Example - Using ORDER BY

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

For example:

SELECT department_id, department_name
FROM departments
WHERE department_id >= 10
UNION ALL
SELECT employee_id, last_name
FROM employees
WHERE last_name = 'Mark'
ORDER BY 2;

In this SQLite 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 department_name / last_name in ascending order, as denoted by the ORDER BY 2.

The department_name / last_name fields are in position #2 in the result set.