How to Use UNION in SQL

The UNION operator is used to combine the result sets of two or more SELECT statements into a single result set. It removes duplicate rows by default.

Syntax:

SELECT column1, column2, ...FROM table1UNIONSELECT column1, column2, ...FROM table2;

Key Points:

• Each SELECT must have the same number of columns.

• Corresponding columns must have compatible data types.

• By default, UNION removes duplicates (like DISTINCT).

• To include duplicates, use UNION ALL.

Example:

Suppose you have two tables: customers_2023 and customers_2024. To get a combined list of all customers:

SELECT customer_id, customer_nameFROM customers_2023UNIONSELECT customer_id, customer_nameFROM customers_2024;

Example with UNION ALL (includes duplicates):

SELECT customer_id, customer_nameFROM customers_2023UNION ALLSELECT customer_id, customer_nameFROM customers_2024;

Notes:

• The order of columns matters.

• You can add ORDER BY at the very end to sort the combined results.

If you want, I can help with examples tailored to your data!