SQL UNION and UNION ALL: combine compatible result sets

Both SELECT branches must return the same number of columns in the same order. Put ORDER BY after the final branch if the combined list needs sorting.

Use UNION when duplicate rows are unwanted. Use UNION ALL when they are meaningful or when each branch already represents a distinct category.

Worked example

Make one outreach list of business customers and German customers.

SELECT name, email FROM customers WHERE segment = 'business'
UNION
SELECT name, email FROM customers WHERE country = 'Germany'
ORDER BY name;

Result on the sample data

name	email
Anna Schmidt	ANNA.SCHMIDT@example.de
Anna Schmidt	anna.schmidt@example.de
Ciara Walsh	ciara.walsh@example.ie
Elena García	elena.garcia@example.es
Emma de Vries	emma.devries@example.nl
Giorgos Ioannou	giorgos.ioannou@example.com
Jonas Becker	jonas.becker@example.com
Marco Bianchi	marco.bianchi@example.it
Mia Weber	mia.weber@example.de
Sean Byrne	sean.byrne@example.ie

Common mistakes

Using UNION ALL for overlapping lists and contacting a person twice.
Trying to combine branches with different column counts.