MySQL — Union

UNION

Combines results from two or more SELECT statements, removing duplicate rows:

SELECT city FROM customers
UNION
SELECT city FROM suppliers;

UNION ALL

Keeps all rows including duplicates — faster than UNION:

SELECT city FROM customers
UNION ALL
SELECT city FROM suppliers;

Rules for UNION

• All SELECT statements must have the same number of columns
• Columns must have compatible data types
• ORDER BY can only appear at the end
• Column names come from the first SELECT

SELECT name, email FROM customers
UNION
SELECT name, email FROM vendors
ORDER BY name;

UNION with WHERE

Each SELECT can have its own WHERE clause:

SELECT name, 'Active' AS status FROM active_users
UNION
SELECT name, 'Inactive' AS status FROM inactive_users;

UNION vs JOIN

Feature	UNION	JOIN
Direction	Vertical (rows)	Horizontal (columns)
Tables	Similar data	Related data
Columns	Same count	Can differ

Mini Practice

Write SQL code that:

1. Uses UNION to combine two result sets
2. Uses UNION ALL to show duplicates
3. Adds ORDER BY at the end
4. Applies different WHERE clauses to each SELECT

Up Next

Continue with Subqueries — nested queries inside other statements.