SQL Set Operation

SQL also supports some of the set operations defined in relational algebra, such as set-union.

UNION §

• Same logic as union in relational algebra
• Requires tables to have the same degree, (i.e. be union-compatible)
• By default, ignores duplicate records. To factor in duplicate records, use UNION ALL

--Ignores duplicate records
SELECT * FROM A
UNION
SELECT * FROM B

--Allows duplicate records
SELECT * FROM A
UNION ALL
SELECT * FROM B