Set Operations

And, Not, Or

• these are put inside the WHERE or HAVING clauses

UNION, INTERSECTION

• these are put between two SELECT clauses
• append all to the keyword to retain duplicates

Impact of null values

• the result of any comparison with a null value is returned as unknown
• false and unknown is false, true and unknown is true (absorption law)
• every other comparison returns unknown
• if unknown is returned to the WHERE clause, then it is treated as false.