Cheatsheet

SQL Set Operations Cheatsheet

UNION, UNION ALL, INTERSECT, and EXCEPT side by side.

The four operators

Operator	Keeps	Duplicates
`UNION ALL`	All rows from both	Kept
`UNION`	All rows from both	Removed
`INTERSECT`	Rows in both	Removed
`EXCEPT`	Rows in the first only	Removed

Set operations combine or compare compatible result sets by row. A UNION stacks rows, while INTERSECT and EXCEPT keep only rows that satisfy the set relationship. Joins, by contrast, combine rows based on a relationship and can add columns from both inputs.

The shape

One ORDER BY, at the very end

select state from customers
union all
select state from suppliers
order by state

Compatibility rules

Every SELECT returns the same number of columns.
Matching columns have compatible types.
Output column names come from the first SELECT.

Common jobs

Stack two sources (keep everything)

select 'customer' as source, state from customers
union all
select 'supplier',            state from suppliers

Values shared by both (overlap)

select state from customers
intersect
select state from suppliers

In the first, not the second (missing list)

select customer_id from customers
except
select customer_id from orders

Watch out

Prefer UNION ALL when duplicate preservation is correct for the question. UNION performs duplicate elimination, which adds work and can change row counts. Put the final ORDER BY after the set expression; without a final ordering, do not rely on result order.

Lesson

SQL WHERE: Filtering Rows

Use WHERE to keep only matching rows.

Reference

SQL WHERE Operators

Comparisons, AND/OR, IN, BETWEEN, and LIKE.

Interview

SQL Window Function Interview Questions

Practice ranking, running totals, and top row per group.

Mistake

Why My Query Returns No Rows

Debug filters that match nothing.