UNION and UNION ALL in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the SQL UNION operator do?

UNION combines the results of two or more SELECT queries into a single result set, removing duplicate rows.

beginner

How does UNION ALL differ from UNION?

UNION ALL combines results from multiple SELECT queries but keeps all duplicates, showing every row from all queries.

intermediate

Can you use UNION or UNION ALL with SELECT queries that have different numbers of columns?

No, all SELECT queries combined with UNION or UNION ALL must have the same number of columns with compatible data types.

intermediate

Why might you choose UNION ALL over UNION in a query?

Use UNION ALL when you want to keep duplicates and improve performance because it does not check for duplicates like UNION does.

beginner

What happens if you use UNION on two queries that return the same rows?

UNION will show each row only once, removing duplicates from the combined result.

Which SQL operator removes duplicate rows when combining results?

A UNION

B UNION ALL

C JOIN

D INTERSECT

UNION removes duplicates, while UNION ALL keeps all rows including duplicates.

If you want to combine results and keep all duplicates, which should you use?

A JOIN

B UNION

C EXCEPT

D UNION ALL

UNION ALL keeps all rows including duplicates.

What must be true about the SELECT statements combined with UNION or UNION ALL?

A They must use WHERE clauses.

B They can have any number of columns.

C They must have the same number of columns with compatible data types.

D They must select from the same table.

All SELECT queries combined must have the same number of columns and compatible types.

Which operator is generally faster because it does not remove duplicates?

A UNION ALL

B UNION

C INTERSECT

D EXCEPT

UNION ALL is faster because it skips the duplicate removal step.

What will the result of UNION be if two queries return identical rows?

A All rows including duplicates

B Only one copy of each duplicate row

C No rows

D An error

UNION removes duplicates, so only one copy of each identical row appears.

Explain the difference between UNION and UNION ALL in SQL.

Think about how duplicates are handled and performance.

What are the requirements for SELECT statements when using UNION or UNION ALL?

Consider the structure of the SELECT queries.