UNION combining result sets in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the SQL UNION operator do?

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

beginner

How does UNION differ from UNION ALL?

UNION removes duplicate rows from the combined result, while UNION ALL includes all rows, even duplicates.

intermediate

What must be true about the SELECT statements used with UNION?

Each SELECT must have the same number of columns, and the columns must have compatible data types in the same order.

intermediate

Can you use ORDER BY with UNION? If yes, where?

Yes, ORDER BY can be used only once at the end of the entire UNION query to sort the combined result set.

intermediate

What happens if the SELECT statements in a UNION have different column names?

The column names in the final result come from the first SELECT statement; later SELECTs' column names are ignored.

What does the UNION operator do in SQL?

A Combines results and keeps duplicates

B Filters rows based on a condition

C Joins tables based on a key

D Combines results and removes duplicates

UNION combines results from multiple SELECTs and removes duplicate rows.

Which of the following is true about SELECT statements in a UNION?

A They must have the same number of columns

B They can have different numbers of columns

C They must have the same column names

D They must come from the same table

All SELECTs in a UNION must have the same number of columns with compatible data types.

What is the difference between UNION and UNION ALL?

A UNION ALL removes duplicates; UNION keeps duplicates

B UNION removes duplicates; UNION ALL keeps duplicates

C UNION sorts results; UNION ALL does not

D UNION works only with two SELECTs; UNION ALL works with many

UNION removes duplicate rows, while UNION ALL includes all rows.

Where can you place ORDER BY when using UNION?

A Only at the end of the entire UNION query

B After each SELECT statement

C Before the first SELECT

D ORDER BY is not allowed with UNION

ORDER BY can only be used once at the end to sort the combined result.

If the first SELECT has columns (id, name) and the second SELECT has columns (user_id, username), what will the final column names be after UNION?

A user_id, name

B user_id, username

C id, name

D id, username

The final column names come from the first SELECT statement.

Explain how the UNION operator works and what rules must be followed when combining SELECT statements.

Think about how two lists of data can be merged without repeats.

Describe the difference between UNION and UNION ALL and when you might use each.

Consider if you want to keep or remove repeated rows.