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

Recall & Review

beginner

What does the UNION operator do in SQL?

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 each query.

intermediate

Can UNION be used 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?

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 duplicate row only once, removing any repeated rows from the combined result.

Which SQL operator removes duplicate rows when combining results?

A UNION ALL

B INTERSECT

C JOIN

D UNION

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

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

A They can have different numbers of columns.

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

C They must select from the same table.

D They must use WHERE clauses.

All SELECT statements combined with UNION or UNION ALL must have the same number of columns and compatible data types.

Which operator is faster when you don't need to remove duplicates?

A UNION ALL

B JOIN

C UNION

D GROUP BY

UNION ALL is faster because it does not check for duplicates.

If two SELECT queries return the same row, how many times will it appear in the result of UNION ALL?

A Twice

B Depends on the database

C Zero times

D Once

UNION ALL includes all rows, so duplicates appear as many times as they occur.

Which SQL operator combines results but does not remove duplicates?

A EXCEPT

B UNION

C UNION ALL

D INTERSECT

UNION ALL combines results and keeps duplicates.

Explain the difference between UNION and UNION ALL in SQL.

Think about how duplicates are handled and when you might want to keep them.

What rules must SELECT queries follow to be combined using UNION or UNION ALL?

Focus on the structure of the SELECT statements.

Practice

1. What is the main difference between UNION and UNION ALL in MySQL?

easy

A. UNION sorts the result, UNION ALL does not.

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

C. UNION only works with numeric columns, UNION ALL works with all data types.

D. UNION removes duplicate rows, UNION ALL keeps all rows including duplicates.

Solution

1. Step 1: Understand UNION behavior

   UNION combines results from two queries and removes duplicate rows to return only unique rows.

2. Step 2: Understand UNION ALL behavior

   UNION ALL combines results but keeps all rows, including duplicates, without filtering.

3. Final Answer:

   UNION removes duplicate rows, UNION ALL keeps all rows including duplicates. -> Option D

4. Quick Check:

   Duplicates removed = UNION, duplicates kept = UNION ALL [OK]

Hint: Remember: UNION = unique rows, UNION ALL = all rows [OK]

Common Mistakes:

• Thinking UNION ALL removes duplicates
• Confusing sorting behavior with UNION
• Assuming UNION only works on numeric data

2. Which of the following is the correct syntax to combine two SELECT queries using UNION ALL in MySQL?

easy

A. SELECT * FROM table1 UNION ALL SELECT * FROM table2;

B. SELECT * FROM table1 UNIONALL SELECT * FROM table2;

C. SELECT * FROM table1 UNION, ALL SELECT * FROM table2;

D. SELECT * FROM table1 UNION; ALL SELECT * FROM table2;

Solution

1. Step 1: Check correct keyword spacing

   The correct keyword is UNION ALL with a space between UNION and ALL.

2. Step 2: Validate SQL syntax

   The syntax must be: SELECT ... UNION ALL SELECT ...; without commas or semicolons between the two SELECTs.

3. Final Answer:

   SELECT * FROM table1 UNION ALL SELECT * FROM table2; -> Option A

4. Quick Check:

   Correct keyword and syntax = SELECT * FROM table1 UNION ALL SELECT * FROM table2; [OK]

Hint: Use space between UNION and ALL, no commas [OK]

Common Mistakes:

• Writing UNIONALL as one word
• Adding commas between SELECT statements
• Using semicolons between SELECTs

3. Given two tables:
Table A:
id | name
1 | Alice
2 | Bob

Table B:
id | name
2 | Bob
3 | Carol

What will be the result of this query?

SELECT id, name FROM A UNION SELECT id, name FROM B;

medium

A. 2 | Bob3 | Carol

B. 1 | Alice2 | Bob2 | Bob3 | Carol

C. 1 | Alice2 | Bob3 | Carol

D. 1 | Alice3 | Carol

Solution

1. Step 1: Combine rows from both tables

   Table A has (1, Alice) and (2, Bob). Table B has (2, Bob) and (3, Carol).

2. Step 2: Apply UNION behavior

   UNION removes duplicates, so (2, Bob) appears only once in the result.

3. Final Answer:

   1 | Alice
   2 | Bob
   3 | Carol -> Option C

4. Quick Check:

   Duplicates removed, unique rows shown [OK]

Hint: UNION removes duplicates, so repeated rows appear once [OK]

Common Mistakes:

• Including duplicate rows with UNION
• Confusing UNION with UNION ALL output
• Ignoring duplicate row removal

4. Consider this query:

SELECT id FROM table1
UNION ALL
SELECT id FROM table2 ORDER BY id;

What is wrong with this query?

medium

A. ORDER BY must be applied only once at the end of the UNION ALL query.

B. UNION ALL cannot be used with ORDER BY.

C. Each SELECT must have its own ORDER BY clause.

D. ORDER BY should come before UNION ALL.

Solution

1. Step 1: Understand ORDER BY with UNION ALL

   ORDER BY applies to the entire combined result, not to individual SELECTs.

2. Step 2: Identify correct placement

   ORDER BY must be placed after the last SELECT and UNION ALL, not inside one SELECT.

3. Final Answer:

   ORDER BY must be applied only once at the end of the UNION ALL query. -> Option A

4. Quick Check:

   ORDER BY after UNION ALL, not inside SELECT [OK]

Hint: Place ORDER BY after all UNION ALL parts [OK]

Common Mistakes:

• Putting ORDER BY inside one SELECT only
• Trying ORDER BY before UNION ALL
• Assuming UNION ALL disallows ORDER BY

5. You have two tables:
Employees:
id | name
1 | John
2 | Jane
3 | Mike

Contractors:
id | name
2 | Jane
4 | Anna

You want to list all people from both tables, including duplicates, and sort by name ascending.
Which query achieves this correctly?

hard

A. SELECT id, name FROM Employees UNION SELECT id, name FROM Contractors ORDER BY name ASC;

B. SELECT id, name FROM Employees UNION ALL SELECT id, name FROM Contractors ORDER BY name ASC;

C. SELECT id, name FROM Employees UNION ALL SELECT id, name FROM Contractors ORDER BY id ASC;

D. SELECT id, name FROM Employees UNION ALL ORDER BY name ASC SELECT id, name FROM Contractors;

Solution

1. Step 1: Choose UNION ALL to keep duplicates

   Since duplicates like Jane (id 2) should appear twice, use UNION ALL.

2. Step 2: Apply ORDER BY after UNION ALL

   ORDER BY must come after the full UNION ALL to sort combined results by name ascending.

3. Final Answer:

   SELECT id, name FROM Employees UNION ALL SELECT id, name FROM Contractors ORDER BY name ASC; -> Option B

4. Quick Check:

   UNION ALL + ORDER BY at end = SELECT id, name FROM Employees UNION ALL SELECT id, name FROM Contractors ORDER BY name ASC; [OK]

Hint: Use UNION ALL for duplicates, ORDER BY after all SELECTs [OK]

Common Mistakes:

• Using UNION instead of UNION ALL (removes duplicates)
• Placing ORDER BY inside one SELECT
• Incorrect syntax with ORDER BY placement