UNION and UNION ALL in MySQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to combine results from two tables without duplicates.

MySQL

SELECT name FROM employees [1] SELECT name FROM managers;

Drag options to blanks, or click blank then click option'

AUNION

BINTERSECT

CUNION ALL

DJOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNION ALL when duplicates should be removed.

Using JOIN which combines rows differently.

The UNION keyword combines results from two queries and removes duplicates.

2fill in blank

medium

Complete the code to combine all rows from two tables, including duplicates.

MySQL

SELECT city FROM customers [1] SELECT city FROM suppliers;

Drag options to blanks, or click blank then click option'

AUNION

BUNION ALL

CINTERSECT

DEXCEPT

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNION which removes duplicates.

Using INTERSECT which returns only common rows.

The UNION ALL keyword combines results from two queries and keeps all duplicates.

3fill in blank

hard

Fix the error in the code to correctly combine two SELECT queries.

MySQL

SELECT id, name FROM products [1] SELECT id, name FROM inventory;

Drag options to blanks, or click blank then click option'

AJOIN

BUNION

CUNION ALL

DWHERE

Attempts:

3 left

💡 Hint

Common Mistakes

Using JOIN which causes syntax error here.

Using WHERE which is invalid between SELECT statements.

To combine all rows including duplicates, use UNION ALL. JOIN is for combining columns, not rows.

4fill in blank

hard

Fill both blanks to combine two queries and order the results by name.

MySQL

SELECT name FROM staff [1] SELECT name FROM contractors [2] name;

Drag options to blanks, or click blank then click option'

AUNION

BORDER BY

CUNION ALL

DGROUP BY

Attempts:

3 left

💡 Hint

Common Mistakes

Placing ORDER BY before UNION.

Using GROUP BY instead of ORDER BY for sorting.

Use UNION to combine results without duplicates, then ORDER BY to sort by name.

5fill in blank

hard

Fill all three blanks to combine two queries, keep duplicates, and sort by city.

MySQL

SELECT city FROM customers [1] SELECT city FROM vendors [2] city [3];

Drag options to blanks, or click blank then click option'

AUNION ALL

BORDER BY

CASC

DGROUP BY

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNION instead of UNION ALL which removes duplicates.

Using GROUP BY which groups rows instead of sorting.

Use UNION ALL to keep duplicates, then ORDER BY city ASC to sort cities alphabetically.

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