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.

Practice

1. What does the SQL UNION operator do when combining results from two SELECT queries?

easy

A. Combines rows but keeps all duplicates

B. Combines rows from both queries and removes duplicate rows

C. Joins tables based on a common column

D. Filters rows based on a condition

Solution

1. Step 1: Understand UNION behavior

   The UNION operator combines rows from two or more SELECT queries into a single result set.

2. Step 2: Check duplicate handling

   By default, UNION removes duplicate rows to ensure unique results.

3. Final Answer:

   Combines rows from both queries and removes duplicate rows -> Option B

4. Quick Check:

   UNION removes duplicates = A [OK]

Hint: UNION removes duplicates by default, unlike UNION ALL [OK]

Common Mistakes:

• Confusing UNION with UNION ALL
• Thinking UNION joins tables by columns
• Assuming UNION filters rows

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

easy

A. SELECT col1 FROM table1 UNION ALL JOIN SELECT col1 FROM table2;

B. SELECT col1 FROM table1 JOIN SELECT col1 FROM table2;

C. SELECT col1 FROM table1 WHERE UNION SELECT col1 FROM table2;

D. SELECT col1 FROM table1 UNION SELECT col1 FROM table2;

Solution

1. Step 1: Review correct UNION syntax

   The correct syntax is to write two SELECT statements separated by the UNION keyword.

2. Step 2: Identify invalid options

   Options B, C, and D misuse JOIN or WHERE with UNION, which is incorrect syntax.

3. Final Answer:

   SELECT col1 FROM table1 UNION SELECT col1 FROM table2; -> Option D

4. Quick Check:

   UNION syntax = SELECT ... UNION SELECT ... [OK]

Hint: UNION joins SELECTs directly, no JOIN or WHERE needed [OK]

Common Mistakes:

• Using JOIN instead of UNION
• Placing WHERE before UNION
• Combining UNION ALL with JOIN incorrectly

3. Given two tables:
Table A: id
1
2
3
Table B: id
2
3
4
What is the result of:
SELECT id FROM A UNION SELECT id FROM B;

medium

A. [1, 2, 3, 4]

B. [1, 2, 2, 3, 3, 4]

C. [2, 3]

D. [1, 4]

Solution

1. Step 1: List rows from both tables

   Table A has ids 1, 2, 3; Table B has ids 2, 3, 4.

2. Step 2: Apply UNION behavior

   UNION combines all rows and removes duplicates, so final list is 1, 2, 3, 4.

3. Final Answer:

   [1, 2, 3, 4] -> Option A

4. Quick Check:

   UNION removes duplicates = [1, 2, 3, 4] [OK]

Hint: UNION removes duplicates, so no repeated ids appear [OK]

Common Mistakes:

• Expecting duplicates to appear
• Confusing UNION with UNION ALL
• Listing only common ids

4. Consider this SQL query:
SELECT name FROM employees UNION SELECT name, department FROM managers;
What is the issue with this query?

medium

A. The number of columns in both SELECT statements differ

B. UNION cannot be used with SELECT statements

C. The query will return duplicate rows

D. The query is missing a WHERE clause

Solution

1. Step 1: Check column counts in SELECTs

   The first SELECT returns 1 column (name), the second returns 2 columns (name, department).

2. Step 2: Understand UNION column rules

   UNION requires all SELECT statements to have the same number of columns with compatible types.

3. Final Answer:

   The number of columns in both SELECT statements differ -> Option A

4. Quick Check:

   UNION needs same columns = C [OK]

Hint: UNION needs same column count in all SELECTs [OK]

Common Mistakes:

• Ignoring column count mismatch
• Thinking UNION filters duplicates only
• Assuming WHERE clause is mandatory

5. You have two tables:
Sales2023(product, amount)
Apple, 100
Banana, 150
Sales2024(product, amount)
Banana, 200
Cherry, 300
Write a query using UNION to list all unique products sold in both years, sorted alphabetically.

hard

A. SELECT product, amount FROM Sales2023 UNION SELECT product, amount FROM Sales2024 ORDER BY amount;

B. SELECT product FROM Sales2023 UNION ALL SELECT product FROM Sales2024 ORDER BY product;

C. SELECT product FROM Sales2023 UNION SELECT product FROM Sales2024 ORDER BY product;

D. SELECT product FROM Sales2023 JOIN Sales2024 ON product ORDER BY product;

Solution

1. Step 1: Select product column from both tables

   We want unique products, so select only the product column from both tables.

2. Step 2: Use UNION to combine and remove duplicates

   UNION combines both lists and removes duplicates, giving unique products.

3. Step 3: Sort results alphabetically

   ORDER BY product sorts the final list alphabetically.

4. Final Answer:

   SELECT product FROM Sales2023 UNION SELECT product FROM Sales2024 ORDER BY product; -> Option C

5. Quick Check:

   UNION + ORDER BY product = B [OK]

Hint: Use UNION to remove duplicates, ORDER BY to sort [OK]

Common Mistakes:

• Using UNION ALL which keeps duplicates
• Selecting amount column when only product needed
• Using JOIN instead of UNION