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.

Practice

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

easy

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

B. UNION requires matching column names, UNION ALL does not.

C. UNION ALL removes duplicates, UNION keeps duplicates.

D. UNION can combine different numbers of columns, UNION ALL cannot.

Solution

1. Step 1: Understand the behavior of UNION

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

2. Step 2: Understand the behavior of UNION ALL

   UNION ALL combines results but keeps all rows, including duplicates.

3. Final Answer:

   UNION removes duplicates, UNION ALL keeps duplicates. -> Option A

4. Quick Check:

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

Hint: UNION removes duplicates; UNION ALL keeps all rows [OK]

Common Mistakes:

• Thinking UNION ALL removes duplicates
• Believing UNION allows different column counts
• Confusing column name requirements

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

easy

A. SELECT name FROM students UNION ALL SELECT name FROM teachers;

B. SELECT name FROM students UNIONALL SELECT name FROM teachers;

C. SELECT name FROM students UNION ALL FROM teachers;

D. SELECT name FROM students UNION ALL SELECT name FROM teachers WHERE;

Solution

1. Step 1: Check the correct keyword spacing

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

2. Step 2: Verify the SELECT statements are complete

   Both queries must be valid SELECT statements with matching column counts and types.

3. Final Answer:

   SELECT name FROM students UNION ALL SELECT name FROM teachers; -> Option A

4. Quick Check:

   Correct syntax uses 'UNION ALL' with space [OK]

Hint: Use space between UNION and ALL, both queries must be valid [OK]

Common Mistakes:

• Writing UNIONALL as one word
• Missing SELECT in second query
• Adding WHERE without condition

3. Given two tables:
Table A:
id | value
1 | 'apple'
2 | 'banana'

Table B:
id | value
2 | 'banana'
3 | 'cherry'

What is the result of this query?

SELECT value FROM A UNION SELECT value FROM B ORDER BY value;

medium

A. apple, banana, banana, cherry

B. apple, banana, cherry, banana

C. banana, cherry

D. apple, banana, cherry

Solution

1. Step 1: Combine values from both tables using UNION

   UNION removes duplicates, so 'banana' appears only once.

2. Step 2: Order the combined results alphabetically

   Ordering by value gives: 'apple', 'banana', 'cherry'.

3. Final Answer:

   apple, banana, cherry -> Option D

4. Quick Check:

   UNION removes duplicates, so only one 'banana' [OK]

Hint: UNION removes duplicates; check ordering carefully [OK]

Common Mistakes:

• Including duplicate 'banana' with UNION
• Ignoring ORDER BY effect
• Confusing UNION with UNION ALL

4. Consider this query:

SELECT id FROM table1 UNION ALL SELECT id FROM table2;

It returns an error. What is the most likely cause?

medium

A. The tables have different names.

B. The two SELECT statements have different numbers of columns.

C. UNION ALL cannot be used with SELECT statements.

D. The query is missing a semicolon.

Solution

1. Step 1: Check column counts in both SELECT statements

   UNION ALL requires both SELECTs to have the same number of columns.

2. Step 2: Identify error cause

   If column counts differ, PostgreSQL throws an error.

3. Final Answer:

   The two SELECT statements have different numbers of columns. -> Option B

4. Quick Check:

   Column count mismatch causes UNION ALL error [OK]

Hint: Ensure both SELECTs have same column count for UNION ALL [OK]

Common Mistakes:

• Thinking table names must match
• Believing UNION ALL disallows SELECT
• Ignoring column count mismatch

5. You want to combine two queries:

SELECT id, name FROM employees WHERE department = 'Sales'
SELECT id, name FROM contractors WHERE department = 'Sales'

and keep all rows including duplicates. Which query correctly does this and why?

hard

A. SELECT id, name FROM employees UNION ALL SELECT id, name FROM contractors; Because WHERE is not needed in second query.

B. SELECT id, name FROM employees UNION SELECT id, name FROM contractors WHERE department = 'Sales'; Because UNION removes duplicates.

C. SELECT id, name FROM employees UNION ALL SELECT id, name FROM contractors WHERE department = 'Sales'; Because UNION ALL keeps duplicates.

D. SELECT id, name FROM employees UNION SELECT id, name FROM contractors; Because UNION ALL is invalid here.

Solution

1. Step 1: Understand requirement to keep duplicates

   To keep duplicates, use UNION ALL because UNION removes duplicates.

2. Step 2: Check WHERE clause usage

   Both queries filter department = 'Sales', so WHERE must be in both queries.

3. Final Answer:

   SELECT id, name FROM employees UNION ALL SELECT id, name FROM contractors WHERE department = 'Sales'; Because UNION ALL keeps duplicates. -> Option C

4. Quick Check:

   Keep duplicates = UNION ALL with correct WHERE clauses [OK]

Hint: Use UNION ALL to keep duplicates; apply WHERE in each SELECT [OK]

Common Mistakes:

• Using UNION instead of UNION ALL
• Omitting WHERE in second query
• Thinking UNION ALL is invalid