UNION and UNION ALL in PostgreSQL

Introduction

UNION and UNION ALL combine results from two or more queries into one list. They help you see data from different places together.

You want to list all customers from two different regions in one list.

You need to combine product names from two tables to see everything available.

You want to merge sales records from two months into a single report.

You want to get all unique email addresses from two contact lists.

You want to include duplicates when combining two lists of orders.

Syntax

PostgreSQL

SELECT column1, column2 FROM table1
UNION
SELECT column1, column2 FROM table2;

UNION removes duplicate rows between the queries.

UNION ALL keeps all rows, including duplicates.

Examples

Combines customer names from two regions, removing duplicates.

PostgreSQL

SELECT name FROM customers_north
UNION
SELECT name FROM customers_south;

Combines customer names from two regions, keeping duplicates.

PostgreSQL

SELECT name FROM customers_north
UNION ALL
SELECT name FROM customers_south;

Lists all unique products from online and store sales.

PostgreSQL

SELECT product FROM products_online
UNION
SELECT product FROM products_store;

Sample Program

This example creates two tables with fruit names. The first query combines them with UNION, showing each fruit once. The second query uses UNION ALL, showing all fruits including duplicates.

PostgreSQL

CREATE TABLE fruits1 (name TEXT);
CREATE TABLE fruits2 (name TEXT);

INSERT INTO fruits1 VALUES ('Apple'), ('Banana'), ('Cherry');
INSERT INTO fruits2 VALUES ('Banana'), ('Date'), ('Apple');

-- Using UNION (removes duplicates)
SELECT name FROM fruits1
UNION
SELECT name FROM fruits2;

-- Using UNION ALL (keeps duplicates)
SELECT name FROM fruits1
UNION ALL
SELECT name FROM fruits2;

Important Notes

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

UNION removes duplicates and sorts the result, so it can be slower than UNION ALL.

Use UNION ALL when you want to keep duplicates and improve performance.

Summary

UNION combines query results and removes duplicates.

UNION ALL combines query results and keeps duplicates.

Both require the same number of columns with matching types.

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