UNION and UNION ALL in PostgreSQL - Mini Project: Build & Apply

Using UNION and UNION ALL in PostgreSQL

📖 Scenario: You work at a bookstore that has two separate tables: one for new arrivals and one for best sellers. You want to create combined lists of books for different purposes.

🎯 Goal: Build SQL queries using UNION and UNION ALL to combine data from two tables: new_arrivals and best_sellers. You will first create the tables, then write queries to combine their data without duplicates and with duplicates.

📋 What You'll Learn

Create two tables named new_arrivals and best_sellers with columns book_id and title.

Insert specific book records into both tables.

Write a query using UNION to combine the two tables without duplicate titles.

Write a query using UNION ALL to combine the two tables including duplicates.

💡 Why This Matters

🌍 Real World

Combining data from multiple sources is common in real-world databases, such as merging product lists, customer records, or event logs.

💼 Career

Understanding how to use UNION and UNION ALL is essential for database querying and reporting tasks in roles like data analyst, database developer, and backend engineer.

1

Create the tables and insert data

Create two tables called new_arrivals and best_sellers with columns book_id (integer) and title (text). Insert these exact rows into new_arrivals: (1, 'The Silent Patient'), (2, 'Where the Crawdads Sing'), (3, 'The Midnight Library'). Insert these exact rows into best_sellers: (10, 'The Midnight Library'), (11, 'Becoming'), (12, 'Educated').

PostgreSQL

-- Create tables new_arrivals and best_sellers and insert the specified rows
-- Your code here

Hint

Use CREATE TABLE statements for both tables. Then use INSERT INTO with the exact values given.

2

Write a UNION query to combine unique book titles

Write a SQL query that selects the title column from new_arrivals and best_sellers and combines them using UNION to show all unique book titles without duplicates.

PostgreSQL

CREATE TABLE new_arrivals (book_id INTEGER, title TEXT);
CREATE TABLE best_sellers (book_id INTEGER, title TEXT);

INSERT INTO new_arrivals (book_id, title) VALUES
  (1, 'The Silent Patient'),
  (2, 'Where the Crawdads Sing'),
  (3, 'The Midnight Library');

INSERT INTO best_sellers (book_id, title) VALUES
  (10, 'The Midnight Library'),
  (11, 'Becoming'),
  (12, 'Educated');

-- Write a query using UNION to combine unique titles
-- Your code here

Hint

Use SELECT title FROM new_arrivals UNION SELECT title FROM best_sellers to combine unique titles.

3

Write a UNION ALL query to combine all book titles including duplicates

Write a SQL query that selects the title column from new_arrivals and best_sellers and combines them using UNION ALL to show all book titles including duplicates.

PostgreSQL

CREATE TABLE new_arrivals (book_id INTEGER, title TEXT);
CREATE TABLE best_sellers (book_id INTEGER, title TEXT);

INSERT INTO new_arrivals (book_id, title) VALUES
  (1, 'The Silent Patient'),
  (2, 'Where the Crawdads Sing'),
  (3, 'The Midnight Library');

INSERT INTO best_sellers (book_id, title) VALUES
  (10, 'The Midnight Library'),
  (11, 'Becoming'),
  (12, 'Educated');

SELECT title FROM new_arrivals
UNION
SELECT title FROM best_sellers;

-- Write a query using UNION ALL to combine all titles including duplicates
-- Your code here

Hint

Use UNION ALL to include duplicates from both tables.

4

Add ORDER BY to sort the combined results

Modify the UNION ALL query to add ORDER BY title ASC at the end to sort the combined list of book titles alphabetically.

PostgreSQL

CREATE TABLE new_arrivals (book_id INTEGER, title TEXT);
CREATE TABLE best_sellers (book_id INTEGER, title TEXT);

INSERT INTO new_arrivals (book_id, title) VALUES
  (1, 'The Silent Patient'),
  (2, 'Where the Crawdads Sing'),
  (3, 'The Midnight Library');

INSERT INTO best_sellers (book_id, title) VALUES
  (10, 'The Midnight Library'),
  (11, 'Becoming'),
  (12, 'Educated');

SELECT title FROM new_arrivals
UNION
SELECT title FROM best_sellers;

SELECT title FROM new_arrivals
UNION ALL
SELECT title FROM best_sellers
-- Add ORDER BY clause to sort titles alphabetically
-- Your code here

Hint

Add ORDER BY title ASC after the UNION ALL query to sort the results alphabetically.

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