Why set operations are needed in SQL

Introduction

Set operations help combine or compare data from two or more tables easily. They let you find common, different, or all data without writing complex code.

You want to find all customers who bought from either store A or store B.

You need to see which products are sold in both online and physical stores.

You want to list employees who work in department X but not in department Y.

You want to merge two lists of emails without duplicates.

Syntax

SQL

SELECT column_list FROM table1
UNION | UNION ALL | INTERSECT | EXCEPT
SELECT column_list FROM table2;

All SELECT statements must have the same number of columns and compatible data types.

UNION removes duplicates, UNION ALL keeps all rows, INTERSECT finds common rows, EXCEPT finds rows in first but not in second.

Examples

Finds all unique customers who bought from store A or B.

SQL

SELECT name FROM customers_in_storeA
UNION
SELECT name FROM customers_in_storeB;

Finds products sold both online and in physical stores.

SQL

SELECT product_id FROM online_products
INTERSECT
SELECT product_id FROM physical_store_products;

Finds employees in department X but not in department Y.

SQL

SELECT employee_id FROM departmentX
EXCEPT
SELECT employee_id FROM departmentY;

Sample Program

This example creates two tables with customer names from two stores. The UNION query lists all unique customers who bought from either store.

SQL

CREATE TABLE storeA_customers (name VARCHAR(20));
CREATE TABLE storeB_customers (name VARCHAR(20));

INSERT INTO storeA_customers VALUES ('Alice'), ('Bob'), ('Charlie');
INSERT INTO storeB_customers VALUES ('Bob'), ('Diana');

SELECT name FROM storeA_customers
UNION
SELECT name FROM storeB_customers;

Important Notes

Set operations work best when columns match in number and type.

UNION removes duplicates, so it can be slower than UNION ALL.

Not all databases support INTERSECT and EXCEPT; check your system.

Summary

Set operations combine or compare data from multiple tables simply.

They help find common, different, or all unique rows.

Use UNION, INTERSECT, and EXCEPT to answer real-world questions about data overlap and difference.

Practice

1. Why do we use set operations like UNION and INTERSECT in SQL?

easy

A. To create new tables permanently

B. To delete rows based on conditions

C. To update values in a single table

D. To combine or compare rows from two or more tables easily

Solution

1. Step 1: Understand the purpose of set operations

   Set operations like UNION and INTERSECT are designed to combine or compare rows from multiple tables.

2. Step 2: Identify what set operations do not do

   They do not create tables, update, or delete rows; those are different SQL commands.

3. Final Answer:

   To combine or compare rows from two or more tables easily -> Option D

4. Quick Check:

   Set operations combine or compare data [OK]

Hint: Set operations combine or compare rows from tables [OK]

Common Mistakes:

• Confusing set operations with data modification commands
• Thinking UNION creates a new permanent table
• Assuming INTERSECT deletes rows

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

easy

A. UNION SELECT * FROM table1, SELECT * FROM table2;

B. SELECT * FROM table1 JOIN UNION SELECT * FROM table2;

C. SELECT * FROM table1 UNION SELECT * FROM table2;

D. SELECT * FROM table1 AND SELECT * FROM table2 UNION;

Solution

1. Step 1: Recall correct UNION syntax

   The correct syntax is to write one SELECT query, then UNION, then another SELECT query.

2. Step 2: Check each option for syntax errors

   SELECT * FROM table1 UNION SELECT * FROM table2; follows the correct syntax. Options A, B, and D have incorrect keywords or order.

3. Final Answer:

   SELECT * FROM table1 UNION SELECT * FROM table2; -> Option C

4. Quick Check:

   Correct UNION syntax = SELECT * FROM table1 UNION SELECT * FROM table2; [OK]

Hint: UNION joins two SELECT queries directly [OK]

Common Mistakes:

• Adding JOIN keyword with UNION
• Using commas instead of UNION
• Placing UNION at the end incorrectly

3. Given two tables:
Table A: {1, 2, 3}
Table B: {2, 3, 4}
What is the result of SELECT * FROM A INTERSECT SELECT * FROM B;?

medium

A. {2, 3}

B. {1, 2, 3}

C. {1, 4}

D. {1, 2, 3, 4}

Solution

1. Step 1: Understand INTERSECT operation

   INTERSECT returns only rows present in both tables.

2. Step 2: Find common elements in Table A and Table B

   Common elements are 2 and 3.

3. Final Answer:

   {2, 3} -> Option A

4. Quick Check:

   INTERSECT = common rows [OK]

Hint: INTERSECT returns only common rows [OK]

Common Mistakes:

• Confusing INTERSECT with UNION
• Including all rows from both tables
• Mixing up EXCEPT with INTERSECT

4. You wrote this SQL query:
SELECT * FROM table1 UNION table2;
What is the error and how to fix it?

medium

A. Missing SELECT before table2; fix by adding SELECT * FROM table2

B. UNION cannot be used with tables; use JOIN instead

C. UNION requires parentheses around queries

D. No error; query runs fine

Solution

1. Step 1: Identify syntax error in UNION usage

   UNION requires two complete SELECT statements, but second query lacks SELECT.

2. Step 2: Correct the query syntax

   Add SELECT * FROM before table2 to fix the error.

3. Final Answer:

   Missing SELECT before table2; fix by adding SELECT * FROM table2 -> Option A

4. Quick Check:

   UNION needs two SELECTs [OK]

Hint: UNION needs two full SELECT queries [OK]

Common Mistakes:

• Omitting SELECT in second query
• Using UNION with tables directly
• Adding unnecessary parentheses

5. You have two customer lists:
List A: customers who bought product X
List B: customers who bought product Y
How do you find customers who bought either product X or Y but not both using set operations?

hard

A. SELECT * FROM A UNION SELECT * FROM B

B. (SELECT * FROM A EXCEPT SELECT * FROM B) UNION (SELECT * FROM B EXCEPT SELECT * FROM A)

C. SELECT * FROM A INTERSECT SELECT * FROM B

D. (SELECT * FROM A EXCEPT SELECT * FROM B) UNION (SELECT * FROM A INTERSECT SELECT * FROM B)

Solution

1. Step 1: Understand the problem

   We want customers who bought product X or Y but not both (exclusive customers).

2. Step 2: Use EXCEPT and UNION to find exclusive customers

   Find customers in A but not in B, and customers in B but not in A, then combine them with UNION.

3. Step 3: Check other options

   SELECT * FROM A UNION SELECT * FROM B gives all customers who bought either product (including both). SELECT * FROM A INTERSECT SELECT * FROM B gives only those who bought both. (SELECT * FROM A EXCEPT SELECT * FROM B) UNION (SELECT * FROM A INTERSECT SELECT * FROM B) gives all customers who bought product X.

4. Final Answer:

   (SELECT * FROM A EXCEPT SELECT * FROM B) UNION (SELECT * FROM B EXCEPT SELECT * FROM A) -> Option B

5. Quick Check:

   Exclusive customers = (A EXCEPT B) UNION (B EXCEPT A) [OK]

Hint: Use EXCEPT both ways, then UNION results [OK]

Common Mistakes:

• Using UNION alone includes both customers
• Using INTERSECT returns only common customers
• Confusing EXCEPT with INTERSECT