INTERSECT for common rows in SQL

Introduction

INTERSECT helps find rows that appear in both tables or queries. It shows only the common data.

You want to find customers who bought products from two different stores.

You want to see which employees are listed in both the sales and support departments.

You want to find common items in two different inventory lists.

You want to compare two lists of students to find those enrolled in both classes.

Syntax

SQL

SELECT column_list FROM table1
INTERSECT
SELECT column_list FROM table2;

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

INTERSECT returns only distinct rows that appear in both queries.

Examples

Finds employee names present in both years.

SQL

SELECT name FROM employees_2023
INTERSECT
SELECT name FROM employees_2024;

Finds products sold in both stores.

SQL

SELECT product_id, product_name FROM store_a
INTERSECT
SELECT product_id, product_name FROM store_b;

Finds cities where customers exist in both regions.

SQL

SELECT city FROM customers_north
INTERSECT
SELECT city FROM customers_south;

Sample Program

This example finds fruit names that are in both fruits_a and fruits_b tables.

SQL

CREATE TABLE fruits_a (name VARCHAR(20));
CREATE TABLE fruits_b (name VARCHAR(20));

INSERT INTO fruits_a VALUES ('Apple'), ('Banana'), ('Cherry');
INSERT INTO fruits_b VALUES ('Banana'), ('Cherry'), ('Date');

SELECT name FROM fruits_a
INTERSECT
SELECT name FROM fruits_b;

Important Notes

INTERSECT removes duplicates automatically; no need to use DISTINCT.

If no rows are common, INTERSECT returns an empty result.

Column order and data types must match in both SELECT statements.

Summary

INTERSECT shows only rows common to both queries.

Use it to compare lists and find shared data.

Both queries must have matching columns and types.

Practice

1. What does the SQL INTERSECT operator do?

easy

A. Combines rows from both queries including duplicates.

B. Returns all rows from the first SELECT query.

C. Returns all rows from the second SELECT query.

D. Returns only the rows common to both SELECT queries.

Solution

1. Step 1: Understand the purpose of INTERSECT

   The INTERSECT operator compares two SELECT queries and returns only the rows that appear in both results.

2. Step 2: Compare with other set operators

   Unlike UNION or UNION ALL, INTERSECT excludes rows not common to both queries.

3. Final Answer:

   Returns only the rows common to both SELECT queries. -> Option D

4. Quick Check:

   INTERSECT = common rows [OK]

Hint: INTERSECT keeps only shared rows between two queries [OK]

Common Mistakes:

• Confusing INTERSECT with UNION which combines all rows
• Thinking INTERSECT returns rows from only one query
• Assuming INTERSECT includes duplicates

2. Which of the following is the correct syntax to find common rows between two tables TableA and TableB with the same columns id and name?

easy

A. SELECT id, name FROM TableA JOIN TableB ON TableA.id = TableB.id;

B. SELECT id, name FROM TableA UNION SELECT id, name FROM TableB;

C. SELECT id, name FROM TableA INTERSECT SELECT id, name FROM TableB;

D. SELECT id, name FROM TableA WHERE id IN TableB;

Solution

1. Step 1: Identify correct INTERSECT syntax

   The INTERSECT operator is used between two SELECT statements with matching columns and types.

2. Step 2: Check each option

   SELECT id, name FROM TableA INTERSECT SELECT id, name FROM TableB; uses INTERSECT correctly. SELECT id, name FROM TableA UNION SELECT id, name FROM TableB; uses UNION which combines rows. SELECT id, name FROM TableA JOIN TableB ON TableA.id = TableB.id; uses JOIN which is different. SELECT id, name FROM TableA WHERE id IN TableB; has incorrect WHERE syntax.

3. Final Answer:

   SELECT id, name FROM TableA INTERSECT SELECT id, name FROM TableB; -> Option C

4. Quick Check:

   Correct INTERSECT syntax = SELECT id, name FROM TableA INTERSECT SELECT id, name FROM TableB; [OK]

Hint: Use INTERSECT between two SELECTs with matching columns [OK]

Common Mistakes:

• Using UNION instead of INTERSECT
• Trying to use WHERE with IN incorrectly
• Confusing JOIN with INTERSECT

3. Given two tables:
Employees1:
id | name
1 | Alice
2 | Bob
3 | Carol

Employees2:
id | name
2 | Bob
3 | Carol
4 | Dave

What is the result of this query?

SELECT id, name FROM Employees1 INTERSECT SELECT id, name FROM Employees2;

medium

A. Rows with Bob and Carol only

B. Rows with Alice, Bob, Carol, and Dave

C. Rows with Alice only

D. Rows with Dave only

Solution

1. Step 1: List rows from both tables

   Employees1 has (1, Alice), (2, Bob), (3, Carol). Employees2 has (2, Bob), (3, Carol), (4, Dave).

2. Step 2: Find common rows

   Common rows are those present in both: (2, Bob) and (3, Carol).

3. Final Answer:

   Rows with Bob and Carol only -> Option A

4. Quick Check:

   INTERSECT returns common rows = Bob, Carol [OK]

Hint: INTERSECT returns only rows present in both tables [OK]

Common Mistakes:

• Including rows unique to one table
• Confusing UNION with INTERSECT
• Ignoring column matching in rows

4. Consider this SQL query:

SELECT id, name FROM Customers INTERSECT SELECT id, name FROM Orders;

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

medium

A. The columns selected have different data types in Customers and Orders.

B. INTERSECT cannot be used with SELECT statements.

C. The tables Customers and Orders do not exist.

D. INTERSECT requires a WHERE clause.

Solution

1. Step 1: Understand INTERSECT requirements

   INTERSECT requires both SELECT queries to have the same number of columns with compatible data types.

2. Step 2: Analyze error causes

   If columns differ in type (e.g., id is integer in one table and string in another), the query errors. Other options are incorrect because INTERSECT is valid syntax, tables must exist to run, and WHERE is optional.

3. Final Answer:

   The columns selected have different data types in Customers and Orders. -> Option A

4. Quick Check:

   Matching column types required for INTERSECT [OK]

Hint: Check column types match for INTERSECT queries [OK]

Common Mistakes:

• Assuming INTERSECT needs WHERE clause
• Thinking INTERSECT is invalid syntax
• Ignoring data type mismatches

5. You have two tables:
ProductsA with columns product_id, name, price
ProductsB with columns product_id, name, price

You want to find products that exist in both tables with the same product_id and name, ignoring price differences.

Which query correctly uses INTERSECT to achieve this?

hard

A. SELECT product_id, name, price FROM ProductsA INTERSECT SELECT product_id, name, price FROM ProductsB;

B. SELECT product_id, name FROM ProductsA INTERSECT SELECT product_id, name FROM ProductsB;

C. SELECT product_id FROM ProductsA INTERSECT SELECT product_id FROM ProductsB;

D. SELECT * FROM ProductsA INTERSECT SELECT * FROM ProductsB;

Solution

1. Step 1: Identify columns to compare

   You want to compare only product_id and name, ignoring price differences.

2. Step 2: Use INTERSECT on matching columns

   SELECT product_id, name FROM ProductsA INTERSECT SELECT product_id, name FROM ProductsB; selects product_id and name from both tables and intersects them, returning only products common by those two columns.

3. Step 3: Analyze other options

   SELECT product_id, name, price FROM ProductsA INTERSECT SELECT product_id, name, price FROM ProductsB; includes price, so products with different prices won't match. SELECT product_id FROM ProductsA INTERSECT SELECT product_id FROM ProductsB; compares only product_id, ignoring name. SELECT * FROM ProductsA INTERSECT SELECT * FROM ProductsB; compares all columns, including price, which is not desired.

4. Final Answer:

   SELECT product_id, name FROM ProductsA INTERSECT SELECT product_id, name FROM ProductsB; -> Option B

5. Quick Check:

   INTERSECT on selected columns matches desired fields [OK]

Hint: Select only columns to compare before INTERSECT [OK]

Common Mistakes:

• Including extra columns that cause mismatches
• Selecting * when columns differ
• Comparing only one column when two are needed