INTERSECT for common rows in SQL - Mini Project: Build & Apply

Find Common Customers Using INTERSECT

📖 Scenario: You work for a company that has two sales databases. Each database has a table of customers who made purchases in different regions. You want to find customers who bought products in both regions.

🎯 Goal: Create two tables named region1_customers and region2_customers with customer names. Then write a query using INTERSECT to find customers common to both regions.

📋 What You'll Learn

Create a table called region1_customers with a column customer_name and insert these exact names: 'Alice', 'Bob', 'Charlie', 'Diana'

Create a table called region2_customers with a column customer_name and insert these exact names: 'Bob', 'Diana', 'Eve', 'Frank'

Write a query using INTERSECT to select customer_name from both tables to find common customers

Complete the final query to show only the common customer names

💡 Why This Matters

🌍 Real World

Companies often need to find customers or data points common to multiple datasets or regions to analyze overlapping markets or shared customers.

💼 Career

Knowing how to use INTERSECT helps database analysts and developers write efficient queries to compare datasets and find common records.

1

Create the region1_customers table and insert data

Create a table called region1_customers with a column customer_name of type VARCHAR(50). Insert these exact customer names: 'Alice', 'Bob', 'Charlie', and 'Diana'.

SQL

-- Create table region1_customers and insert customers
-- Your code here

Hint

Use CREATE TABLE to make the table and INSERT INTO to add rows.

2

Create the region2_customers table and insert data

Create a table called region2_customers with a column customer_name of type VARCHAR(50). Insert these exact customer names: 'Bob', 'Diana', 'Eve', and 'Frank'.

SQL

CREATE TABLE region1_customers (customer_name VARCHAR(50));
INSERT INTO region1_customers (customer_name) VALUES
('Alice'),
('Bob'),
('Charlie'),
('Diana');

-- Create table region2_customers and insert customers
-- Your code here

Hint

Repeat the table creation and insertion steps for the second region.

3

Write the INTERSECT query to find common customers

Write a SQL query that selects customer_name from region1_customers and uses INTERSECT with a select of customer_name from region2_customers to find customers present in both tables.

SQL

CREATE TABLE region1_customers (customer_name VARCHAR(50));
INSERT INTO region1_customers (customer_name) VALUES
('Alice'),
('Bob'),
('Charlie'),
('Diana');

CREATE TABLE region2_customers (customer_name VARCHAR(50));
INSERT INTO region2_customers (customer_name) VALUES
('Bob'),
('Diana'),
('Eve'),
('Frank');

-- Write INTERSECT query to find common customers
-- Your code here

Hint

Use INTERSECT between two SELECT statements to get common rows.

4

Complete the query to show only common customer names

Ensure the final query selects customer_name from region1_customers and uses INTERSECT with the select from region2_customers to display only the customers who appear in both tables.

SQL

CREATE TABLE region1_customers (customer_name VARCHAR(50));
INSERT INTO region1_customers (customer_name) VALUES
('Alice'),
('Bob'),
('Charlie'),
('Diana');

CREATE TABLE region2_customers (customer_name VARCHAR(50));
INSERT INTO region2_customers (customer_name) VALUES
('Bob'),
('Diana'),
('Eve'),
('Frank');

-- Finalize the query to show common customers
-- Your code here

Hint

The query is complete when it uses INTERSECT to find common customer names.

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