INTERSECT and EXCEPT in PostgreSQL

Introduction

Use INTERSECT and EXCEPT to find common or different data between two lists or tables easily.

You want to find items that appear in both of two lists, like customers who bought both product A and product B.

You want to find items in one list but not in another, like employees who worked last year but not this year.

You want to compare two sets of data to see what is shared or what is missing.

You want to clean up duplicates by finding common entries between two sources.

You want to check differences between two reports or tables quickly.

Syntax

PostgreSQL

SELECT column_list FROM table1
INTERSECT
SELECT column_list FROM table2;

SELECT column_list FROM table1
EXCEPT
SELECT column_list FROM table2;

Both queries must select the same number of columns with compatible data types.

INTERSECT returns rows common to both queries; EXCEPT returns rows in the first query but not in the second.

Examples

Finds employees who worked in both 2023 and 2024.

PostgreSQL

SELECT name FROM employees_2023
INTERSECT
SELECT name FROM employees_2024;

Finds products sold in January but not in February.

PostgreSQL

SELECT product_id FROM sales_january
EXCEPT
SELECT product_id FROM sales_february;

Finds cities where customers live in both US and Canada.

PostgreSQL

SELECT city FROM customers_us
INTERSECT
SELECT city FROM customers_canada;

Sample Program

This example creates two fruit lists and shows how to find common fruits and fruits only in the first list.

PostgreSQL

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

INSERT INTO fruits1 VALUES ('apple'), ('banana'), ('cherry');
INSERT INTO fruits2 VALUES ('banana'), ('cherry'), ('date');

-- Find fruits in both lists
SELECT name FROM fruits1
INTERSECT
SELECT name FROM fruits2;

-- Find fruits in fruits1 but not in fruits2
SELECT name FROM fruits1
EXCEPT
SELECT name FROM fruits2;

Important Notes

INTERSECT and EXCEPT automatically remove duplicates in the results.

Order of rows in the result is not guaranteed unless you add ORDER BY.

Use UNION if you want all rows from both queries without filtering.

Summary

INTERSECT finds common rows between two queries.

EXCEPT finds rows in the first query that are not in the second.

Both require the same number and type of columns selected.

Practice

1. What does the INTERSECT operator do in PostgreSQL?

easy

A. Returns rows only in the second query

B. Returns all rows from the first query

C. Combines all rows from both queries without duplicates

D. Returns rows common to both queries

Solution

1. Step 1: Understand INTERSECT purpose

   INTERSECT returns only rows that appear in both query results.

2. Step 2: Compare with other set operations

   Unlike UNION or EXCEPT, INTERSECT finds common rows, not all or unique rows.

3. Final Answer:

   Returns rows common to both queries -> Option D

4. Quick Check:

   INTERSECT = common rows [OK]

Hint: INTERSECT = find common rows between two queries [OK]

Common Mistakes:

• Confusing INTERSECT with UNION
• Thinking INTERSECT returns all rows from first query
• Mixing INTERSECT with EXCEPT

2. Which of the following is the correct syntax to find rows in table1 but not in table2 using EXCEPT?

easy

A. SELECT * FROM table1 INTERSECT SELECT * FROM table2;

B. SELECT * FROM table1 EXCEPT SELECT * FROM table2;

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

D. SELECT * FROM table1 EXCEPT ALL SELECT * FROM table2;

Solution

1. Step 1: Identify EXCEPT syntax

   The correct syntax is: SELECT ... EXCEPT SELECT ... to get rows in first query not in second.

2. Step 2: Check options for correct usage

   SELECT * FROM table1 EXCEPT SELECT * FROM table2; matches correct syntax; others misuse INTERSECT, UNION, or EXCEPT ALL (which is valid but different).

3. Final Answer:

   SELECT * FROM table1 EXCEPT SELECT * FROM table2; -> Option B

4. Quick Check:

   EXCEPT syntax = SELECT ... EXCEPT SELECT ... [OK]

Hint: Use EXCEPT between two SELECTs to find rows only in first [OK]

Common Mistakes:

• Using INTERSECT instead of EXCEPT
• Adding UNION incorrectly
• Using EXCEPT ALL when not needed

3. Given these tables:
table_a:
id
1
2
3

table_b:
id
2
3
4

What is the result of:
SELECT id FROM table_a INTERSECT SELECT id FROM table_b;

medium

A. [1, 2, 3]

B. [1]

C. [2, 3]

D. [4]

Solution

1. Step 1: List rows from both tables

   table_a has ids 1, 2, 3; table_b has ids 2, 3, 4.

2. Step 2: Find common ids using INTERSECT

   Common ids are 2 and 3 only.

3. Final Answer:

   [2, 3] -> Option C

4. Quick Check:

   INTERSECT = common rows = [2, 3] [OK]

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

Common Mistakes:

• Including ids only in one table
• Confusing INTERSECT with UNION
• Expecting all ids combined

4. What is wrong with this query?

SELECT id, name FROM table1 EXCEPT SELECT id FROM table2;

medium

A. The number of columns in both SELECTs differ

B. EXCEPT cannot be used with SELECT statements

C. The query is missing a WHERE clause

D. The table names are invalid

Solution

1. Step 1: Check column counts in both SELECTs

   First SELECT returns two columns (id, name), second returns one column (id).

2. Step 2: Understand EXCEPT requirements

   EXCEPT requires both SELECTs to have the same number and compatible types of columns.

3. Final Answer:

   The number of columns in both SELECTs differ -> Option A

4. Quick Check:

   EXCEPT needs equal columns [OK]

Hint: EXCEPT requires same number of columns in both SELECTs [OK]

Common Mistakes:

• Ignoring column count mismatch
• Thinking EXCEPT needs WHERE clause
• Assuming table names cause error

5. You have two tables:
employees with columns id, name
contractors with columns id, name
You want to find people who are either employees or contractors but not both.
Which query correctly returns this list?

hard

A. SELECT id, name FROM employees EXCEPT SELECT id, name FROM contractors UNION SELECT id, name FROM contractors EXCEPT SELECT id, name FROM employees;

B. SELECT id, name FROM employees INTERSECT SELECT id, name FROM contractors;

C. SELECT id, name FROM employees UNION SELECT id, name FROM contractors;

D. SELECT id, name FROM employees EXCEPT SELECT id, name FROM contractors;

Solution

1. Step 1: Understand the requirement

   We want people in employees or contractors but not in both (symmetric difference).

2. Step 2: Use EXCEPT and UNION to find symmetric difference

   EXCEPT finds those in one table but not the other; UNION combines both exclusive sets.

3. Step 3: Analyze options

   SELECT id, name FROM employees EXCEPT SELECT id, name FROM contractors UNION SELECT id, name FROM contractors EXCEPT SELECT id, name FROM employees; correctly combines EXCEPT in both directions with UNION to get symmetric difference.

4. Final Answer:

   SELECT id, name FROM employees EXCEPT SELECT id, name FROM contractors UNION SELECT id, name FROM contractors EXCEPT SELECT id, name FROM employees; -> Option A

5. Quick Check:

   Symmetric difference = EXCEPT + UNION [OK]

Hint: Symmetric difference = (A EXCEPT B) UNION (B EXCEPT A) [OK]

Common Mistakes:

• Using only INTERSECT or UNION
• Using EXCEPT one way only
• Confusing symmetric difference with intersection