Why INTERSECT and EXCEPT in PostgreSQL? - Purpose & Use Cases

The Big Idea

Discover how to instantly find what two lists share or don't share without tedious checking!

The Scenario

Imagine you have two lists of friends from different social events written on paper. You want to find who attended both events or who attended only the first event. Doing this by hand means checking each name one by one, which is tiring and easy to mess up.

The Problem

Manually comparing lists is slow and mistakes happen easily. You might miss names, count duplicates, or forget to check all entries. This wastes time and causes frustration, especially with long lists.

The Solution

Using INTERSECT and EXCEPT in SQL lets the database quickly find common or unique entries between two sets. It does the hard work instantly and accurately, so you get reliable results without the hassle.

Before vs After

✗ Before

Check each name in list1 against list2 one by one.

✓ After

SELECT name FROM list1 INTERSECT SELECT name FROM list2;
SELECT name FROM list1 EXCEPT SELECT name FROM list2;

What It Enables

It enables fast and error-free comparison of data sets to find common or unique records effortlessly.

Real Life Example

A company wants to find customers who bought both product A and product B (INTERSECT) or those who bought product A but not product B (EXCEPT) to target marketing campaigns.

Key Takeaways

Manual comparison is slow and error-prone.

INTERSECT finds common data between sets.

EXCEPT finds data in one set but not the other.

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