INTERSECT and EXCEPT in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this INTERSECT query?

Given two tables Employees_A and Employees_B with the following data:

Employees_A
id | name
1 | Alice
2 | Bob
3 | Carol

Employees_B
id | name
2 | Bob
3 | Carol
4 | Dave

What will be the result of this query?

SELECT id, name FROM Employees_A
INTERSECT
SELECT id, name FROM Employees_B;

A

id | name
2  | Bob
3  | Carol

B

id | name
4  | Dave

C

id | name
1  | Alice

D

id | name
1  | Alice
2  | Bob
3  | Carol
4  | Dave

💡 Hint

INTERSECT returns only rows that appear in both queries.

The INTERSECT operator returns only the rows that exist in both Employees_A and Employees_B. Here, Bob and Carol are common in both tables.

❓ query_result

intermediate

What rows does this EXCEPT query return?

Given two tables Products_2023 and Products_2024 with data:

Products_2023
id | product
1 | Chair
2 | Table
3 | Lamp

Products_2024
id | product
2 | Table
3 | Lamp
4 | Sofa

What is the output of this query?

SELECT id, product FROM Products_2023
EXCEPT
SELECT id, product FROM Products_2024;

A

id | product
2  | Table
3  | Lamp

B

id | product
4  | Sofa

C

id | product
1  | Chair

D

id | product
1  | Chair
4  | Sofa

💡 Hint

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

The EXCEPT operator returns rows from Products_2023 that do not appear in Products_2024. Only 'Chair' is unique to Products_2023.

🧠 Conceptual

advanced

Which statement about INTERSECT and EXCEPT is true?

Consider the following statements about INTERSECT and EXCEPT in SQL:

A) INTERSECT returns all rows from the first query plus rows from the second.
B) EXCEPT returns rows that appear in both queries.
C) INTERSECT returns only rows common to both queries.
D) EXCEPT returns all rows from the second query that are not in the first.

Which statement is correct?

A EXCEPT returns rows that appear in both queries.

B INTERSECT returns only rows common to both queries.

C INTERSECT returns all rows from the first query plus rows from the second.

D EXCEPT returns all rows from the second query that are not in the first.

💡 Hint

Think about what INTERSECT and EXCEPT do with sets of rows.

INTERSECT returns only the rows that exist in both queries. EXCEPT returns rows from the first query that do not appear in the second.

📝 Syntax

advanced

Which query will cause a syntax error?

Which of the following SQL queries using INTERSECT or EXCEPT will cause a syntax error in PostgreSQL?

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

B SELECT * FROM table1 EXCEPT SELECT * FROM table2;

C SELECT id FROM table1 INTERSECT SELECT id FROM table2;

D SELECT id FROM table1 INTERSECT SELECT id FROM table2 WHERE id > 10;

💡 Hint

Check if the number and order of columns match in both queries.

Option A selects two columns in the first query but only one column in the second, causing a syntax error because INTERSECT and EXCEPT require the same number of columns with compatible types.

❓ optimization

expert

How to optimize this EXCEPT query for large tables?

You have two large tables, Orders_2023 and Orders_2024, both with millions of rows and the same schema.

You want to find orders in 2023 that are not in 2024 using:

SELECT * FROM Orders_2023 EXCEPT SELECT * FROM Orders_2024;

Which optimization will most improve performance?

A Add indexes on all columns used in the SELECT * for both tables.

B Rewrite the query using LEFT JOIN and IS NULL to find missing rows.

C Use EXCEPT ALL instead of EXCEPT to avoid duplicate elimination.

D Increase the work_mem setting in PostgreSQL to allow more memory for sorting.

💡 Hint

Think about how EXCEPT works internally with sorting and memory.

EXCEPT requires sorting or hashing to remove duplicates. Increasing work_mem allows PostgreSQL to perform these operations in memory, reducing disk I/O and improving performance.