INTERSECT and EXCEPT in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the SQL INTERSECT operator do?

The INTERSECT operator returns only the rows that appear in both query results. It shows common rows between two SELECT statements.

beginner

What is the purpose of the EXCEPT operator in SQL?

The EXCEPT operator returns rows from the first query that are not present in the second query. It shows what is unique to the first SELECT statement.

intermediate

Can INTERSECT and EXCEPT be used with different numbers of columns in the SELECT statements?

No. Both SELECT statements must have the same number of columns with compatible data types for INTERSECT and EXCEPT to work.

intermediate

How does INTERSECT differ from JOIN in SQL?

INTERSECT returns rows common to both queries as whole rows, while JOIN combines rows based on matching column values, often producing more columns.

beginner

If you want to find customers who bought product A but not product B, which operator would you use?

You would use the EXCEPT operator to find customers in the product A list who are not in the product B list.

What does the INTERSECT operator return?

A Rows unique to the first query

B All rows from the first query

C Rows common to both SELECT queries

D All rows from the second query

INTERSECT returns only the rows that appear in both SELECT query results.

Which operator returns rows from the first query that are not in the second?

A EXCEPT

B INTERSECT

C UNION

D JOIN

EXCEPT returns rows from the first query that do not appear in the second query.

Can INTERSECT be used if the two SELECT statements have different numbers of columns?

A No, they must have the same number of columns

B Yes, always

C Only if columns have the same names

D Only if columns are numeric

Both SELECT statements must have the same number of columns with compatible data types for INTERSECT.

Which SQL operator would you use to find rows unique to the first query?

A INTERSECT

B EXCEPT

C UNION

D CROSS JOIN

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

What is a key difference between INTERSECT and JOIN?

A INTERSECT combines columns, JOIN returns common rows

B Both do the same thing

C JOIN returns unique rows, INTERSECT returns all rows

D INTERSECT returns common rows, JOIN combines rows based on matching columns

INTERSECT returns rows common to both queries as whole rows, JOIN combines rows based on matching column values.

Explain how the INTERSECT operator works in SQL and give a simple example.

Think about finding common friends in two friend lists.

Describe the EXCEPT operator and when you would use it.

Imagine subtracting one list from another.

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