Why set operations matter in PostgreSQL - Challenge Your Understanding

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this UNION query?

Consider two tables: students_a and students_b, each with a single column name.
students_a has names: Alice, Bob, Charlie.
students_b has names: Bob, Diana, Edward.
What is the result of this query?

SELECT name FROM students_a
UNION
SELECT name FROM students_b;

PostgreSQL

CREATE TABLE students_a(name TEXT);
INSERT INTO students_a VALUES ('Alice'), ('Bob'), ('Charlie');
CREATE TABLE students_b(name TEXT);
INSERT INTO students_b VALUES ('Bob'), ('Diana'), ('Edward');

SELECT name FROM students_a
UNION
SELECT name FROM students_b;

A Alice, Charlie, Diana, Edward

B Alice, Bob, Charlie, Diana, Edward

C Bob, Diana, Edward

D Alice, Bob, Charlie, Bob, Diana, Edward

💡 Hint

UNION removes duplicates and combines all unique rows from both queries.

The UNION operation combines rows from both tables but removes duplicates. Bob appears in both tables but only once in the result.

❓ query_result

intermediate

What does EXCEPT return in this query?

Given the same tables students_a and students_b as before,
what is the output of this query?

SELECT name FROM students_a
EXCEPT
SELECT name FROM students_b;

PostgreSQL

CREATE TABLE students_a(name TEXT);
INSERT INTO students_a VALUES ('Alice'), ('Bob'), ('Charlie');
CREATE TABLE students_b(name TEXT);
INSERT INTO students_b VALUES ('Bob'), ('Diana'), ('Edward');

SELECT name FROM students_a
EXCEPT
SELECT name FROM students_b;

A Alice, Bob, Charlie, Diana, Edward

B Bob

C Diana, Edward

D Alice, Charlie

💡 Hint

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

Bob is in both tables, so it is excluded. Alice and Charlie are only in students_a, so they appear.

📝 Syntax

advanced

Which query correctly uses INTERSECT to find common names?

You want to find names that appear in both students_a and students_b. Which query is correct?

A SELECT name FROM students_a INTERSECT SELECT name FROM students_b;

B SELECT name FROM students_a UNION INTERSECT SELECT name FROM students_b;

C SELECT name FROM students_a INTERSECT ALL SELECT name FROM students_b;

D SELECT name FROM students_a EXCEPT INTERSECT SELECT name FROM students_b;

💡 Hint

INTERSECT combines two queries and returns only rows present in both.

Option A is the correct syntax for INTERSECT. Options B and D have invalid syntax. Option A uses INTERSECT ALL, which is supported in PostgreSQL but preserves duplicate rows if they exist in both result sets.

❓ optimization

advanced

Which approach is more efficient to find unique names from two large tables?

You have two large tables with millions of rows each. You want to get all unique names from both tables.
Which query is generally more efficient in PostgreSQL?

A Use INTERSECT to find unique names.

B Use UNION ALL followed by SELECT DISTINCT on the combined result.

C Use UNION which removes duplicates automatically.

D Use EXCEPT to remove duplicates.

💡 Hint

UNION removes duplicates during the operation, avoiding extra steps.

UNION is optimized to remove duplicates during the merge. UNION ALL with DISTINCT requires an extra step and can be slower. INTERSECT and EXCEPT do not return all unique names.

🧠 Conceptual

expert

Why do set operations require queries to have the same number of columns and compatible types?

Set operations like UNION, INTERSECT, and EXCEPT combine results from multiple queries.
Why must these queries have the same number of columns and compatible data types?

A Because set operations combine rows vertically, each column must match in position and type to merge correctly.

B Because set operations join tables horizontally, columns can differ in number and type.

C Because PostgreSQL automatically converts all data types to text during set operations.

D Because set operations only work on numeric columns.

💡 Hint

Think about how rows from different queries stack on top of each other.

Set operations stack rows from queries. Each column position must match in number and type to combine rows properly. Different numbers or incompatible types cause errors.

Practice

1. What is the main purpose of set operations like UNION and INTERSECT in PostgreSQL?

easy

A. To delete rows from multiple tables

B. To create new tables from existing ones

C. To update data in a table

D. To combine or compare results from multiple SELECT queries

Solution

1. Step 1: Understand set operations

   Set operations like UNION and INTERSECT combine or compare results from multiple SELECT queries.

2. Step 2: Identify their purpose

   They help find unique, common, or different data across queries without modifying tables.

3. Final Answer:

   To combine or compare results from multiple SELECT queries -> Option D

4. Quick Check:

   Set operations = combine/compare SELECT results [OK]

Hint: Set operations combine query results, not modify tables [OK]

Common Mistakes:

• Thinking set operations modify tables
• Confusing set operations with DML commands
• Assuming set operations create new tables

2. Which of the following is the correct syntax to combine two SELECT queries using UNION in PostgreSQL?

easy

A. SELECT col1 FROM table1 JOIN SELECT col1 FROM table2;

B. SELECT col1 FROM table1 UNION SELECT col1 FROM table2;

C. SELECT col1 FROM table1 + SELECT col1 FROM table2;

D. SELECT col1 FROM table1 INTERSECT SELECT col1 FROM table2;

Solution

1. Step 1: Recall UNION syntax

   The correct syntax uses UNION keyword between two SELECT statements with matching columns.

2. Step 2: Check options

   Only SELECT col1 FROM table1 UNION SELECT col1 FROM table2; is correct. JOIN is for table joins; + is invalid syntax; INTERSECT is a different operation.

3. Final Answer:

   SELECT col1 FROM table1 UNION SELECT col1 FROM table2; -> Option B

4. Quick Check:

   UNION syntax = SELECT ... UNION SELECT ... [OK]

Hint: UNION joins SELECTs with UNION keyword, no + or JOIN [OK]

Common Mistakes:

• Using + instead of UNION
• Confusing JOIN with UNION
• Mixing INTERSECT with UNION syntax

3. Given these tables:
table1: (id) values (1), (2), (3);
table2: (id) values (2), (3), (4);
What is the result of this query?

SELECT id FROM table1 INTERSECT SELECT id FROM table2;

medium

A. [2, 3]

B. [1, 2, 3, 4]

C. [1]

D. [4]

Solution

1. Step 1: Understand INTERSECT operation

   INTERSECT returns only values present in both queries.

2. Step 2: Compare table1 and table2 ids

   table1 has 1,2,3; table2 has 2,3,4; common values are 2 and 3.

3. Final Answer:

   [2, 3] -> Option A

4. Quick Check:

   INTERSECT = common values [OK]

Hint: INTERSECT returns only common rows between queries [OK]

Common Mistakes:

• Including all values from both tables
• Confusing INTERSECT with UNION
• Returning values only from one table

4. Consider this query:

SELECT id FROM table1 UNION ALL SELECT id FROM table2;

It returns duplicate rows. How can you fix it to return only unique rows?

medium

A. Replace UNION ALL with UNION

B. Add DISTINCT inside each SELECT

C. Use INTERSECT instead of UNION ALL

D. Add GROUP BY id at the end

Solution

1. Step 1: Understand UNION ALL vs UNION

   UNION ALL returns all rows including duplicates; UNION removes duplicates.

2. Step 2: Fix query to remove duplicates

   Replacing UNION ALL with UNION removes duplicates automatically.

3. Final Answer:

   Replace UNION ALL with UNION -> Option A

4. Quick Check:

   UNION removes duplicates, UNION ALL does not [OK]

Hint: Use UNION (not UNION ALL) to remove duplicates [OK]

Common Mistakes:

• Using UNION ALL when duplicates are unwanted
• Adding DISTINCT inside each SELECT unnecessarily
• Using INTERSECT which returns only common rows

5. You want to find all unique customer IDs who made purchases in either 2022 or 2023 from two tables:
purchases_2022(customer_id) and purchases_2023(customer_id).
Which query correctly returns this list without duplicates?

hard

A. SELECT customer_id FROM purchases_2022 UNION ALL SELECT customer_id FROM purchases_2023;

B. SELECT customer_id FROM purchases_2022 INTERSECT SELECT customer_id FROM purchases_2023;

C. SELECT customer_id FROM purchases_2022 UNION SELECT customer_id FROM purchases_2023;

D. SELECT DISTINCT customer_id FROM purchases_2022, purchases_2023;

Solution

1. Step 1: Understand the goal

   We want all unique customer IDs from either year, combined without duplicates.

2. Step 2: Evaluate options

   SELECT customer_id FROM purchases_2022 INTERSECT SELECT customer_id FROM purchases_2023; returns only customers in both years (common). SELECT customer_id FROM purchases_2022 UNION SELECT customer_id FROM purchases_2023; returns all unique customers from both years. SELECT customer_id FROM purchases_2022 UNION ALL SELECT customer_id FROM purchases_2023; returns all including duplicates. SELECT DISTINCT customer_id FROM purchases_2022, purchases_2023; creates a cross join, not union.

3. Final Answer:

   SELECT customer_id FROM purchases_2022 UNION SELECT customer_id FROM purchases_2023; -> Option C

4. Quick Check:

   UNION = unique combined rows [OK]

Hint: Use UNION to combine unique rows from multiple tables [OK]

Common Mistakes:

• Using INTERSECT to get all customers
• Using UNION ALL causing duplicates
• Using cross join instead of union