Why set operations matter in PostgreSQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to combine two tables and remove duplicates.

PostgreSQL

SELECT column_name FROM table1 [1] SELECT column_name FROM table2;

Drag options to blanks, or click blank then click option'

AJOIN

BUNION

CINTERSECT

DEXCEPT

Attempts:

3 left

💡 Hint

Common Mistakes

Using JOIN instead of UNION causes syntax errors.

Using INTERSECT returns only common rows, not all combined.

The UNION operation combines results from two queries and removes duplicates.

2fill in blank

medium

Complete the code to find common rows between two tables.

PostgreSQL

SELECT column_name FROM table1 [1] SELECT column_name FROM table2;

Drag options to blanks, or click blank then click option'

AJOIN

BUNION

CEXCEPT

DINTERSECT

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNION returns all unique rows, not just common ones.

Using EXCEPT returns rows in first query but not second.

The INTERSECT operation returns only rows that appear in both queries.

3fill in blank

hard

Fix the error in the code to find rows in table1 but not in table2.

PostgreSQL

SELECT column_name FROM table1 [1] SELECT column_name FROM table2;

Drag options to blanks, or click blank then click option'

AEXCEPT

BUNION

CINTERSECT

DJOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using INTERSECT returns common rows, not unique ones.

Using UNION returns all unique rows combined.

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

4fill in blank

hard

Complete the code to combine two tables including duplicates.

PostgreSQL

SELECT column_name FROM table1 [1] SELECT column_name FROM table2 ;;

Drag options to blanks, or click blank then click option'

AUNION ALL

BUNION

CINTERSECT

D;

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNION removes duplicates.

Missing semicolon causes syntax errors.

The UNION ALL operation combines results from two queries including duplicates.

5fill in blank

hard

Fill all three blanks to find rows in table1 not in table2 and order results.

PostgreSQL

SELECT column_name FROM table1 [1] SELECT column_name FROM table2 [2] ORDER BY [3];

Drag options to blanks, or click blank then click option'

AEXCEPT

B;

Ccolumn_name

DUNION

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNION returns all unique rows, not just differences.

Forgetting semicolon causes errors.

Ordering by a non-existent column causes errors.

The EXCEPT operation returns rows from the first query that are not in the second. ORDER BY column_name sorts the results.

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