Why set operations are needed in SQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to combine two tables and get all unique rows.

SQL

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

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

AUNION

BJOIN

CINTERSECT

DWHERE

Attempts:

3 left

💡 Hint

Common Mistakes

Using JOIN instead of UNION causes a different result.

Using WHERE is incorrect for combining tables.

The UNION operation combines rows from two tables and removes duplicates, giving all unique rows.

2fill in blank

medium

Complete the code to find rows common to both tables.

SQL

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

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

AINTERSECT

BJOIN

CEXCEPT

DUNION

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNION returns all rows, not just common ones.

Using EXCEPT returns rows only in the first table.

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

3fill in blank

hard

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

SQL

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

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

AUNION

BINTERSECT

CJOIN

DEXCEPT

Attempts:

3 left

💡 Hint

Common Mistakes

Using INTERSECT returns common rows, not unique ones.

Using UNION combines all rows, including duplicates.

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

4fill in blank

hard

Fill both blanks to combine two tables and keep all rows including duplicates.

SQL

SELECT * FROM table1 [1] SELECT * FROM table2 [2] id;

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

AUNION ALL

BUNION

CORDER BY

DWHERE

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNION removes duplicates.

Using WHERE is not valid here.

UNION ALL combines all rows from both tables including duplicates. ORDER BY sorts the final result.

5fill in blank

hard

Fill all three blanks to find rows in table1 not in table2 and sort by id.

SQL

SELECT * FROM table1 [1] SELECT * FROM table2 [2] id [3];

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

AEXCEPT

BORDER BY

CASC

DDESC

Attempts:

3 left

💡 Hint

Common Mistakes

Using DESC sorts in descending order, not ascending.

Using UNION returns all rows, not just unique ones.

EXCEPT finds rows in table1 not in table2. ORDER BY sorts the result by id in ascending order (ASC).

Practice

1. Why do we use set operations like UNION and INTERSECT in SQL?

easy

A. To create new tables permanently

B. To delete rows based on conditions

C. To update values in a single table

D. To combine or compare rows from two or more tables easily

Solution

1. Step 1: Understand the purpose of set operations

   Set operations like UNION and INTERSECT are designed to combine or compare rows from multiple tables.

2. Step 2: Identify what set operations do not do

   They do not create tables, update, or delete rows; those are different SQL commands.

3. Final Answer:

   To combine or compare rows from two or more tables easily -> Option D

4. Quick Check:

   Set operations combine or compare data [OK]

Hint: Set operations combine or compare rows from tables [OK]

Common Mistakes:

• Confusing set operations with data modification commands
• Thinking UNION creates a new permanent table
• Assuming INTERSECT deletes rows

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

easy

A. UNION SELECT * FROM table1, SELECT * FROM table2;

B. SELECT * FROM table1 JOIN UNION SELECT * FROM table2;

C. SELECT * FROM table1 UNION SELECT * FROM table2;

D. SELECT * FROM table1 AND SELECT * FROM table2 UNION;

Solution

1. Step 1: Recall correct UNION syntax

   The correct syntax is to write one SELECT query, then UNION, then another SELECT query.

2. Step 2: Check each option for syntax errors

   SELECT * FROM table1 UNION SELECT * FROM table2; follows the correct syntax. Options A, B, and D have incorrect keywords or order.

3. Final Answer:

   SELECT * FROM table1 UNION SELECT * FROM table2; -> Option C

4. Quick Check:

   Correct UNION syntax = SELECT * FROM table1 UNION SELECT * FROM table2; [OK]

Hint: UNION joins two SELECT queries directly [OK]

Common Mistakes:

• Adding JOIN keyword with UNION
• Using commas instead of UNION
• Placing UNION at the end incorrectly

3. Given two tables:
Table A: {1, 2, 3}
Table B: {2, 3, 4}
What is the result of SELECT * FROM A INTERSECT SELECT * FROM B;?

medium

A. {2, 3}

B. {1, 2, 3}

C. {1, 4}

D. {1, 2, 3, 4}

Solution

1. Step 1: Understand INTERSECT operation

   INTERSECT returns only rows present in both tables.

2. Step 2: Find common elements in Table A and Table B

   Common elements are 2 and 3.

3. Final Answer:

   {2, 3} -> Option A

4. Quick Check:

   INTERSECT = common rows [OK]

Hint: INTERSECT returns only common rows [OK]

Common Mistakes:

• Confusing INTERSECT with UNION
• Including all rows from both tables
• Mixing up EXCEPT with INTERSECT

4. You wrote this SQL query:
SELECT * FROM table1 UNION table2;
What is the error and how to fix it?

medium

A. Missing SELECT before table2; fix by adding SELECT * FROM table2

B. UNION cannot be used with tables; use JOIN instead

C. UNION requires parentheses around queries

D. No error; query runs fine

Solution

1. Step 1: Identify syntax error in UNION usage

   UNION requires two complete SELECT statements, but second query lacks SELECT.

2. Step 2: Correct the query syntax

   Add SELECT * FROM before table2 to fix the error.

3. Final Answer:

   Missing SELECT before table2; fix by adding SELECT * FROM table2 -> Option A

4. Quick Check:

   UNION needs two SELECTs [OK]

Hint: UNION needs two full SELECT queries [OK]

Common Mistakes:

• Omitting SELECT in second query
• Using UNION with tables directly
• Adding unnecessary parentheses

5. You have two customer lists:
List A: customers who bought product X
List B: customers who bought product Y
How do you find customers who bought either product X or Y but not both using set operations?

hard

A. SELECT * FROM A UNION SELECT * FROM B

B. (SELECT * FROM A EXCEPT SELECT * FROM B) UNION (SELECT * FROM B EXCEPT SELECT * FROM A)

C. SELECT * FROM A INTERSECT SELECT * FROM B

D. (SELECT * FROM A EXCEPT SELECT * FROM B) UNION (SELECT * FROM A INTERSECT SELECT * FROM B)

Solution

1. Step 1: Understand the problem

   We want customers who bought product X or Y but not both (exclusive customers).

2. Step 2: Use EXCEPT and UNION to find exclusive customers

   Find customers in A but not in B, and customers in B but not in A, then combine them with UNION.

3. Step 3: Check other options

   SELECT * FROM A UNION SELECT * FROM B gives all customers who bought either product (including both). SELECT * FROM A INTERSECT SELECT * FROM B gives only those who bought both. (SELECT * FROM A EXCEPT SELECT * FROM B) UNION (SELECT * FROM A INTERSECT SELECT * FROM B) gives all customers who bought product X.

4. Final Answer:

   (SELECT * FROM A EXCEPT SELECT * FROM B) UNION (SELECT * FROM B EXCEPT SELECT * FROM A) -> Option B

5. Quick Check:

   Exclusive customers = (A EXCEPT B) UNION (B EXCEPT A) [OK]

Hint: Use EXCEPT both ways, then UNION results [OK]

Common Mistakes:

• Using UNION alone includes both customers
• Using INTERSECT returns only common customers
• Confusing EXCEPT with INTERSECT