What is SQL - Interactive Quiz & Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all columns from the table named 'employees'.

SQL

SELECT [1] FROM employees;

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

A*

BALL

Ceverything

Dcolumns

Attempts:

3 left

💡 Hint

Common Mistakes

Using words like ALL or columns instead of *

Leaving the blank empty

The asterisk (*) means select all columns from the table.

2fill in blank

medium

Complete the code to filter rows where the age is greater than 30.

SQL

SELECT * FROM users WHERE age [1] 30;

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

A=

B>

C<

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' which selects only age exactly 30

Using '<' which selects ages less than 30

The '>' operator means 'greater than', so it filters ages above 30.

3fill in blank

hard

Fix the error in the code to count the number of rows in the 'orders' table.

SQL

SELECT COUNT([1]) FROM orders;

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

Aall

Border_id

Crows

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using a column name that might have NULLs and miss some rows

Using words like 'all' or 'rows' which are not valid here

Using COUNT(*) counts all rows regardless of column values.

4fill in blank

hard

Fill both blanks to select the name and age from 'students' where age is less than 20.

SQL

SELECT [1], age FROM students WHERE age [2] 20;

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

Aname

B>

C<

Dstudent_name

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '<' for filtering age

Using wrong column names like 'student_name'

We select the 'name' column and filter ages less than 20 using '<'.

5fill in blank

hard

Fill all three blanks to select the city and count of customers from 'customers' grouped by city having more than 5 customers.

SQL

SELECT [1], COUNT([2]) FROM customers GROUP BY [3] HAVING COUNT([2]) > 5;

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

Acity

B*

Dcustomer_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using different columns for grouping and selecting

Counting a specific column that might have NULLs

We select 'city', count the 'customer_id' column, group by 'city' to get counts per city, and filter groups with more than 5 customers.

Practice

1. What is the main purpose of SQL?

easy

A. To design website layouts

B. To manage and manipulate data in databases

C. To write computer games

D. To create graphic designs

Solution

1. Step 1: Understand SQL's role

   SQL is a language used to work with data stored in databases.

2. Step 2: Compare options

   Only To manage and manipulate data in databases describes managing and manipulating data, which is what SQL does.

3. Final Answer:

   To manage and manipulate data in databases -> Option B

4. Quick Check:

   SQL = Data management [OK]

Hint: SQL is for data, not design or games [OK]

Common Mistakes:

• Confusing SQL with programming languages
• Thinking SQL is for web design
• Mixing SQL with graphic tools

2. Which of the following is a correct SQL command to get data from a table named users?

easy

A. GET * FROM users;

B. FETCH ALL users;

C. SELECT * FROM users;

D. SHOW DATA users;

Solution

1. Step 1: Recall SQL syntax for retrieving data

   The correct command to get all data from a table is SELECT * FROM table_name;.

2. Step 2: Match options with correct syntax

   Only SELECT * FROM users; matches the correct SQL syntax for selecting data.

3. Final Answer:

   SELECT * FROM users; -> Option C

4. Quick Check:

   SELECT = retrieve data [OK]

Hint: Use SELECT to get data from tables [OK]

Common Mistakes:

• Using GET instead of SELECT
• Using FETCH or SHOW incorrectly
• Missing FROM keyword

3. What will this SQL query return?

SELECT name FROM employees WHERE age > 30;

medium

A. All employee names with age less than 30

B. An error because of wrong syntax

C. All employee names regardless of age

D. All employee names with age greater than 30

Solution

1. Step 1: Understand the SELECT and WHERE clause

   The query selects the name column from employees where the age is greater than 30.

2. Step 2: Interpret the condition

   The condition age > 30 means only employees older than 30 are included.

3. Final Answer:

   All employee names with age greater than 30 -> Option D

4. Quick Check:

   WHERE age > 30 filters older employees [OK]

Hint: WHERE filters rows by condition [OK]

Common Mistakes:

• Confusing > with <
• Ignoring WHERE clause
• Assuming all rows are returned

4. Identify the error in this SQL statement:

SELEC name FROM customers;

medium

A. Incorrect keyword 'SELEC' instead of 'SELECT'

B. Missing semicolon at the end

C. Table name 'customers' is invalid

D. No columns specified to select

Solution

1. Step 1: Check SQL keywords

   The keyword to get data is SELECT, but here it is misspelled as SELEC.

2. Step 2: Verify other parts

   The table name and column are valid, and the semicolon is optional in many systems.

3. Final Answer:

   Incorrect keyword 'SELEC' instead of 'SELECT' -> Option A

4. Quick Check:

   Correct spelling of SELECT is required [OK]

Hint: Check spelling of SQL keywords carefully [OK]

Common Mistakes:

• Misspelling keywords
• Assuming semicolon is mandatory
• Confusing table and column names

5. You want to add a new user named 'Alice' with age 28 to a table users with columns name and age. Which SQL command should you use?

hard

A. INSERT INTO users (name, age) VALUES ('Alice', 28);

B. UPDATE users SET name = 'Alice', age = 28;

C. SELECT name, age FROM users WHERE name = 'Alice';

D. DELETE FROM users WHERE name = 'Alice';

Solution

1. Step 1: Understand the goal

   You want to add a new row (user) to the table, so you need to insert data.

2. Step 2: Identify the correct command

   INSERT INTO adds new rows; UPDATE changes existing rows; SELECT reads data; DELETE removes data.

3. Final Answer:

   INSERT INTO users (name, age) VALUES ('Alice', 28); -> Option A

4. Quick Check:

   INSERT adds new data [OK]

Hint: Use INSERT to add new rows [OK]

Common Mistakes:

• Using UPDATE to add data
• Confusing SELECT with INSERT
• Deleting instead of adding