Why normalization matters in SQL - Test Your Understanding

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'

Aemployees

BALL

C*

Dcolumns

Attempts:

3 left

💡 Hint

Common Mistakes

Using the table name instead of '*' to select all columns.

Using 'ALL' which is not valid in this context.

The asterisk (*) selects all columns from the table.

2fill in blank

medium

Complete the code to find duplicate employee names in the 'employees' table.

SQL

SELECT name, COUNT(*) FROM employees GROUP BY name HAVING COUNT(*) [1] 1;

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

A=

B>

C<=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' which only finds names appearing exactly once.

Using '<=' which includes names appearing once or less.

We use '>' to find names that appear more than once, indicating duplicates.

3fill in blank

hard

Fix the error in the query to select unique department names from 'employees'.

SQL

SELECT DISTINCT [1] FROM employees;

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

Adepartment-name

Bdepartment name

CdepartmentName

Ddepartment_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using column names with spaces or dashes without quotes.

Using camelCase when the column uses underscores.

Column names cannot have spaces or dashes unless quoted. 'department_id' is a valid column name.

4fill in blank

hard

Fill both blanks to create a query that lists employee names and their department names by joining 'employees' and 'departments'.

SQL

SELECT employees.name, departments.[1] FROM employees JOIN departments ON employees.[2] = departments.id;

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

Aname

Bdepartment_id

Cid

Ddept_name

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong column names for join or select.

Mixing up foreign key and primary key columns.

The department name column is 'dept_name'. The join uses 'department_id' from employees to match 'id' in departments.

5fill in blank

hard

Fill all three blanks to write a query that counts employees per department, showing department name and count, only for departments with more than 5 employees.

SQL

SELECT departments.[1], COUNT(employees.[2]) FROM employees JOIN departments ON employees.[3] = departments.id GROUP BY departments.[1] HAVING COUNT(employees.[2]) > 5;

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

Adept_name

Bid

Cdepartment_id

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong columns for counting or joining.

Not grouping by department name.

We select 'dept_name' from departments, count employee 'id's, and join on 'department_id'.