DISTINCT for unique values in MySQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select unique city names from the customers table.

MySQL

SELECT [1] city FROM customers;

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

AONLY

BDISTINCT

CSINGLE

DUNIQUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNIQUE instead of DISTINCT, which is not valid in SELECT clause.

Omitting DISTINCT and getting duplicate city names.

The DISTINCT keyword returns only unique values in the selected column.

2fill in blank

medium

Complete the code to find unique product categories from the products table.

MySQL

SELECT [1] category FROM products;

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

ADISTINCT

BUNIQUE

CSINGLE

DONLY

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNIQUE which is invalid in SELECT clause.

Forgetting DISTINCT and getting repeated categories.

DISTINCT is the correct keyword to get unique values in SQL.

3fill in blank

hard

Fix the error in the query to select unique customer names.

MySQL

SELECT [1] name FROM customers;

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

ADISTINCT

BUNIQUE

CSINGLE

DONLY

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNIQUE instead of DISTINCT causes syntax error.

Leaving out DISTINCT and getting duplicates.

The keyword DISTINCT must be used to get unique names. UNIQUE is not valid here.

4fill in blank

hard

Fill both blanks to select unique countries and order them alphabetically.

MySQL

SELECT [1] country FROM customers ORDER BY country [2];

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

ADISTINCT

BDESC

CASC

DUNIQUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNIQUE instead of DISTINCT.

Using DESC instead of ASC for alphabetical order.

Use DISTINCT to get unique countries and ASC to order them from A to Z.

5fill in blank

hard

Fill all three blanks to select unique departments, count employees, and show only those with more than 5 employees.

MySQL

SELECT [1] department, COUNT(*) AS emp_count FROM employees GROUP BY [2] HAVING emp_count [3] 5;

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

ADISTINCT

Bdepartment

C>

DUNIQUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNIQUE instead of DISTINCT.

Grouping by wrong column.

Using wrong comparison operator.

Use DISTINCT to select unique departments, group by department, and filter groups with employee count greater than 5 using >.

Practice

1. What does the DISTINCT keyword do in a MySQL query?

easy

A. It sorts the results in ascending order.

B. It updates values in a column.

C. It counts the number of rows in a table.

D. It returns only unique values by removing duplicates.

Solution

1. Step 1: Understand the purpose of DISTINCT

   The DISTINCT keyword is used to filter out duplicate rows in the result set.

2. Step 2: Compare with other options

   Sorting is done by ORDER BY, counting by COUNT(), and updating by UPDATE statement, so these are incorrect.

3. Final Answer:

   It returns only unique values by removing duplicates. -> Option D

4. Quick Check:

   DISTINCT = unique values [OK]

Hint: DISTINCT removes duplicates, giving unique results only [OK]

Common Mistakes:

• Confusing DISTINCT with ORDER BY
• Thinking DISTINCT counts rows
• Using DISTINCT to update data

2. Which of the following is the correct syntax to select unique city names from a table called customers?

easy

A. SELECT DISTINCT city FROM customers;

B. SELECT city UNIQUE FROM customers;

C. SELECT UNIQUE city FROM customers;

D. SELECT city FROM customers DISTINCT;

Solution

1. Step 1: Recall correct syntax for unique selection

   MySQL uses DISTINCT keyword immediately after SELECT to get unique values.

2. Step 2: Check each option

   SELECT DISTINCT city FROM customers; uses correct syntax: SELECT DISTINCT city FROM customers; others are invalid syntax.

3. Final Answer:

   SELECT DISTINCT city FROM customers; -> Option A

4. Quick Check:

   Syntax for unique values = SELECT DISTINCT [OK]

Hint: Use SELECT DISTINCT column FROM table for unique values [OK]

Common Mistakes:

• Using UNIQUE instead of DISTINCT in SELECT
• Placing DISTINCT after FROM clause
• Misplacing keywords causing syntax errors

3. Given the table orders with a column product_id containing values: [1, 2, 2, 3, 1, 4], what will the query SELECT DISTINCT product_id FROM orders; return?

medium

A. [1, 2, 3, 4]

B. [1, 2, 2, 3, 1, 4]

C. [2, 3, 4]

D. [1, 3, 4]

Solution

1. Step 1: Identify duplicates in product_id

   The values are 1, 2, 2, 3, 1, 4. Duplicates are 1 and 2 repeated.

2. Step 2: Apply DISTINCT to remove duplicates

   Removing duplicates leaves unique values: 1, 2, 3, 4.

3. Final Answer:

   [1, 2, 3, 4] -> Option A

4. Quick Check:

   DISTINCT removes duplicates = unique list [OK]

Hint: DISTINCT returns only one of each repeated value [OK]

Common Mistakes:

• Expecting all original values including duplicates
• Confusing DISTINCT with sorting
• Ignoring repeated values in output

4. What is wrong with this query if you want unique customer names from the customers table?
SELECT DISTINCT name, age FROM customers;
Assuming some customers share the same name but have different ages.

medium

A. It will cause a syntax error.

B. It will return unique names only, ignoring age.

C. It will return unique combinations of name and age, not just names.

D. It will return duplicate names.

Solution

1. Step 1: Understand DISTINCT with multiple columns

   DISTINCT applies to the entire row of selected columns, so it returns unique pairs of name and age.

2. Step 2: Analyze the effect on names

   If customers share the same name but different ages, each pair is unique, so duplicates in names alone are not removed.

3. Final Answer:

   It will return unique combinations of name and age, not just names. -> Option C

4. Quick Check:

   DISTINCT on multiple columns = unique rows [OK]

Hint: DISTINCT on multiple columns checks all columns together [OK]

Common Mistakes:

• Thinking DISTINCT applies to each column separately
• Expecting only unique names when selecting multiple columns
• Assuming syntax error for multiple columns with DISTINCT

5. You have a table sales with columns region and salesperson. Some salespersons work in multiple regions. Which query will list all unique salespersons regardless of region?

hard

A. SELECT DISTINCT region, salesperson FROM sales;

B. SELECT DISTINCT salesperson FROM sales;

C. SELECT salesperson FROM sales GROUP BY region;

D. SELECT salesperson FROM sales WHERE DISTINCT region;

Solution

1. Step 1: Identify the goal

   We want unique salespersons only, ignoring regions.

2. Step 2: Analyze each query

   SELECT DISTINCT region, salesperson FROM sales; returns unique pairs of region and salesperson, so duplicates by salesperson remain if regions differ. SELECT DISTINCT salesperson FROM sales; returns unique salespersons only. SELECT salesperson FROM sales GROUP BY region; groups by region, not salesperson, so duplicates remain. SELECT salesperson FROM sales WHERE DISTINCT region; is invalid syntax.

3. Final Answer:

   SELECT DISTINCT salesperson FROM sales; -> Option B

4. Quick Check:

   Distinct on salesperson column only = unique salespersons [OK]

Hint: Use DISTINCT on the single column you want unique values for [OK]

Common Mistakes:

• Selecting multiple columns with DISTINCT when only one is needed
• Using GROUP BY on wrong column
• Writing invalid WHERE DISTINCT syntax