ORDER BY single column in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this query ordering by age ascending?

Given the table People with columns id, name, and age, what will be the order of names after running this query?

SELECT name FROM People ORDER BY age ASC;

SQL

CREATE TABLE People (id INT, name VARCHAR(20), age INT);
INSERT INTO People VALUES (1, 'Alice', 30), (2, 'Bob', 25), (3, 'Charlie', 35);

A Bob, Alice, Charlie

B Alice, Bob, Charlie

C Charlie, Alice, Bob

D Bob, Charlie, Alice

💡 Hint

ORDER BY age ASC sorts from smallest to largest age.

The query orders the rows by the age column in ascending order, so the names appear from youngest to oldest: Bob (25), Alice (30), Charlie (35).

❓ query_result

intermediate

What is the output order when ordering by salary descending?

Consider the table Employees with columns emp_id, emp_name, and salary. What is the order of emp_name after running this query?

SELECT emp_name FROM Employees ORDER BY salary DESC;

SQL

CREATE TABLE Employees (emp_id INT, emp_name VARCHAR(20), salary INT);
INSERT INTO Employees VALUES (1, 'John', 5000), (2, 'Jane', 7000), (3, 'Doe', 6000);

A John, Jane, Doe

B John, Doe, Jane

C Doe, Jane, John

D Jane, Doe, John

💡 Hint

ORDER BY salary DESC sorts from highest to lowest salary.

The query sorts employees by salary in descending order, so the highest salary comes first: Jane (7000), Doe (6000), John (5000).

📝 Syntax

advanced

Which query will cause a syntax error?

Identify the query that will cause a syntax error when ordering by the column score.

A SELECT * FROM Results ORDER BY score;

B SELECT * FROM Results ORDER BY score DESC;

C SELECT * FROM Results ORDER BY score ASCENDING;

D SELECT * FROM Results ORDER BY score ASC;

💡 Hint

Check the valid keywords for sorting order in SQL.

The keyword 'ASCENDING' is not valid in SQL. The correct keyword is 'ASC'. Options A, B, and C are valid syntax.

❓ optimization

advanced

Which query is more efficient for ordering by a single indexed column?

Given a large table Sales with an index on the date column, which query will likely perform best when ordering by date?

A SELECT * FROM Sales ORDER BY date + 0;

B SELECT * FROM Sales ORDER BY date;

C SELECT * FROM Sales ORDER BY date DESC;

D SELECT * FROM Sales ORDER BY CAST(date AS VARCHAR);

💡 Hint

Using the indexed column directly helps the database use the index.

Ordering by the indexed column directly (option B) allows the database to use the index efficiently. Options A and D apply expressions on the column, preventing index use. Option B is also efficient but descending order may require scanning index in reverse; still efficient but option B is the simplest ascending order.

🧠 Conceptual

expert

What happens if you ORDER BY a column with NULL values?

Consider a table Products with a column price that contains some NULL values. What is the default behavior of ORDER BY price ASC regarding NULLs?

A NULL values appear last in the result set.

B NULL values appear first in the result set.

C NULL values are excluded from the result set.

D The query raises an error due to NULL values.

💡 Hint

Think about how SQL treats NULLs in sorting by default.

By default, when ordering ascending, NULL values appear last because NULL is treated as unknown and considered greater than any non-NULL value. They are not excluded or cause errors.

Practice

1. What does the SQL clause ORDER BY do when used with a single column?

easy

A. It deletes rows that have duplicate values in that column.

B. It filters the rows to only show those with unique values in that column.

C. It groups the rows by the values in that column.

D. It sorts the query results based on the values in that column.

Solution

1. Step 1: Understand the purpose of ORDER BY

   The ORDER BY clause is used to arrange the rows returned by a query in a specific order based on one or more columns.

2. Step 2: Focus on single column sorting

   When used with a single column, it sorts the results either ascending (default) or descending based on that column's values.

3. Final Answer:

   It sorts the query results based on the values in that column. -> Option D

4. Quick Check:

   ORDER BY = Sort results [OK]

Hint: ORDER BY arranges rows by column values [OK]

Common Mistakes:

• Confusing ORDER BY with filtering or grouping
• Thinking ORDER BY removes duplicates
• Assuming ORDER BY changes data in the table

2. Which of the following is the correct syntax to sort the table Employees by the column LastName in ascending order?

easy

A. SELECT * FROM Employees ORDER BY LastName DESC;

B. SELECT * FROM Employees SORT BY LastName;

C. SELECT * FROM Employees ORDER BY LastName ASC;

D. SELECT * FROM Employees ORDER LastName;

Solution

1. Step 1: Recall correct ORDER BY syntax

   The correct syntax to sort results is ORDER BY column_name. To specify ascending order, use ASC (which is also default).

2. Step 2: Check each option

   SELECT * FROM Employees ORDER BY LastName ASC; uses correct syntax with ORDER BY LastName ASC. SELECT * FROM Employees SORT BY LastName; uses invalid keyword SORT BY. SELECT * FROM Employees ORDER LastName; misses BY. SELECT * FROM Employees ORDER BY LastName DESC; sorts descending, not ascending.

3. Final Answer:

   SELECT * FROM Employees ORDER BY LastName ASC; -> Option C

4. Quick Check:

   ORDER BY + column + ASC = correct syntax [OK]

Hint: Use ORDER BY column ASC for ascending sort [OK]

Common Mistakes:

• Using SORT BY instead of ORDER BY
• Omitting BY keyword
• Confusing ASC with DESC

3. Given the table Products with columns ProductID and Price, what will be the order of ProductID after running this query?

SELECT ProductID FROM Products ORDER BY Price DESC;

Assume the data:
ProductID: 1, 2, 3
Price: 10, 30, 20

medium

A. [1, 3, 2]

B. [2, 3, 1]

C. [2, 1, 3]

D. [3, 2, 1]

Solution

1. Step 1: Understand ORDER BY Price DESC

   The query sorts rows by Price in descending order, so highest price first.

2. Step 2: Sort the data by Price descending

   Prices are 10 (ProductID 1), 30 (ProductID 2), 20 (ProductID 3). Descending order: 30, 20, 10. Corresponding ProductIDs: 2, 3, 1.

3. Final Answer:

   [2, 3, 1] -> Option B

4. Quick Check:

   Price DESC sorts highest to lowest [OK]

Hint: Sort by column DESC to get highest values first [OK]

Common Mistakes:

• Mixing ascending and descending order
• Sorting by ProductID instead of Price
• Assuming default ascending order

4. Identify the error in this SQL query:

SELECT Name FROM Customers ORDER LastName;

medium

A. Missing BY keyword after ORDER

B. ORDER BY cannot be used with SELECT

C. Name column does not exist

D. ORDER BY requires two columns

Solution

1. Step 1: Check ORDER BY syntax

   The correct syntax requires the keyword BY after ORDER. The query uses ORDER LastName which is invalid.

2. Step 2: Verify other parts

   The SELECT statement is valid, and Name is assumed to exist. ORDER BY can be used with SELECT. Only one column is needed for ORDER BY.

3. Final Answer:

   Missing BY keyword after ORDER -> Option A

4. Quick Check:

   ORDER BY requires BY keyword [OK]

Hint: ORDER BY must include BY keyword [OK]

Common Mistakes:

• Omitting BY after ORDER
• Thinking ORDER BY needs multiple columns
• Assuming ORDER BY can't be used with SELECT

5. You have a table Students with columns StudentID, Name, and Score. You want to list all students ordered by their Score from highest to lowest, but if two students have the same score, order them alphabetically by Name. Which query correctly achieves this?

hard

A. SELECT * FROM Students ORDER BY Score DESC, Name ASC;

B. SELECT * FROM Students ORDER BY Name ASC, Score DESC;

C. SELECT * FROM Students ORDER BY Score ASC, Name DESC;

D. SELECT * FROM Students ORDER BY Score DESC;

Solution

1. Step 1: Understand sorting requirements

   Primary sort is by Score descending (highest first). Secondary sort for ties is by Name ascending (alphabetical).

2. Step 2: Analyze each option

   SELECT * FROM Students ORDER BY Score DESC, Name ASC; orders by Score DESC then Name ASC, matching requirements. SELECT * FROM Students ORDER BY Name ASC, Score DESC; reverses order priorities. SELECT * FROM Students ORDER BY Score ASC, Name DESC; sorts Score ascending and Name descending, wrong directions. SELECT * FROM Students ORDER BY Score DESC; only sorts by Score DESC, no tie-breaker.

3. Final Answer:

   SELECT * FROM Students ORDER BY Score DESC, Name ASC; -> Option A

4. Quick Check:

   ORDER BY primary DESC, secondary ASC for tie-break [OK]

Hint: Use ORDER BY primary DESC, secondary ASC for tie-breaks [OK]

Common Mistakes:

• Reversing sort order directions
• Not adding secondary column for tie-break
• Using only one column when two are needed