SELECT specific columns in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Selecting specific columns from a table

Given a table Employees with columns id, name, department, and salary, what will be the output of this query?

SELECT name, salary FROM Employees;

SQL

CREATE TABLE Employees (id INT, name VARCHAR(50), department VARCHAR(50), salary INT);
INSERT INTO Employees VALUES (1, 'Alice', 'HR', 50000), (2, 'Bob', 'IT', 60000), (3, 'Charlie', 'Finance', 55000);

A [{"id": 1, "name": "Alice"}, {"id": 2, "name": "Bob"}, {"id": 3, "name": "Charlie"}]

B [{"name": "Alice", "salary": 50000}, {"name": "Bob", "salary": 60000}, {"name": "Charlie", "salary": 55000}]

C [{"department": "HR", "salary": 50000}, {"department": "IT", "salary": 60000}, {"department": "Finance", "salary": 55000}]

D [{"id": 1, "department": "HR"}, {"id": 2, "department": "IT"}, {"id": 3, "department": "Finance"}]

💡 Hint

Look at the columns listed after SELECT. Only those columns will appear in the output.

The query selects only the name and salary columns from the Employees table. So the output contains only those two columns for all rows.

❓ query_result

intermediate

Selecting columns with WHERE clause

What will be the output of this query on the Employees table?

SELECT id, name FROM Employees WHERE salary > 55000;

SQL

CREATE TABLE Employees (id INT, name VARCHAR(50), department VARCHAR(50), salary INT);
INSERT INTO Employees VALUES (1, 'Alice', 'HR', 50000), (2, 'Bob', 'IT', 60000), (3, 'Charlie', 'Finance', 55000);

A [{"id": 3, "name": "Charlie"}]

B [{"id": 1, "name": "Alice"}, {"id": 2, "name": "Bob"}]

C [{"id": 2, "name": "Bob"}]

D [{"id": 2, "name": "Bob"}, {"id": 3, "name": "Charlie"}]

💡 Hint

Only rows where salary is greater than 55000 are included.

The query filters rows with salary > 55000. Only Bob has salary 60000, so only his id and name are returned.

📝 Syntax

advanced

Identify the syntax error in selecting columns

Which option contains a syntax error when selecting specific columns from the Employees table?

A SELECT id name FROM Employees;

B SELECT id, name FROM Employees;

C SELECT name, salary FROM Employees;

D SELECT department FROM Employees;

💡 Hint

Check if commas separate the column names correctly.

Option A misses a comma between id and name, causing a syntax error.

❓ optimization

advanced

Optimizing column selection for performance

You want to retrieve only the name and department columns from a large Employees table. Which query is best for performance?

A SELECT * FROM Employees;

B SELECT id, name, department FROM Employees;

C SELECT name, department, salary FROM Employees;

D SELECT name, department FROM Employees;

💡 Hint

Select only the columns you need to reduce data transfer.

Option D selects only the needed columns, reducing data size and improving performance.

🧠 Conceptual

expert

Understanding column selection with aliases

Consider this query:

SELECT name AS employee_name, salary AS employee_salary FROM Employees;

What will be the column names in the output?

A ["employee_name", "employee_salary"]

B ["name AS employee_name", "salary AS employee_salary"]

C ["name", "salary"]

D ["employee", "salary"]

💡 Hint

Look at the AS keyword which renames columns in the output.

The AS keyword renames the columns in the result set to employee_name and employee_salary.