SELECT all columns in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this query?

Given a table Employees with columns id, name, and department, what will this query return?

SELECT * FROM Employees WHERE department = 'Sales';

SQL

SELECT * FROM Employees WHERE department = 'Sales';

A All columns of employees who work in the Sales department

B Only the 'department' column for employees in Sales

C All employees regardless of department

D Syntax error due to missing column names

💡 Hint

The asterisk (*) means select all columns, but the WHERE clause filters rows.

The query selects all columns (*) but only rows where the department is 'Sales'. So the output includes all columns for those employees.

❓ query_result

intermediate

What does this query return?

Consider a table Products with columns product_id, name, and price. What will this query output?

SELECT * FROM Products WHERE price > 100;

SQL

SELECT * FROM Products WHERE price > 100;

A All columns of products priced above 100

B Only the 'price' column for products above 100

C All products regardless of price

D An error because * cannot be used with WHERE

💡 Hint

The * selects all columns, and WHERE filters rows by price.

The query returns all columns for products where the price is greater than 100.

📝 Syntax

advanced

Which query is syntactically correct to select all columns from the table Orders?

Choose the correct SQL query that selects all columns from the Orders table.

A SELECT * FROM Orders WHERE;

B SELECT ALL COLUMNS FROM Orders;

C SELECT ALL FROM Orders;

D SELECT * FROM Orders;

💡 Hint

The standard way to select all columns uses * after SELECT.

Only option D uses the correct syntax: SELECT * FROM Orders; Options B and C use invalid keywords, and D has an incomplete WHERE clause.

❓ optimization

advanced

Which query is more efficient to select all columns from Customers with country 'USA'?

You want to get all columns for customers from the USA. Which query is better for performance?

A SELECT * FROM Customers WHERE country = 'USA';

B SELECT id, name, email, country FROM Customers WHERE country = 'USA';

C SELECT * FROM Customers;

D SELECT country FROM Customers WHERE country = 'USA';

💡 Hint

Selecting only needed columns can improve performance.

Option B explicitly selects needed columns and filters by country, which can be more efficient than selecting all columns with *. Option B selects all columns which may be unnecessary. Option B returns all customers. Option B returns only country column.

🧠 Conceptual

expert

What happens if you use SELECT * in a join query with tables Authors and Books?

Given two tables, Authors (id, name) and Books (id, title, author_id), what will this query return?

SELECT * FROM Authors JOIN Books ON Authors.id = Books.author_id;

SQL

SELECT * FROM Authors JOIN Books ON Authors.id = Books.author_id;

A Only columns from Books table for matching rows

B Only columns from Authors table for matching rows

C All columns from both tables for matching author-book pairs

D Syntax error due to ambiguous column names

💡 Hint

SELECT * returns all columns from all tables in the FROM and JOIN clauses.

The query returns all columns from both Authors and Books for rows where Authors.id matches Books.author_id. There is no syntax error; columns with same names are returned with table prefixes.