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.

Practice

1. What does the SQL query SELECT * FROM employees; do?

easy

A. It deletes all data from the employees table.

B. It creates a new employees table.

C. It updates all columns in the employees table.

D. It retrieves all columns and all rows from the employees table.

Solution

1. Step 1: Understand the SELECT * syntax

   The asterisk (*) means select all columns from the specified table.

2. Step 2: Apply to the employees table

   The query fetches every column and every row from the employees table.

3. Final Answer:

   It retrieves all columns and all rows from the employees table. -> Option D

4. Quick Check:

   SELECT * = all columns and rows [OK]

Hint: SELECT * means get everything from the table [OK]

Common Mistakes:

• Thinking SELECT * deletes or updates data
• Confusing SELECT * with creating tables
• Assuming it selects only one column

2. Which of the following is the correct syntax to select all columns from a table named products?

easy

A. SELECT * FROM products;

B. SELECT products.*;

C. SELECT columns FROM products;

D. SELECT ALL FROM products;

Solution

1. Step 1: Recall the correct SELECT syntax

   The correct way to select all columns is using SELECT * FROM table_name.

2. Step 2: Check each option

   SELECT * FROM products; matches the correct syntax exactly.

3. Final Answer:

   SELECT * FROM products; -> Option A

4. Quick Check:

   SELECT * FROM table = correct syntax [OK]

Hint: Use SELECT * FROM table_name to get all columns [OK]

Common Mistakes:

• Using SELECT ALL instead of SELECT *
• Forgetting FROM keyword
• Trying to select 'columns' literally

3. Given the table students with columns id, name, and age, what will the query SELECT * FROM students WHERE age > 20; return?

medium

A. All students regardless of age.

B. Only the age column for students older than 20.

C. All columns for students whose age is greater than 20.

D. An error because * cannot be used with WHERE.

Solution

1. Step 1: Understand SELECT * with WHERE clause

   SELECT * fetches all columns, and WHERE filters rows based on condition.

2. Step 2: Apply condition age > 20

   Only rows where age is greater than 20 are returned, with all columns shown.

3. Final Answer:

   All columns for students whose age is greater than 20. -> Option C

4. Quick Check:

   SELECT * + WHERE filters rows [OK]

Hint: SELECT * returns all columns; WHERE filters rows [OK]

Common Mistakes:

• Thinking WHERE limits columns instead of rows
• Assuming * selects only one column
• Believing * cannot be used with WHERE

4. Identify the error in this SQL query: SELECT * employees;

medium

A. Table name should be in quotes.

B. Missing FROM keyword before table name.

C. Using * is not allowed in SELECT.

D. SELECT cannot be used without WHERE.

Solution

1. Step 1: Check SQL SELECT syntax

   The correct syntax requires FROM before the table name.

2. Step 2: Identify missing keyword

   The query misses FROM between SELECT * and employees.

3. Final Answer:

   Missing FROM keyword before table name. -> Option B

4. Quick Check:

   SELECT * FROM table is correct syntax [OK]

Hint: Always use FROM before table name in SELECT [OK]

Common Mistakes:

• Omitting FROM keyword
• Thinking * is invalid
• Adding unnecessary quotes around table name

5. You want to quickly check all data in a large table named orders. Which query is best to avoid slowing down your database?

hard

A. SELECT * FROM orders LIMIT 10;

B. SELECT * FROM orders;

C. SELECT order_id FROM orders;

D. DELETE FROM orders WHERE order_id < 10;

Solution

1. Step 1: Understand SELECT * impact on large tables

   SELECT * fetches all rows and columns, which can be slow on big tables.

2. Step 2: Use LIMIT to reduce rows fetched

   Adding LIMIT 10 fetches only 10 rows, making the query faster and safer.

3. Final Answer:

   SELECT * FROM orders LIMIT 10; -> Option A

4. Quick Check:

   LIMIT reduces rows, speeding up SELECT * [OK]

Hint: Use LIMIT with SELECT * on big tables to speed queries [OK]

Common Mistakes:

• Running SELECT * without LIMIT on large tables
• Confusing DELETE with SELECT
• Selecting only one column when all data is needed