SELECT all columns in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the SQL query SELECT * FROM table_name; do?

It selects and returns all columns and all rows from the table named table_name.

beginner

In SQL, what symbol is used to select all columns from a table?

The asterisk symbol * is used to select all columns.

intermediate

Why might you want to avoid using SELECT * in large databases?

Because it returns all columns, which can be slow and use more resources if the table has many columns or rows. Selecting only needed columns is more efficient.

beginner

Write a SQL query to select all columns from a table named employees.

SELECT * FROM employees;

beginner

What is the difference between SELECT * and SELECT column1, column2?

SELECT * returns all columns, while SELECT column1, column2 returns only the specified columns.

What does the asterisk (*) mean in the SQL query SELECT * FROM customers;?

A Select all columns from the customers table

B Select all rows from the customers table

C Select only the first column from the customers table

D Delete all data from the customers table

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

Which SQL query selects all columns from the table named orders?

A SELECT columns FROM orders;

B SELECT all FROM orders;

C SELECT * FROM orders;

D SELECT orders.*;

The correct syntax to select all columns is SELECT * FROM table_name;

If you want to select only the columns 'name' and 'age' from a table, which query is correct?

A SELECT * FROM table_name;

B SELECT name AND age FROM table_name;

C SELECT name age FROM table_name;

D SELECT name, age FROM table_name;

To select specific columns, list them separated by commas after SELECT.

What is a potential downside of using SELECT * in a query?

A It only returns one row

B It can slow down the query by returning unnecessary data

C It deletes data from the table

D It requires specifying column names

Selecting all columns can return more data than needed, slowing down performance.

Which part of the SQL query specifies the table to select data from?

A FROM

B WHERE

C SELECT

D ORDER BY

The FROM clause specifies the table to select data from.

Explain how to select all columns from a table in SQL and why you might want to be careful using it.

Think about the * symbol and performance.

Write a simple SQL query to get all data from a table called 'products'.

Use the wildcard * to select all columns.

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