First query execution in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the SELECT statement in SQL?

The SELECT statement is used to retrieve data from a database table. It lets you choose which columns and rows to see.

beginner

What does this query do?
SELECT * FROM users;

This query retrieves all columns and all rows from the table named users. The asterisk (*) means 'all columns'.

beginner

Why do we end SQL statements with a semicolon (;)?

The semicolon marks the end of an SQL statement. It tells the database that the command is complete and ready to run.

beginner

What is a database table?

A database table is like a spreadsheet with rows and columns. It stores data in an organized way so you can find and use it easily.

beginner

What happens when you run your first SQL query?

When you run your first SQL query, the database reads your command, finds the data you asked for, and shows it to you.

What does the SELECT statement do?

A Retrieves data from a database

B Deletes data from a database

C Updates data in a database

D Creates a new database

The SELECT statement is used to get data from a database table.

What does the asterisk (*) mean in SELECT * FROM table_name;?

A Select only one column

B Select no columns

C Select all columns

D Select only rows with *

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

Why do SQL statements end with a semicolon (;)?

A To mark the end of the statement

B To start the statement

C To comment the statement

D To ignore the statement

The semicolon tells the database that the SQL command is complete.

What is a database table similar to?

A A video file

B A spreadsheet with rows and columns

C A photo album

D A text document

A database table organizes data like a spreadsheet with rows and columns.

What happens when you run your first SQL query?

A Nothing happens

B The database deletes all data

C The database crashes

D The database shows the data you asked for

Running a query asks the database to find and show the requested data.

Explain what happens when you run your first SQL query.

Think about how the database processes your command and shows you data.

Describe the role of the SELECT statement in SQL.

Focus on what <code>SELECT</code> does in a query.

Practice

1. What does the SELECT statement do in MySQL?

easy

A. It retrieves data from a database table.

B. It deletes data from a database table.

C. It updates data in a database table.

D. It creates a new database table.

Solution

1. Step 1: Understand the purpose of SELECT

   The SELECT statement is used to get or read data from a table in a database.

2. Step 2: Compare with other SQL commands

   DELETE removes data, UPDATE changes data, and CREATE makes new tables, so they are different from SELECT.

3. Final Answer:

   It retrieves data from a database table. -> Option A

4. Quick Check:

   SELECT = retrieve data [OK]

Hint: SELECT means get data from a table [OK]

Common Mistakes:

• Confusing SELECT with DELETE or UPDATE
• Thinking SELECT creates tables
• Mixing SELECT with data modification commands

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

easy

A. SELECT users FROM *;

B. SELECT * FROM users;

C. SELECT * users FROM;

D. FROM users SELECT *;

Solution

1. Step 1: Recall the basic SELECT syntax

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

2. Step 2: Check each option

   SELECT * FROM users; matches the correct syntax. Others have wrong order or misplaced keywords.

3. Final Answer:

   SELECT * FROM users; -> Option B

4. Quick Check:

   SELECT * FROM table_name; is correct [OK]

Hint: SELECT * FROM table_name; gets all columns [OK]

Common Mistakes:

• Swapping SELECT and FROM keywords
• Omitting the FROM keyword
• Placing * in wrong position

3. Given a table products with columns id, name, and price, what will this query return?

SELECT name FROM products;

medium

A. All product names from the products table.

B. All product prices from the products table.

C. All columns from the products table.

D. An error because price column is missing.

Solution

1. Step 1: Understand the SELECT clause

   The query selects only the 'name' column from the products table.

2. Step 2: Check what is returned

   It returns all rows but only the 'name' column values, not price or id.

3. Final Answer:

   All product names from the products table. -> Option A

4. Quick Check:

   SELECT name returns names only [OK]

Hint: SELECT column_name returns that column's data [OK]

Common Mistakes:

• Assuming all columns are returned
• Thinking missing columns cause errors
• Confusing column names in SELECT

4. What is wrong with this query?

SELECT FROM users;

medium

A. FROM keyword should be after WHERE.

B. Table name is incorrect.

C. Missing column names or * after SELECT.

D. SELECT cannot be used with FROM.

Solution

1. Step 1: Check the SELECT syntax

   SELECT must be followed by column names or * to specify what to retrieve.

2. Step 2: Identify the error in the query

   The query has SELECT but no columns or * after it, causing a syntax error.

3. Final Answer:

   Missing column names or * after SELECT. -> Option C

4. Quick Check:

   SELECT needs columns or * [OK]

Hint: Always specify columns or * after SELECT [OK]

Common Mistakes:

• Leaving SELECT empty
• Misplacing FROM keyword
• Thinking FROM is optional

5. You want to see all columns from a table employees but only for employees with the job title 'Manager'. Which query should you use?

hard

A. SELECT * WHERE job_title = 'Manager' FROM employees;

B. SELECT job_title FROM employees WHERE job_title = 'Manager';

C. SELECT * FROM employees;

D. SELECT * FROM employees WHERE job_title = 'Manager';

Solution

1. Step 1: Identify the need to filter rows

   We want all columns but only rows where job_title is 'Manager', so we need a WHERE clause.

2. Step 2: Check query syntax and logic

   SELECT * FROM employees WHERE job_title = 'Manager'; correctly uses SELECT * FROM employees WHERE job_title = 'Manager'; which returns all columns filtered by job title.

3. Final Answer:

   SELECT * FROM employees WHERE job_title = 'Manager'; -> Option D

4. Quick Check:

   Use WHERE to filter rows [OK]

Hint: Use WHERE to filter rows after FROM [OK]

Common Mistakes:

• Omitting WHERE clause for filtering
• Placing WHERE before FROM
• Selecting only job_title instead of all columns