Why SELECT retrieves data in MySQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all columns from the table named 'employees'.

MySQL

SELECT [1] FROM employees;

Drag options to blanks, or click blank then click option'

A*

BALL

CCOLUMNS

DEVERYTHING

Attempts:

3 left

💡 Hint

Common Mistakes

Using words like ALL or COLUMNS instead of *

Forgetting to specify what to select

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

2fill in blank

medium

Complete the code to select only the 'name' column from the 'employees' table.

MySQL

SELECT [1] FROM employees;

Drag options to blanks, or click blank then click option'

Aall

Bname

Cnames

Demployee_name

Attempts:

3 left

💡 Hint

Common Mistakes

Using plural or different column names like 'names' or 'employee_name'

Using keywords like 'all' instead of a column name

Use the exact column name 'name' to select that column.

3fill in blank

hard

Fix the error in the code to select the 'age' column from 'employees'.

MySQL

SELECT [1] FROM employees;

Drag options to blanks, or click blank then click option'

A*

Bage;

Cage

Dages

Attempts:

3 left

💡 Hint

Common Mistakes

Adding a semicolon after the column name inside the SELECT clause

Using wrong column names like 'ages'

Do not put a semicolon inside the SELECT clause. The semicolon ends the statement.

4fill in blank

hard

Fill both blanks to select the 'salary' column from 'employees' where salary is greater than 50000.

MySQL

SELECT [1] FROM employees WHERE salary [2] 50000;

Drag options to blanks, or click blank then click option'

Asalary

B>

C<

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' instead of 'salary' in SELECT

Using '<' instead of '>' in the WHERE clause

Select the 'salary' column and use '>' to filter salaries greater than 50000.

5fill in blank

hard

Fill all three blanks to select 'name' and 'age' from 'employees' where age is less than 30.

MySQL

SELECT [1], [2] FROM employees WHERE age [3] 30;

Drag options to blanks, or click blank then click option'

Aname

Bage

C<

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '<' in the WHERE clause

Swapping the order of columns (though this is not an error, but less common)

Select 'name' and 'age' columns and filter rows where age is less than 30 using '<'.

Practice

1. What is the main purpose of the SELECT statement in MySQL?

easy

A. To retrieve data from one or more tables

B. To delete data from a table

C. To update existing data in a table

D. To create a new table in the database

Solution

1. Step 1: Understand the role of SELECT

   The SELECT statement is designed to fetch or retrieve data stored in tables.

2. Step 2: Compare with other SQL commands

   DELETE removes data, UPDATE changes data, and CREATE TABLE makes new tables, so these are not for retrieving data.

3. Final Answer:

   To retrieve data from one or more tables -> Option A

4. Quick Check:

   SELECT = retrieve data [OK]

Hint: SELECT always fetches data, not changes or deletes [OK]

Common Mistakes:

• Confusing SELECT with DELETE or UPDATE
• Thinking SELECT creates tables
• Assuming SELECT modifies data

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

easy

A. SELECT employees FROM *;

B. SELECT ALL employees;

C. SELECT * FROM employees;

D. GET * FROM employees;

Solution

1. Step 1: Recall correct SELECT syntax

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

2. Step 2: Check each option

   SELECT * FROM employees; matches the correct syntax. The other options are invalid: table name before *, using ALL incorrectly, and using GET instead of SELECT.

3. Final Answer:

   SELECT * FROM employees; -> Option C

4. Quick Check:

   SELECT * FROM table = correct syntax [OK]

Hint: Use * to select all columns in SELECT [OK]

Common Mistakes:

• Using GET instead of SELECT
• Placing table name before *
• Using ALL keyword incorrectly

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

SELECT name FROM products WHERE price > 100;

medium

A. All product names with price less than 100

B. All product names with price greater than 100

C. All columns for products priced over 100

D. An error because price comparison is invalid

Solution

1. Step 1: Analyze the SELECT clause

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

2. Step 2: Understand the WHERE condition

   The condition price > 100 filters rows where price is greater than 100.

3. Final Answer:

   All product names with price greater than 100 -> Option B

4. Quick Check:

   SELECT name WHERE price > 100 = product names over 100 [OK]

Hint: WHERE filters rows; SELECT chooses columns [OK]

Common Mistakes:

• Confusing greater than with less than
• Expecting all columns instead of just name
• Thinking the query causes an error

4. Identify the error in this query:

SELECT FROM customers WHERE age > 30;

medium

A. Missing column names after SELECT

B. Incorrect WHERE clause syntax

C. Table name should be after WHERE

D. No error, query is correct

Solution

1. Step 1: Check SELECT syntax

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

2. Step 2: Review the query structure

   The query has SELECT but no columns listed before FROM, which is invalid syntax.

3. Final Answer:

   Missing column names after SELECT -> Option A

4. Quick Check:

   SELECT needs columns or * [OK]

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

Common Mistakes:

• Leaving SELECT empty
• Placing table name after WHERE
• Assuming WHERE syntax is wrong

5. You want to see only the email addresses of customers who live in 'New York' from the customers table. Which query correctly retrieves this data?

hard

A. SELECT email FROM customers WHERE city == 'New York';

B. SELECT * FROM customers WHERE city = New York;

C. SELECT email WHERE city = 'New York' FROM customers;

D. SELECT email FROM customers WHERE city = 'New York';

Solution

1. Step 1: Identify correct SELECT and WHERE syntax

   The correct syntax is SELECT column FROM table WHERE condition;. The city value must be in quotes.

2. Step 2: Evaluate each option

   SELECT email FROM customers WHERE city = 'New York'; uses correct syntax and quotes around 'New York'. SELECT * FROM customers WHERE city = New York; misses quotes, causing error. SELECT email WHERE city = 'New York' FROM customers; has wrong clause order. SELECT email FROM customers WHERE city == 'New York'; uses double equals which is invalid in SQL.

3. Final Answer:

   SELECT email FROM customers WHERE city = 'New York'; -> Option D

4. Quick Check:

   Correct syntax with quotes and order [OK]

Hint: Use single quotes for string values in WHERE [OK]

Common Mistakes:

• Omitting quotes around string values
• Using == instead of = in WHERE
• Placing WHERE before FROM