Why SELECT is the most important command in SQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

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

SQL

SELECT [1] FROM employees;

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

A*

BALL

CEVERY

DCOLUMNS

Attempts:

3 left

💡 Hint

Common Mistakes

Using words like ALL or COLUMNS instead of * causes errors.

Leaving the blank empty will cause a syntax error.

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

2fill in blank

medium

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

SQL

SELECT [1] FROM students;

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

Anames

Bname

Cstudent_name

Dall_names

Attempts:

3 left

💡 Hint

Common Mistakes

Using plural or different column names that don't exist.

Using reserved words or made-up names.

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 the 'users' table.

SQL

SELECT [1] FROM users;

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

Aage

Buser_age

Cages

DAge

Attempts:

3 left

💡 Hint

Common Mistakes

Using plural or different column names.

Capitalizing column names when not needed.

The correct column name is 'age'. Using 'ages' or 'user_age' will cause errors.

4fill in blank

hard

Fill both blanks to select the 'id' and 'email' columns from the 'contacts' table.

SQL

SELECT [1], [2] FROM contacts;

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

Aid

Bemail

Cname

Dphone

Attempts:

3 left

💡 Hint

Common Mistakes

Using columns that don't exist in the table.

Forgetting the comma between column names.

To select multiple columns, list them separated by commas. Here, 'id' and 'email' are the correct columns.

5fill in blank

hard

Fill all three blanks to select 'product_name', 'price', and 'stock' columns from the 'inventory' table.

SQL

SELECT [1], [2], [3] FROM inventory;

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

Aproduct_name

Bprice

Cstock

Dquantity

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong column names like 'quantity' instead of 'stock'.

Missing commas between column names.

List the exact column names separated by commas to select multiple columns.

Practice

1. Why is the SELECT command considered the most important in SQL?

easy

A. Because it retrieves data from the database for viewing.

B. Because it deletes data from the database.

C. Because it creates new tables in the database.

D. Because it updates existing data in the database.

Solution

1. Step 1: Understand the purpose of SELECT

   The SELECT command is used to get data from tables so users can see it.

2. Step 2: Compare with other commands

   DELETE removes data, CREATE makes tables, UPDATE changes data, but SELECT only shows data.

3. Final Answer:

   Because it retrieves data from the database for viewing. -> Option A

4. Quick Check:

   SELECT = retrieve data [OK]

Hint: SELECT is for viewing data, not changing it [OK]

Common Mistakes:

• Confusing SELECT with DELETE or UPDATE
• Thinking SELECT changes data
• Mixing SELECT with CREATE commands

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

easy

A. GET * FROM employees;

B. SELECT ALL FROM employees;

C. SELECT * FROM employees;

D. SHOW * FROM employees;

Solution

1. Step 1: Recall standard SELECT syntax

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

2. Step 2: Check each option

   SELECT * FROM employees; matches the correct syntax. Options A, B, and D use incorrect keywords.

3. Final Answer:

   SELECT * FROM employees; -> Option C

4. Quick Check:

   SELECT * FROM table = correct syntax [OK]

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

Common Mistakes:

• Using GET or SHOW instead of SELECT
• Omitting FROM keyword
• Using ALL without SELECT

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 product names regardless of price.

D. An error because price comparison is invalid.

Solution

1. Step 1: Understand the WHERE clause

   The WHERE clause filters rows where price is greater than 100.

2. Step 2: Understand SELECT columns

   Only the name column is selected, so the output shows product names matching the filter.

3. Final Answer:

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

4. Quick Check:

   WHERE price > 100 filters rows [OK]

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

Common Mistakes:

• Confusing greater than (>) with less than (<)
• Expecting all columns instead of only name
• Thinking query causes error

4. What is wrong with this SQL query?

SELECT name age FROM users;

medium

A. Missing WHERE clause.

B. Incorrect table name.

C. SELECT cannot have more than one column.

D. Missing comma between column names.

Solution

1. Step 1: Check column list syntax

   Multiple columns must be separated by commas in SELECT.

2. Step 2: Identify the error

   The query misses a comma between 'name' and 'age', causing syntax error.

3. Final Answer:

   Missing comma between column names. -> Option D

4. Quick Check:

   Columns need commas in SELECT [OK]

Hint: Separate columns with commas in SELECT [OK]

Common Mistakes:

• Forgetting commas between columns
• Assuming WHERE is always required
• Thinking SELECT allows no commas

5. You want to see the names and salaries of employees who earn more than 5000 from the employees table. Which query correctly does this?

hard

A. SELECT name, salary FROM employees WHERE salary > 5000;

B. SELECT name salary FROM employees WHERE salary > 5000;

C. SELECT * FROM employees WHERE salary < 5000;

D. SELECT name, salary FROM employees;

Solution

1. Step 1: Identify needed columns and condition

   You need name and salary columns, filtered by salary greater than 5000.

2. Step 2: Check each option

   SELECT name, salary FROM employees WHERE salary > 5000; selects correct columns and filters salary > 5000. SELECT name salary FROM employees WHERE salary > 5000; misses comma, causing error. SELECT * FROM employees WHERE salary < 5000; filters salary less than 5000, wrong condition. SELECT name, salary FROM employees; selects all employees without filter.

3. Final Answer:

   SELECT name, salary FROM employees WHERE salary > 5000; -> Option A

4. Quick Check:

   SELECT columns WHERE condition = correct query [OK]

Hint: Use WHERE to filter rows, commas to separate columns [OK]

Common Mistakes:

• Missing commas between columns
• Using wrong comparison operators
• Omitting WHERE clause when filtering