Tables, rows, and columns concept in SQL - Interactive Code Practice

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'

Acolumns

B*

CALL

Drows

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'ALL' instead of '*'

Writing 'columns' which is not valid SQL syntax

The asterisk (*) selects 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'

Aname

Ball

Cnames

Dstudent_name

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'names' which is plural and may not exist

Using 'student_name' which is a different column

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 'persons' table.

SQL

SELECT [1] FROM persons;

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

Aages

BAge

Cage

Dperson_age

Attempts:

3 left

💡 Hint

Common Mistakes

Using plural form 'ages'

Using a different column name like 'person_age'

The column name is 'age'. Using 'ages' or other variants causes 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 not in the table like 'name' or 'phone'

Forgetting the comma between column names

To select multiple columns, list them separated by commas.

5fill in blank

hard

Fill all three blanks to select 'first_name', 'last_name', and 'age' columns from the 'users' table.

SQL

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

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

Afirst_name

Blast_name

Cage

Demail

Attempts:

3 left

💡 Hint

Common Mistakes

Including columns not asked for like 'email'

Missing commas between column names

List all desired columns separated by commas to select them.

Practice

1. What does a row represent in a database table?

easy

A. A group of tables linked together

B. The name of the table

C. The type of data stored in the table

D. A single record containing data for each column

Solution

1. Step 1: Understand the structure of a table

   A table is made up of rows and columns where columns define data types and rows hold data.

2. Step 2: Identify what a row represents

   A row is one complete record with values for each column in the table.

3. Final Answer:

   A single record containing data for each column -> Option D

4. Quick Check:

   Row = single record [OK]

Hint: Rows = records, columns = data types [OK]

Common Mistakes:

• Confusing rows with columns
• Thinking rows are table names
• Mixing up rows with database schemas

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

easy

A. GET * FROM Employees;

B. SELECT ALL COLUMNS Employees;

C. SELECT * FROM Employees;

D. SELECT COLUMNS FROM Employees;

Solution

1. Step 1: Recall SQL syntax for selecting all columns

   The standard SQL command to select all columns is SELECT * FROM table_name;.

2. Step 2: Match the correct syntax

   SELECT * FROM Employees; matches the correct syntax exactly.

3. Final Answer:

   SELECT * FROM Employees; -> Option C

4. Quick Check:

   SELECT * = all columns [OK]

Hint: Use * to select all columns [OK]

Common Mistakes:

• Using GET instead of SELECT
• Writing SELECT ALL COLUMNS which is invalid
• Omitting FROM keyword

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

SELECT name FROM Students WHERE age > 20;

medium

A. Only the names of students older than 20

B. All columns for students older than 20

C. Only the ages of students older than 20

D. An error because age is not selected

Solution

1. Step 1: Analyze the SELECT clause

   The query selects only the name column from the table.

2. Step 2: Analyze the WHERE condition

   It filters rows where age is greater than 20, so only those students are included.

3. Final Answer:

   Only the names of students older than 20 -> Option A

4. Quick Check:

   SELECT name WHERE age > 20 = names only [OK]

Hint: SELECT columns control output, WHERE filters rows [OK]

Common Mistakes:

• Thinking all columns are returned
• Assuming age column is returned without selecting it
• Believing query causes error due to WHERE clause

4. What is wrong with this SQL query?

SELECT id, name FROM Students WHERE age = 'twenty';

medium

A. Using quotes around a number in WHERE clause causes error

B. Comparing age to a string 'twenty' may cause no matches

C. Selecting multiple columns requires commas

D. Missing FROM keyword before table name

Solution

1. Step 1: Check the WHERE clause comparison

   The query compares age to the string 'twenty', but age is numeric.

2. Step 2: Understand effect of comparing number to string

   This will not cause a syntax error but likely returns zero rows because no numeric age equals 'twenty'.

3. Final Answer:

   Comparing age to a string 'twenty' may cause no matches -> Option B

4. Quick Check:

   Wrong data type comparison = no results [OK]

Hint: Match data types in WHERE clause [OK]

Common Mistakes:

• Thinking quotes cause syntax error
• Assuming query returns matching rows
• Ignoring data type mismatch

5. You have a table Orders with columns order_id, customer_id, and amount. You want to find the total amount spent by each customer. Which SQL query correctly groups and sums the data?

hard

A. SELECT customer_id, SUM(amount) FROM Orders GROUP BY customer_id;

B. SELECT order_id, SUM(amount) FROM Orders GROUP BY order_id;

C. SELECT customer_id, amount FROM Orders WHERE SUM(amount) > 100;

D. SELECT customer_id, amount FROM Orders GROUP BY amount;

Solution

1. Step 1: Identify the goal

   We want total amount spent per customer, so we must group by customer_id.

2. Step 2: Use aggregation function

   SUM(amount) calculates total amount per group.

3. Step 3: Check query correctness

   SELECT customer_id, SUM(amount) FROM Orders GROUP BY customer_id; correctly groups by customer_id and sums amount.

4. Final Answer:

   SELECT customer_id, SUM(amount) FROM Orders GROUP BY customer_id; -> Option A

5. Quick Check:

   Group by customer_id + SUM(amount) = total per customer [OK]

Hint: Group by key, aggregate values with SUM [OK]

Common Mistakes:

• Grouping by wrong column
• Using WHERE with aggregate functions incorrectly
• Selecting columns not in GROUP BY without aggregation