Relational model mental model in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the SQL query 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

BALL

CEVERY

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using words like ALL or COLUMNS instead of the symbol *.

Forgetting to specify columns or using invalid keywords.

In SQL, * means select all columns from the table.

2fill in blank

medium

Complete the SQL query to get only the distinct department names from the 'departments' table.

SQL

SELECT [1] department_name FROM departments;

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

ADISTINCT

BONLY

CSINGLE

DUNIQUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNIQUE instead of DISTINCT in SELECT statements.

Using ONLY or SINGLE which are not valid SQL keywords here.

The keyword DISTINCT returns unique values in SQL.

3fill in blank

hard

Fix the error in the SQL query to count the number of employees in each department.

SQL

SELECT department_id, COUNT([1]) FROM employees GROUP BY department_id;

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

Aemployee_id

B*

Cdepartment_id

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Counting a specific column that might have NULLs, causing wrong counts.

Using department_id inside COUNT() which counts the group key, not rows.

Using COUNT(*) counts all rows per group, which is the correct way to count employees.

4fill in blank

hard

Fill both blanks to create a query that finds employees with salary greater than 50000.

SQL

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

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

A>

B50000

C<

D100000

Attempts:

3 left

💡 Hint

Common Mistakes

Using less than operator instead of greater than.

Using wrong salary values.

The condition salary > 50000 filters employees earning more than 50000.

5fill in blank

hard

Fill all three blanks to create a query that lists employee names and their department names by joining 'employees' and 'departments' tables.

SQL

SELECT e.name AS employee_name, d.name AS department_name FROM employees e [1] departments d ON e.[2] = d.[3];

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

AJOIN

Bdepartment_id

DLEFT

Attempts:

3 left

💡 Hint

Common Mistakes

Using LEFT instead of JOIN without ON clause.

Using wrong column names for joining.

Use JOIN to combine tables on matching department_id columns.

Practice

1. In the relational model, what does a table represent?

easy

A. A collection of related records about a thing

B. A single piece of data

C. A programming function

D. A type of database software

Solution

1. Step 1: Understand the relational model basics

   In the relational model, data is organized into tables that represent entities or things.

2. Step 2: Identify what a table holds

   A table holds many rows, each row is a record about the thing the table represents.

3. Final Answer:

   A collection of related records about a thing -> Option A

4. Quick Check:

   Table = collection of records [OK]

Hint: Tables = things, rows = records, columns = details [OK]

Common Mistakes:

• Confusing a table with a single data value
• Thinking a table is a program or software
• Mixing up rows and columns

2. Which of the following is the correct way to describe a primary key in a table?

easy

A. A column or set of columns that uniquely identifies each row

B. A column that stores only numbers

C. A column that can have duplicate values

D. A column used to sort the table

Solution

1. Step 1: Recall the role of a primary key

   A primary key uniquely identifies each record in a table, so no two rows share the same primary key value.

2. Step 2: Check the options

   Only A column or set of columns that uniquely identifies each row describes uniqueness and identification, others describe unrelated properties.

3. Final Answer:

   A column or set of columns that uniquely identifies each row -> Option A

4. Quick Check:

   Primary key = unique identifier [OK]

Hint: Primary key means unique row ID [OK]

Common Mistakes:

• Thinking primary key allows duplicates
• Confusing primary key with sorting or data type
• Assuming primary key is always numeric

3. Given a table Employees with columns EmpID (primary key), Name, and Department, what will the following SQL query return?

SELECT Name FROM Employees WHERE Department = 'Sales';

medium

A. Only employees with no department assigned

B. All employee names regardless of department

C. All employee names who work in the Sales department

D. An error because Department is not a primary key

Solution

1. Step 1: Understand the SELECT and WHERE clauses

   The query selects the Name column from Employees where Department equals 'Sales'.

2. Step 2: Determine the output

   It returns all employee names whose Department is exactly 'Sales'. It does not return others or cause errors.

3. Final Answer:

   All employee names who work in the Sales department -> Option C

4. Quick Check:

   WHERE filters rows, SELECT chooses columns [OK]

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

Common Mistakes:

• Thinking it returns all employees ignoring WHERE
• Assuming Department must be primary key to filter
• Expecting error due to non-primary key in WHERE

4. Consider this SQL snippet:

CREATE TABLE Products (ProductID INT, Name VARCHAR(50), Price DECIMAL);
ALTER TABLE Products ADD PRIMARY KEY (ProductID);
INSERT INTO Products VALUES (1, 'Pen', 1.5), (1, 'Pencil', 0.5);

What is the problem with this code?

medium

A. Price column cannot be DECIMAL

B. It tries to insert duplicate primary key values

C. The primary key is not defined correctly

D. The table has no columns

Solution

1. Step 1: Check primary key uniqueness

   Primary key ProductID must be unique. The insert tries to add two rows with ProductID = 1.

2. Step 2: Identify the error cause

   Inserting duplicate primary keys violates uniqueness, causing an error.

3. Final Answer:

   It tries to insert duplicate primary key values -> Option B

4. Quick Check:

   Primary key must be unique [OK]

Hint: Primary keys cannot repeat values [OK]

Common Mistakes:

• Ignoring duplicate key violation
• Thinking DECIMAL is invalid for Price
• Assuming primary key syntax is wrong

5. You have two tables: Orders(OrderID, CustomerID, Amount) and Customers(CustomerID, Name). Which relational model concept explains why CustomerID appears in both tables?

hard

A. Column aliasing to rename data

B. Primary key duplication for faster queries

C. Data redundancy to store copies

D. Foreign key linking related records between tables

Solution

1. Step 1: Understand the role of CustomerID in both tables

   CustomerID uniquely identifies customers in Customers and links orders to customers in Orders.

2. Step 2: Identify relational model concept

   This linking is done by a foreign key, which references the primary key in another table to connect related data.

3. Final Answer:

   Foreign key linking related records between tables -> Option D

4. Quick Check:

   Foreign key = link between tables [OK]

Hint: Foreign key connects tables via shared column [OK]

Common Mistakes:

• Confusing foreign key with primary key duplication
• Thinking it's just data redundancy
• Mixing foreign key with column aliasing