Relational model mental model in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a relation in the relational model?

A relation is like a table with rows and columns where data is stored. Each row is a record, and each column is an attribute.

beginner

What does a tuple represent in the relational model?

A tuple is a single row in a relation, representing one record or data item.

beginner

Explain the concept of attributes in the relational model.

Attributes are the columns in a relation. They define the type of data stored, like name, age, or price.

intermediate

What is a primary key in the relational model?

A primary key is a unique attribute or set of attributes that identify each tuple (row) in a relation (table).

intermediate

How does the relational model ensure data integrity?

It uses rules like primary keys to avoid duplicates and foreign keys to maintain valid links between tables.

In the relational model, what does a relation correspond to?

A A programming function

B A table with rows and columns

C A single data value

D A database user

A relation is a table that organizes data into rows and columns.

What is a tuple in the relational model?

A A row in a table

B A column in a table

C A database schema

D A type of query

A tuple is a single row representing one record in a table.

Which of the following best describes an attribute?

A A table name

B A database user

C A query command

D A column in a table

Attributes are columns that define the data types stored in a table.

What is the purpose of a primary key?

A To uniquely identify each row

B To store large text data

C To link two databases

D To encrypt data

Primary keys uniquely identify each row in a table to avoid duplicates.

How does the relational model maintain data integrity?

A By deleting old data automatically

B By encrypting all data

C By using primary and foreign keys

D By limiting user access only

Primary and foreign keys enforce rules that keep data consistent and linked properly.

Describe the main components of the relational model and how they relate to a real-life spreadsheet.

Think about how a spreadsheet organizes data in rows and columns.

Explain why primary keys are important in the relational model and give an example.

Consider how you identify a person uniquely in a list.

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