Relational database concepts in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a relational database?

A relational database stores data in tables made of rows and columns. Each table represents an entity, and relationships between tables are made using keys.

beginner

What is a primary key in a relational database?

A primary key is a column or set of columns that uniquely identifies each row in a table. It ensures no two rows have the same key value.

intermediate

Explain the concept of foreign key.

A foreign key is a column in one table that refers to the primary key in another table. It creates a link between the two tables to maintain data consistency.

intermediate

What is normalization in relational databases?

Normalization is the process of organizing data to reduce duplication and improve data integrity by dividing tables and defining relationships.

beginner

What does SQL stand for and what is its role?

SQL stands for Structured Query Language. It is used to communicate with and manage data in relational databases by writing queries.

Which of the following uniquely identifies a row in a table?

A Primary key

B Foreign key

C Index

D Column name

The primary key uniquely identifies each row in a table.

What does a foreign key do?

A Sorts data alphabetically

B Creates a new table

C Deletes duplicate rows

D Links one table to another

A foreign key links one table to another by referring to the primary key.

Normalization helps to:

A Reduce data duplication

B Increase data duplication

C Make data harder to access

D Store data in one big table

Normalization organizes data to reduce duplication and improve integrity.

Which language is used to query relational databases?

A HTML

B CSS

C SQL

D JavaScript

SQL is the language used to query and manage relational databases.

In a table, columns represent:

A Individual records

B Attributes or fields

C Database names

D User permissions

Columns represent attributes or fields describing data in each row.

Describe the role of primary and foreign keys in relational databases.

Think about how tables connect and keep data unique.

Explain why normalization is important in database design.

Consider how data is stored efficiently and cleanly.

Practice

1. What is the main purpose of a primary key in a relational database table?

easy

A. To speed up database backups

B. To store large text data

C. To link two tables together

D. To uniquely identify each record in the table

Solution

1. Step 1: Understand the role of primary keys

   A primary key is a column or set of columns that uniquely identifies each row in a table.

2. Step 2: Compare options with the definition

   Only To uniquely identify each record in the table describes this unique identification role correctly.

3. Final Answer:

   To uniquely identify each record in the table -> Option D

4. Quick Check:

   Primary key = unique record ID [OK]

Hint: Primary key means unique ID for each row [OK]

Common Mistakes:

• Confusing primary key with foreign key
• Thinking primary key stores large data
• Assuming primary key speeds backups

2. Which of the following is the correct syntax to declare a primary key on the column id in a MySQL table?

easy

A. PRIMARY KEY (id);

B. KEY PRIMARY id;

C. PRIMARY KEY id;

D. PRIMARY id KEY;

Solution

1. Step 1: Recall MySQL primary key syntax

   In MySQL, the primary key is declared using PRIMARY KEY (column_name).

2. Step 2: Match syntax with options

   PRIMARY KEY (id); matches the correct syntax with parentheses around the column name.

3. Final Answer:

   PRIMARY KEY (id); -> Option A

4. Quick Check:

   Primary key syntax uses parentheses [OK]

Hint: Primary key syntax uses parentheses around column [OK]

Common Mistakes:

• Omitting parentheses around column name
• Swapping order of keywords
• Using incorrect keywords like KEY PRIMARY

3. Given two tables: Customers(id, name) and Orders(order_id, customer_id), which SQL query correctly retrieves all orders with the customer names?

medium

A. SELECT Orders.order_id, Customers.name FROM Orders JOIN Customers ON Orders.id = Customers.customer_id;

B. SELECT Orders.order_id, Customers.name FROM Orders JOIN Customers ON Orders.customer_id = Customers.id;

C. SELECT order_id, name FROM Orders, Customers WHERE Orders.id = Customers.id;

D. SELECT order_id, name FROM Orders LEFT JOIN Customers ON Orders.customer_id = Customers.order_id;

Solution

1. Step 1: Identify the correct join condition

   The foreign key customer_id in Orders links to id in Customers.

2. Step 2: Check each option's join condition

   SELECT Orders.order_id, Customers.name FROM Orders JOIN Customers ON Orders.customer_id = Customers.id; correctly joins on Orders.customer_id = Customers.id. Others use wrong columns or join types.

3. Final Answer:

   SELECT Orders.order_id, Customers.name FROM Orders JOIN Customers ON Orders.customer_id = Customers.id; -> Option B

4. Quick Check:

   Join on foreign key = primary key [OK]

Hint: Join tables on foreign key = primary key [OK]

Common Mistakes:

• Swapping join columns incorrectly
• Using WHERE instead of JOIN condition properly
• Mixing up LEFT JOIN with INNER JOIN

4. You wrote this SQL to create a foreign key: FOREIGN KEY (customer_id) REFERENCES Customers(customer_id). But it gives an error. What is the likely cause?

medium

A. You must declare foreign keys before primary keys

B. Foreign keys cannot reference columns in the same table

C. The referenced column customer_id does not exist or is not a primary key in Customers

D. Foreign keys must reference columns named 'id' only

Solution

1. Step 1: Understand foreign key requirements

   A foreign key must reference a column that exists and is a primary or unique key in the referenced table.

2. Step 2: Analyze the error cause

   If Customers has no customer_id column or it is not a key, the foreign key declaration fails.

3. Final Answer:

   The referenced column customer_id does not exist or is not a primary key in Customers -> Option C

4. Quick Check:

   Foreign key must reference existing primary key [OK]

Hint: Foreign key must reference existing primary key column [OK]

Common Mistakes:

• Referencing non-existent columns
• Assuming foreign keys must reference 'id' column
• Incorrect order of key declarations

5. You have two tables: Employees(emp_id, name) and Departments(dept_id, dept_name, manager_id). The manager_id in Departments references emp_id in Employees. Which of these statements is TRUE about this relationship?

hard

A. An employee can manage multiple departments, but a department can have only one manager

B. Each department must have exactly one manager who is an employee

C. A department can have multiple managers if multiple employees share the same emp_id

D. The manager_id column in Departments can be NULL only if emp_id is NULL in Employees

Solution

1. Step 1: Understand foreign key relationship

   The manager_id in Departments points to emp_id in Employees, linking each department to one employee as manager.

2. Step 2: Analyze relationship cardinality

   One department has one manager, but an employee can manage many departments (one-to-many).

3. Step 3: Evaluate options

   An employee can manage multiple departments, but a department can have only one manager correctly states this one-to-many relationship. Each department must have exactly one manager who is an employee is wrong because departments can have one manager, but not necessarily exactly one if NULL allowed. A department can have multiple managers if multiple employees share the same emp_id is wrong because emp_id is unique. The manager_id column in Departments can be NULL only if emp_id is NULL in Employees is incorrect because NULL in manager_id is independent of emp_id NULL.

4. Final Answer:

   An employee can manage multiple departments, but a department can have only one manager -> Option A

5. Quick Check:

   One-to-many foreign key = An employee can manage multiple departments, but a department can have only one manager [OK]

Hint: Foreign key one-to-many means one employee, many departments [OK]

Common Mistakes:

• Assuming multiple managers per department
• Confusing NULL constraints
• Thinking emp_id can be duplicated