Primary keys and uniqueness in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a primary key in a database table?

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

beginner

Can a primary key column contain NULL values?

No, a primary key column cannot contain NULL values because it must uniquely identify every row, and NULL means unknown or missing.

intermediate

What is the difference between a primary key and a unique constraint?

A primary key uniquely identifies rows and does not allow NULLs. A unique constraint also enforces uniqueness but allows NULL values depending on the database system.

beginner

Why is it important to have a primary key in a table?

Having a primary key helps to quickly find, update, or delete specific rows. It also helps maintain data integrity by preventing duplicate rows.

beginner

How do you define a primary key when creating a table in SQL?

You define a primary key using the PRIMARY KEY keyword in the CREATE TABLE statement, for example: CREATE TABLE Students (ID INT PRIMARY KEY, Name VARCHAR(50));

Which of the following is true about a primary key?

A It must be unique and cannot contain NULL values.

B It can contain duplicate values.

C It can contain multiple NULL values.

D It is optional for every table.

A primary key uniquely identifies each row and cannot have NULL values.

What happens if you try to insert a duplicate value into a primary key column?

A The database allows it without error.

B The database converts the duplicate to NULL.

C The database rejects the insert and shows an error.

D The duplicate overwrites the existing row.

Primary keys enforce uniqueness, so duplicates cause an error.

Which SQL keyword is used to enforce uniqueness on a column but allow NULL values?

A CHECK

B PRIMARY KEY

C FOREIGN KEY

D UNIQUE

The UNIQUE constraint enforces uniqueness but can allow NULLs.

Can a table have more than one primary key?

A Yes, multiple primary keys are allowed.

B No, only one primary key per table is allowed.

C Yes, but only if they are composite keys.

D No, primary keys are optional.

A table can have only one primary key, which can be a single or composite key.

What is a composite primary key?

A A primary key made of multiple columns combined.

B A primary key that allows NULL values.

C A primary key that is automatically generated.

D A primary key that references another table.

A composite primary key uses more than one column to uniquely identify rows.

Explain what a primary key is and why it is important in a database table.

Think about how you identify a person uniquely in real life.

Describe the difference between a primary key and a unique constraint.

Consider how strict each rule is about NULL values.

Practice

1. Which of the following is TRUE about a PRIMARY KEY in a SQL table?

easy

A. It allows duplicate values but no NULLs.

B. It uniquely identifies each row and cannot contain NULL values.

C. It can contain multiple NULL values.

D. It is used only for indexing and does not enforce uniqueness.

Solution

1. Step 1: Understand the role of PRIMARY KEY

   A PRIMARY KEY uniquely identifies each row in a table and does not allow NULL values to ensure every row is distinct.

2. Step 2: Compare options with PRIMARY KEY properties

   It uniquely identifies each row and cannot contain NULL values. correctly states uniqueness and no NULLs. Other options contradict these rules.

3. Final Answer:

   It uniquely identifies each row and cannot contain NULL values. -> Option B

4. Quick Check:

   PRIMARY KEY = unique + no NULL [OK]

Hint: Primary key = unique + no NULL values [OK]

Common Mistakes:

• Thinking primary keys allow NULLs
• Confusing primary key with unique constraint
• Assuming primary key allows duplicates

2. Which SQL statement correctly defines a column id as a primary key in a table?

easy

A. CREATE TABLE users (id INT, name VARCHAR(50) PRIMARY KEY);

B. CREATE TABLE users (id INT UNIQUE, name VARCHAR(50));

C. CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(50));

D. CREATE TABLE users (id INT NOT NULL, name VARCHAR(50));

Solution

1. Step 1: Identify correct PRIMARY KEY syntax

   The correct syntax to define a primary key inline is column_name data_type PRIMARY KEY.

2. Step 2: Check each option

   CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(50)); uses correct syntax. CREATE TABLE users (id INT UNIQUE, name VARCHAR(50)); uses UNIQUE, not PRIMARY KEY. CREATE TABLE users (id INT NOT NULL, name VARCHAR(50)); only uses NOT NULL. CREATE TABLE users (id INT, name VARCHAR(50) PRIMARY KEY); incorrectly assigns PRIMARY KEY to the wrong column.

3. Final Answer:

   CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(50)); -> Option C

4. Quick Check:

   PRIMARY KEY syntax = column_name data_type PRIMARY KEY [OK]

Hint: PRIMARY KEY goes right after column type [OK]

Common Mistakes:

• Using UNIQUE instead of PRIMARY KEY
• Placing PRIMARY KEY on wrong column
• Omitting PRIMARY KEY keyword

3. Given the table employees with id as PRIMARY KEY, what happens if you run this SQL?

INSERT INTO employees (id, name) VALUES (1, 'Alice');
INSERT INTO employees (id, name) VALUES (1, 'Bob');

medium

A. The second insert fails due to duplicate primary key.

B. Both rows are inserted successfully.

C. The second insert overwrites the first row.

D. Both inserts fail because primary keys cannot be integers.

Solution

1. Step 1: Understand PRIMARY KEY uniqueness constraint

   PRIMARY KEY enforces unique values. The first insert adds id=1 successfully.

2. Step 2: Analyze second insert with duplicate id

   The second insert tries to add id=1 again, violating uniqueness, causing an error.

3. Final Answer:

   The second insert fails due to duplicate primary key. -> Option A

4. Quick Check:

   Duplicate PRIMARY KEY value causes insert failure [OK]

Hint: Duplicate primary keys cause insert errors [OK]

Common Mistakes:

• Assuming duplicates are allowed in primary key
• Thinking second insert overwrites first
• Believing primary keys cannot be integers

4. You have this table definition:

CREATE TABLE products (
  product_id INT UNIQUE,
  name VARCHAR(100)
);

Which problem will occur if you try to insert two rows with product_id as NULL?

medium

A. Both rows insert successfully because UNIQUE allows multiple NULLs.

B. The second insert fails because UNIQUE does not allow duplicate NULLs.

C. Syntax error because NULL is not allowed in UNIQUE columns.

D. Both inserts fail because product_id is not a PRIMARY KEY.

Solution

1. Step 1: Understand UNIQUE constraint with NULLs

   UNIQUE allows only one NULL in some databases, but standard SQL allows multiple NULLs because NULL means unknown.

2. Step 2: Apply to given inserts

   In most SQL systems, inserting multiple rows with NULL in a UNIQUE column succeeds because NULLs are not considered equal.

3. Final Answer:

   Both rows insert successfully because UNIQUE allows multiple NULLs. -> Option A

4. Quick Check:

   UNIQUE allows multiple NULLs in most SQL [OK]

Hint: UNIQUE allows multiple NULLs, PRIMARY KEY does not [OK]

Common Mistakes:

• Assuming UNIQUE disallows multiple NULLs
• Confusing UNIQUE with PRIMARY KEY NULL rules
• Expecting syntax error on NULL in UNIQUE

5. You want to create a table orders where each order has a unique order_id and a unique tracking_number that can be NULL. Which SQL definition is correct?

hard

A. CREATE TABLE orders (order_id INT UNIQUE, tracking_number VARCHAR(20) PRIMARY KEY);

B. CREATE TABLE orders (order_id INT, tracking_number VARCHAR(20) UNIQUE PRIMARY KEY);

C. CREATE TABLE orders (order_id INT PRIMARY KEY UNIQUE, tracking_number VARCHAR(20) NOT NULL UNIQUE);

D. CREATE TABLE orders (order_id INT PRIMARY KEY, tracking_number VARCHAR(20) UNIQUE);

Solution

1. Step 1: Define order_id as PRIMARY KEY

   order_id must be unique and not NULL, so PRIMARY KEY is appropriate.

2. Step 2: Define tracking_number as UNIQUE and nullable

   tracking_number should be unique but can be NULL, so UNIQUE constraint without NOT NULL is correct.

3. Step 3: Evaluate options

   CREATE TABLE orders (order_id INT PRIMARY KEY, tracking_number VARCHAR(20) UNIQUE); matches requirements. CREATE TABLE orders (order_id INT UNIQUE, tracking_number VARCHAR(20) PRIMARY KEY); wrongly assigns PRIMARY KEY to tracking_number. CREATE TABLE orders (order_id INT PRIMARY KEY UNIQUE, tracking_number VARCHAR(20) NOT NULL UNIQUE); makes tracking_number NOT NULL, disallowing NULLs. CREATE TABLE orders (order_id INT, tracking_number VARCHAR(20) UNIQUE PRIMARY KEY); has invalid syntax combining UNIQUE and PRIMARY KEY on tracking_number.

4. Final Answer:

   CREATE TABLE orders (order_id INT PRIMARY KEY, tracking_number VARCHAR(20) UNIQUE); -> Option D

5. Quick Check:

   PRIMARY KEY unique + NOT NULL, UNIQUE allows NULL [OK]

Hint: PRIMARY KEY = unique+not null; UNIQUE allows NULL [OK]

Common Mistakes:

• Making UNIQUE column NOT NULL when NULLs allowed
• Assigning PRIMARY KEY to wrong column
• Combining UNIQUE and PRIMARY KEY on same column incorrectly