Primary key declaration in MySQL - 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

How do you declare a primary key when creating a table in MySQL?

You declare a primary key using the PRIMARY KEY keyword after the column definition or at the end of the column list in the CREATE TABLE statement.

beginner

Can a primary key column contain NULL values?

No, a primary key column cannot contain NULL values because it must uniquely identify each row.

beginner

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

The database will reject the insert and return an error because primary keys must be unique.

beginner

Show an example of declaring a primary key on a single column in MySQL.

Example:<br>

CREATE TABLE users (<br>  id INT NOT NULL,<br>  name VARCHAR(100),<br>  PRIMARY KEY (id)<br>);

Which of the following is true about a primary key?

A It allows NULL values.

B It can have duplicate values.

C It is optional and does not affect uniqueness.

D It uniquely identifies each row and cannot be NULL.

A primary key must uniquely identify each row and cannot contain NULL values.

How do you declare a primary key on the 'id' column when creating a table?

A PRIMARY KEY id;

B UNIQUE KEY id;

C PRIMARY KEY (id);

D KEY PRIMARY id;

The correct syntax is PRIMARY KEY (id) inside the CREATE TABLE statement.

What will happen if you insert two rows with the same primary key value?

A Both rows will be inserted.

B The second insert will fail with an error.

C The database will automatically change the second value.

D The first row will be deleted.

Primary key values must be unique, so the second insert will fail.

Can a table have more than one primary key?

A No, a table can have only one primary key.

B Yes, multiple primary keys are allowed.

C Yes, but only if they are on different columns.

D Only if the keys are composite keys.

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

Which keyword is used to declare a primary key in MySQL?

A PRIMARY KEY

B UNIQUE

C FOREIGN KEY

D INDEX

The PRIMARY KEY keyword declares the primary key constraint.

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

Think about how you find a unique person in a list.

Describe how to declare a primary key in a MySQL CREATE TABLE statement with an example.

Look at the structure of CREATE TABLE and where PRIMARY KEY fits.