PRIMARY KEY constraint 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 and that the key is never NULL.

beginner

Can a PRIMARY KEY column contain NULL values?

No, a PRIMARY KEY column cannot contain NULL values. Every row must have a unique and non-null value for the PRIMARY KEY.

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How many PRIMARY KEYs can a table have?

A table can have only one PRIMARY KEY. This key can be made of one or multiple columns (called a composite key).

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What happens if you try to insert a duplicate value into a PRIMARY KEY column?

The database will reject the insert and return an error because PRIMARY KEY values must be unique.

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How do you define a PRIMARY KEY when creating a table in SQL?

You can define a PRIMARY KEY using the syntax: <br>CREATE TABLE table_name (id INT PRIMARY KEY, ...); or by adding a constraint: <br>CREATE TABLE table_name (id INT, ..., PRIMARY KEY (id));

What is the main purpose of a PRIMARY KEY in a table?

A To uniquely identify each row

B To allow duplicate values

C To store large text data

D To allow NULL values

A PRIMARY KEY uniquely identifies each row and does not allow duplicates or NULLs.

How many PRIMARY KEYs can a table have?

A One

B Two

C As many as columns

D None

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

Which of these is true about PRIMARY KEY columns?

A They can contain NULL values

B They can have duplicate values

C They must be unique and not NULL

D They are optional

PRIMARY KEY columns must have unique values and cannot be NULL.

What happens if you insert a row with a duplicate PRIMARY KEY value?

A The old row is deleted

B The row is inserted successfully

C The duplicate is ignored

D The database throws an error

Inserting a duplicate PRIMARY KEY value causes an error because uniqueness is enforced.

Which SQL keyword is used to define a PRIMARY KEY?

A UNIQUE

B PRIMARY KEY

C FOREIGN KEY

D INDEX

The PRIMARY KEY keyword defines the primary key constraint on a column or columns.

Explain what a PRIMARY KEY is and why it is important in a database table.

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

Describe how to define a PRIMARY KEY when creating a new table in SQL.

Use CREATE TABLE statement with PRIMARY KEY.