Primary Key

• In MySQL, a primary key is a column or a set of columns that uniquely identifies each row in the table. A primary key column must contain unique values.
• If the primary key consists of multiple columns, the combination of values in these columns must be unique. Additionally, a primary key column cannot contain NULL.
• A table can have either zero or one primary key, but not more than one.

Defining a single-column primary key

You define it for a table when you Create Tables.

CREATE TABLE table_name(
	column1 datatype PRIMARY KEY,
	column2 datatype,
	...
);

In this syntax, you define the PRIMARY KEY constraint as a column constraint.

Additionally, you can put the PRIMARY KEY at the end of the column list:

CREATE TABLE table_name(
	column1 datatype,
	column2 datatype,
	...,
	PRIMARY KEY(column1)
);

In this syntax, you define the PRIMARY KEY constraint as a table constraint.

Defining a multi-column primary key

If the primary key consists of two or more columns, you need to use a table constraint to define the primary key:

CREATE TABLE table_name(
   column1 datatype,
   column2 datatype,
   column3 datatype,
   ...,
   PRIMARY KEY(column1, column2)
);

Adding a primary key to an existing table

We will use ALTER

ALTER TABLE table_name
ADD PRIMARY KEY(column1, column2, ...);

Removing a primary key

We will use DROP

ALTER TABLE table_name
DROP PRIMARY KEY;