Most of the time, INT datatype is sufficient but when too many records are to be stored at a large scale then the column of the primary key should be defined as BIGINT data type. While assigning the data type to the column of the primary key, we need to be careful because it should contain all the rows that we wish to insert and hence the sufficient number of keys should be available and the range of the data type of the column needs to be defined accordingly.Whenever a primary key is defined on the table, MySQL automatically internally creates an index named PRIMARY on the columns on which the primary key is defined.Most often the column on which the primary key is defined is kept of integer type and AUTO_INCREMENT property is assigned to it as it will lead to the generation of the values of the column uniquely and sequential fashion. A table can have only one column on which the AUTO_INCREMENT property can be assigned.This will lead to faster and quick retrieval of the resultset. Most often, the datatype of the column that is used to define the primary key is kept integer because MySQL works very fast with integers.Any table in MySQL can have only one or none of the primary key defined on it.If we try inserting or updating such columns with NULL value then MySQL will issue an error. Hence, the NULL value cannot be stored in the columns that define the primary key. When the primary key is defined on the column then implicitly the NOT NULL constraint is applied for the column or columns on which the primary key constraint is defined.When a single column is used for defining the primary key then it must contain the unique values stored in each of the rows of the table while in case of defining the primary key on multiple columns the combination of the value of all the columns on which the primary key is defined should have unique value together.
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