Rules for Primary Key

Primary Key in MySQL

A MySQL Primary Key is a unique column/field in a table that should not contain duplicate or NULL values and is used to identify each record in the table uniquely....

Rules for Primary Key

Each value in the primary key column must be unique. No Null values are allowed in the primary key column. Primary key value should remain constant throughout the lifespan of the record. Primary keys can be either system-generated such as auto-incremented integers or natural keys such as existing data that uniquely identifies a record (for example- Email)....

How to Add Primary Key in MySQL Table

There are two ways in MySQL to create primary key in a table:...

Using PRIMARY KEY on CREATE TABLE

You can create Primary Key while creating a Table using the CREATE TABLE statement....

Using PRIMARY KEY on ALTER TABLE

You can add a primary key in an already existing table by using the ALTER TABLE statement. If the table has already one primary key then you can’t add another by using the ALTER TABLE statement....

Drop Primary Key

To drop/remove a primary key from a table you can use the ALTER TABLE statement....

Difference between Primary Key and Unique Key

Primary Key Unique Key Each value in the primary key column must be unique. Each value in the unique key column must be unique, but a table can have multiple unique keys. The primary key column cannot contain NULL values. The unique key column allows one NULL value. A table can have only one primary key. A table can have more than one unique key. By default, a primary key creates a clustered index (affects physical storage order of data). The default index for a unique key is non-clustered....

Conclusion

In conclusion, a primary key play an important role in uniquely identifying and distinguishing individual records within a database. This key attribute ensures data integrity and helps us to establish relationships between different tables....