Add primary key to existing table

There is already an primary key in your table. You can't just add primary key,otherwise will cause error. **Because there is one primary key for sql table.**

First, you have to drop your old primary key.

MySQL:

ALTER TABLE Persion
DROP PRIMARY KEY;

SQL Server / Oracle / MS Access:

ALTER TABLE Persion
DROP CONSTRAINT 'constraint name';

> You have to find the constraint name in your table. If you had
> given constraint name when you created table,you can easily use
> the constraint name(ex:PK_Persion).

Second,Add primary key.

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persion ADD PRIMARY KEY (PersionId,Pname,PMID);

or the better one below

ALTER TABLE Persion ADD CONSTRAINT PK_Persion PRIMARY KEY (PersionId,Pname,PMID);
This can set constraint name by developer. It's more easily to maintain the table.



I got a little confuse when i have looked all answers. So I research some document to find every detail. Hope this answer can help other SQL beginner.

Reference:https://www.w3schools.com/sql/sql_primarykey.asp

alter table[Person]
add ID int primary key IDENTITY (1,1)

This will add primary key and indentity seed and populate the new column.