Add primary key to existing table

drop constraint and recreate it

alter table Persion drop CONSTRAINT <constraint_name>alter table Persion add primary key (persionId,Pname,PMID)

edit:

you can find the constraint name by using the query below:

select OBJECT_NAME(OBJECT_ID) AS NameofConstraintFROM sys.objectswhere OBJECT_NAME(parent_object_id)='Persion'and type_desc LIKE '%CONSTRAINT'

I think something like this should work

-- drop current primary key constraintALTER TABLE dbo.persion DROP CONSTRAINT PK_persionId;GO-- add new auto incremented fieldALTER TABLE dbo.persion ADD pmid BIGINT IDENTITY;GO-- create new primary key constraintALTER TABLE dbo.persion ADD CONSTRAINT PK_persionId PRIMARY KEY NONCLUSTERED (pmid, persionId);GO

-- create new primary key constraintALTER TABLE dbo.persion ADD CONSTRAINT PK_persionId PRIMARY KEY NONCLUSTERED (pmid, persionId);

is a better solution because you have control over the naming of the primary_key.

It's better than just using

ALTER TABLE Persion ADD PRIMARY KEY(persionId,Pname,PMID)

which yeilds randomized names and can cause problems when scripting out or comparing databases