Answers for "how to add a primary key to an existing table"

add primary key to existing table sql

alter table Persion add primary key (persionId,Pname,PMID)

add primary key to table

ALTER TABLE Persons

ADD PRIMARY KEY (ID);

sql server add primary key to existing table with data

ALTER TABLE Production.TransactionHistoryArchive
   ADD CONSTRAINT PK_TransactionHistoryArchive_TransactionID PRIMARY KEY CLUSTERED (TransactionID);