PostgreSQL Alter table add primary key to existing table

Add primary key to existing table

How to add primary key to existing table. Alter syntax and example.

Syntax

ALTER TABLE tabel_name 
ADD PRIMARY KEY (column_name);

Create table

CREATE TABLE users(
  userid numeric,
  username varchar(100),
  userpass varchar(250),
  usermail varchar(50)  
  );    

Example Add primary key

ALTER TABLE users 
ADD PRIMARY KEY (userid);