postgresql primary key
CREATE TABLE users (
id SERIAL PRIMARY KEY, # Primary key
username VARCHAR(50),
col_name3 data_type,
…
);
CREATE TABLE posts (
id SERIAL PRIMARY KEY, # Primary key
url VARCHAR(200),
user_id INTEGER REFERENCES users(id), # foreign key
col_name3 data_type,
col_name4 data_type key_type,
…
);