custom primary key postgres code example

Example 1: postgres alter table add primary key

-- To set a column as primary key:
ALTER TABLE distributors ADD PRIMARY KEY (tablename);

Example 2: postgres composite primary key

CREATE TABLE course_grades (
    quarter_id INTEGER,
    course_id TEXT,
    student_id INTEGER,
    grade INTEGER,
    PRIMARY KEY(quarter_id, course_id, student_id)
);