PRIMARY KEY

Implies the behaviors of UNIQUE and NOT NULL, that is:

enforces that there are no duplicates.
implies NOT NULL.
creates an index.

Unlike UNIQUE, only one column per table can be marked as PRIMARY KEY.

Example:

# ERROR: can only be used for a single col
CREATE TABLE t (
    id INT PRIMARY KEY,
    val VARCHAR(16) PRIMARY KEY
);
CREATE TABLE t (
    id INT PRIMARY KEY,
    val VARCHAR(16)
);
INSERT INTO t VALUES (1, 'one');
# ERROR: no dupes
INSERT INTO t VALUES (1, 'one2');
# ERROR: cannot be null
INSERT INTO t VALUES (NULL, 'null');
SELECT * FROM t;
DROP TABLE t;