MySQL's Constraints

1. Common constraint types

2. PRIMARY KEY constraint: The data that requires primary key columns is unique and cannot be empty, and the primary key can uniquely identify a record. A table has a maximum of one primary KEY constraint.

1). The primary key can be divided into single-Chini and multi-field federated primary keys.

2). The primary key can be added at the time of the table, or can be added later: ALTER TABLE user modify column name type primary key;

3). When you add a PRIMARY KEY constraint, you cannot have data in the database.

MySQL's Constraints