Five constraints
1. Primary key constraint primay key Constraint : uniqueness and non-emptiness
2. The only constraint of Unique Constraint : uniqueness may be empty but only one
3. Check constraints Check Constraint: limit the scope of the series of data format
4. Default constraint Default OCnstraint : the default value of the data
5. Foreign key constraint Foreign keyConstraint: Need to establish a relationship between the two tables and reference the columns of the main table
Syntax example of the five constraints:
1. Add primary key constraints
Alter table table name add Constraint primary key name primary key ( field )
2. Add unique constraints
Alter table table name add Constraint constraint name unique ( field )
3. Add default constraints
Alter table table name add Constraint constraint name default (default content) for field name
4. Add check constraints
Alter table table name add Constraint constraint name check ( field expression )
5. Add foreign key constraints
Alter table table name add COnstraint constraint name foreign key ( field ) references table name (field name)