Constraints

 Constraints keep user data constrained, thereby helping to prevent invalid data from being entered into the database. Defining a data type for a column is a constraint in itself. For example, a column of type DATE constrains the column to valid dates.

This chapter covers a variety of constraints. We have already shown DEFAULT  can be specified at table creation. Constraints are defined at table creation in a similar way.

 

• NOT NULL
• UNIQUE
• PRIMARY KEY
• Foreign Key/REFERENCES
  • Modification of Primary Key Row
  • Multicolumn Primary Keys
  • Handling NULL Values in the Foreign Key
  • Frequency of Foreign Key Checking
• CHECK
• Summary