Overview of Integrity Constraints

Overview of Integrity Constraints

You can define integrity constraints to enforce business rules on data in your tables. Business rules specify conditions and relationships that must always be true, or must always be false. Because each company defines its own policies about things like salaries, employee numbers, inventory tracking, and so on, you can specify a different set of rules for each database table.

When an integrity constraint applies to a table, all data in the table must conform to the corresponding rule. When you issue a SQL statement that modifies data in the table, Oracle Database ensures that the new data satisfies the integrity constraint, without the need to do any checking within your program.

When to Enforce Business Rules with Integrity Constraints

You can enforce rules by defining integrity constraints more reliably than by adding logic to your application. Oracle Database can check that all the data in a table obeys an integrity constraint faster than an application can.

Example of an Integrity Constraint for a Business Rule

To ensure that each employee works for a valid department, first create a rule that all values in the department table are unique:

ALTER TABLE Dept_tab

ADD PRIMARY KEY (Deptno);

Then, create a rule that every department listed in the employee table must match one of the values in the department table:

ALTER TABLE Emp_tab

ADD FOREIGN KEY (Deptno) REFERENCES Dept_tab(Deptno);

When you add a new employee record to the table, Oracle Database automatically checks that its department number appears in the department table.

To enforce this rule without integrity constraints, you can use a trigger to query the department table and test that each new employee's department is valid. But this method is less reliable than the integrity constraint. SELECT in Oracle Database uses "consistent read", so the query might miss uncommitted changes from other transactions.

3-2 Oracle Database Application Developer's Guide - Fundamentals