Types of Integrity Constraints

Oracle query optimizer can use declarations to learn more about data to improve overall query performance. (Also, taking integrity rules out of application code and database triggers guarantees that checks are only made when necessary.)

Flexibility for Data Loads and Identification of Integrity Violations

You can disable integrity constraints temporarily so that large amounts of data can be loaded without the overhead of constraint checking. When the data load is complete, you can easily enable the integrity constraints, and you can automatically report any new rows that violate integrity constraints to a separate exceptions table.

The Performance Cost of Integrity Constraints

The advantages of enforcing data integrity rules come with some loss in performance. In general, the cost of including an integrity constraint is, at most, the same as executing a SQL statement that evaluates the constraint.

Types of Integrity Constraints

You can use the following integrity constraints to impose restrictions on the input of column values:

NOT NULL Integrity Constraints

By default, all columns in a table allow nulls. Null means the absence of a value. A NOT NULL constraint requires a column of a table contain no null values. For example, you can define a NOT NULL constraint to require that a value be input in the last_name column for every row of the employees table.

Figure 21–2 illustrates a NOT NULL integrity constraint.

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Types of Integrity Constraints

Figure 21–2 NOT NULL Integrity Constraints

UNIQUE Key Integrity Constraints

A UNIQUE key integrity constraint requires that every value in a column or set of columns (key) be unique—that is, no two rows of a table have duplicate values in a specified column or set of columns.

For example, in Figure 21–3 a UNIQUE key constraint is defined on the DNAME column of the departments table to disallow rows with duplicate department names.

21-8 Oracle9i Database Concepts

Types of Integrity Constraints

Figure 21–3 A UNIQUE Key Constraint

UNIQUE Key Constraint

(no row may duplicate a value in the constraint's column)

Table DEPT

Unique Keys

The columns included in the definition of the UNIQUE key constraint are called the unique key. Unique key is often incorrectly used as a synonym for the terms

UNIQUE key constraint or UNIQUE index. However, note that key refers only to the column or set of columns used in the definition of the integrity constraint.

If the UNIQUE key consists of more than one column, that group of columns is said to be a composite unique key. For example, in Figure 21–4 the customer table has a UNIQUE key constraint defined on the composite unique key: the area and phone columns.

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Types of Integrity Constraints

This UNIQUE key constraint lets you enter an area code and telephone number any number of times, but the combination of a given area code and given telephone number cannot be duplicated in the table. This eliminates unintentional duplication of a telephone number.

UNIQUE Key Constraints and Indexes

Oracle enforces unique integrity constraints with indexes. For example, in Figure 21–4, Oracle enforces the UNIQUE key constraint by implicitly creating a unique index on the composite unique key. Therefore, composite UNIQUE key constraints have the same limitations imposed on composite indexes: up to 32 columns can constitute a composite unique key.

21-10 Oracle9i Database Concepts