A constraint is a rule that the database manager enforces.

There are four types of constraints:

• A unique constraint is a rule that forbids duplicate values in one or more columns within a table. Unique and primary keys are the supported unique constraints. For example, a unique constraint can be defined on the supplier identifier in the supplier table to ensure that the same supplier identifier is not given to two suppliers.
  A unique constraint is the rule that the values of a key are valid only if they are unique within a table. Unique constraints are optional and can be defined in the CREATE TABLE or ALTER TABLE statement using the PRIMARY KEY clause or the UNIQUE clause. The columns specified in a unique constraint must be defined as NOT NULL. The database manager uses a unique index to enforce the uniqueness of the key during changes to the columns of the unique constraint.
  A table can have an arbitrary number of unique constraints, with at most one unique constraint defined as the primary key. A table cannot have more than one unique constraint on the same set of columns.
  A unique constraint that is referenced by the foreign key of a referential constraint is called the parent key.
  When a unique constraint is defined in a CREATE TABLE statement, a unique index is automatically created by the database manager and designated as a primary or unique system-required index.
  When a unique constraint is defined in an ALTER TABLE statement and an index exists on the same columns, that index is designated as unique and system-required. If such an index does not exist, the unique index is automatically created by the database manager and designated as a primary or unique system-required index.
  Note that there is a distinction between defining a unique constraint and creating a unique index. Although both enforce uniqueness, a unique index allows nullable columns and generally cannot be used as a parent key.
• A referential constraint is a logical rule about values in one or more columns in one or more tables. For example, a set of tables shares information about a corporation's suppliers. Occasionally, a supplier's name changes. You can define a referential constraint stating that the ID of the supplier in a table must match a supplier ID in the supplier information. This constraint prevents insert, update, or delete operations that would otherwise result in missing supplier information.
  Referential integrity is the state of a database in which all values of all foreign keys are valid. A foreign keyis a column or a set of columns in a table whose values are required to match at least one primary key or unique key value of a row in its parent table. A referential constraint is the rule that the values of the foreign key are valid only if one of the following conditions is true:
  They appear as values of a parent key.
  Some component of the foreign key is null.
  The table containing the parent key is called the parent table of the referential constraint, and the table containing the foreign key is said to be a dependent of that table.
  Referential constraints are optional and can be defined in the CREATE TABLE statement or the ALTER TABLE statement. Referential constraints are enforced by the database manager during the execution of INSERT, UPDATE, DELETE, ALTER TABLE, ADD CONSTRAINT, and SET INTEGRITY statements.
  Referential constraints with a delete or an update rule of RESTRICT are enforced before all other referential constraints. Referential constraints with a delete or an update rule of NO ACTION behave like RESTRICT in most cases.
  Note that referential constraints, check constraints, and triggers can be combined.
  Referential integrity rules involve the following concepts and terminology:
• A table check constraint sets restrictions on data added to a specific table. For example, a table check constraint can ensure that the salary level for an employee is at least $20,000 whenever salary data is added or updated in a table containing personnel information.
  A table check constraint is a rule that specifies the values allowed in one or more columns of every row in a table. A constraint is optional, and can be defined using the CREATE TABLE or the ALTER TABLE statement. Specifying table check constraints is done through a restricted form of a search condition. One of the restrictions is that a column name in a table check constraint on table T must identify a column of table T.
  A table can have an arbitrary number of table check constraints. A table check constraint is enforced by applying its search condition to each row that is inserted or updated. An error occurs if the result of the search condition is false for any row.
  When one or more table check constraints is defined in the ALTER TABLE statement for a table with existing data, the existing data is checked against the new condition before the ALTER TABLE statement completes. The SET INTEGRITY statement can be used to put the table in check pending state, which allows the ALTER TABLE statement to proceed without checking the data.
• An informational constraint is a rule that can be used by the SQL compiler, but that is not enforced by the database manager.
  An informational constraint is a rule that can be used by the SQL compiler to improve the access path to data. Informational constraints are not enforced by the database manager, and are not used for additional verification of data; rather, they are used to improve query performance.
  Use the CREATE TABLE or ALTER TABLE statement to define a referential or table check constraint, specifying constraint attributes that determine whether or not the database manager is to enforce the constraint and whether or not the constraint is to be used for query optimization.

Referential and table check constraints can be turned on or off. It is generally a good idea, for example, to turn off the enforcement of a constraint when large amounts of data are loaded into a database.