A CONSTRAINT clause is an optional part of a CREATE TABLE statement or an ALTER TABLE statement. A constraint is a rule to which data must conform. Constraint names are optional.
See CREATE TABLE statement and rrefsqlj81859.html# for details on those statements.
Column-level constraints refer to a single column in the table and do not specify a column name (except check constraints). They refer to the column that they follow.
Table-level constraints refer to one or more columns in the table. Table-level constraints specify the names of the columns to which they apply. Table-level CHECK constraints can refer to 0 or more columns in the table.
Specifies that this column cannot hold NULL values (constraints of this type are not nameable).
Specifies the column that uniquely identifies a row in the table. The identified columns must be defined as NOT NULL.
Specifies that values in the column must be unique.
Specifies that the values in the column must correspond to values in a referenced primary key or unique key column or that they are NULL.
Specifies rules for values in the column.
Specifies the column or columns that uniquely identify a row in the table. NULL values are not allowed.
Specifies that values in the columns must be unique.
Specifies that the values in the columns must correspond to values in referenced primary key or unique columns or that they are NULL.
Specifies a wide range of rules for values in the table.
Column constraints and table constraints have the same function; the difference is in where you specify them. Table constraints allow you to specify more than one column in a PRIMARY KEY, UNIQUE, CHECK, or FOREIGN KEY constraint definition. Column-level constraints (except for check constraints) refer to only one column.
A constraint operates with the privileges of the owner of the constraint. See "Using SQL standard authorization" and "Privileges on views, triggers, and constraints" in the Derby Developer's Guide for details.
Constraints can be deferred, meaning that Derby does not check constraints immediately. By default, a constraint is checked as soon as a statement completes. Deferrable constraints allow temporary breaches of constraints for more flexible insert and update operations.
When a deferrable constraint's constraint mode is DEFERRED before execution of a statement starts, the checking of the constraint does not take place at the end of the statement execution as usual, but only when it is explicitly or implicitly requested using one of the following mechanisms:
The point at which a deferrable constraint is checked is referred to as the deferred checking time.
If the constraint mode of a constraint is IMMEDIATE before a call to a stored procedure or function, and the stored procedure or function sets the constraint mode of that constraint to DEFERRED, the constraint mode is implicitly reset to IMMEDIATE on return from the stored procedure. This happens because the constraint mode is pushed on a stack when we enter the stored procedure or function (as are other session state variables, like the current role). If a constraint violation happens as a result, the transaction is rolled back and an exception is thrown.
See Referential actions for information about the behavior of deferrable foreign keys.
A primary key defines the set of columns that uniquely identifies rows in a table.
When you create a primary key constraint, none of the columns included in the primary key can have NULL constraints; that is, they must not permit NULL values.
ALTER TABLE ADD PRIMARY KEY allows you to include existing columns in a primary key if they were first defined as NOT NULL. NULL values are not allowed. If the column(s) contain NULL values, the system will not add the primary key constraint. See ALTER TABLE statement for more information.
A table can have at most one PRIMARY KEY constraint.
A UNIQUE constraint defines a set of columns that uniquely identify rows in a table only if all the key values are not NULL. If one or more key parts are NULL, duplicate keys are allowed.
For example, if there is a UNIQUE constraint on col1 and col2 of a table, the combination of the values held by col1 and col2 will be unique as long as these values are not NULL. If one of col1 and col2 holds a NULL value, there can be another identical row in the table.
A table can have multiple UNIQUE constraints.
Foreign keys provide a way to enforce the referential integrity of a database. A foreign key is a column or group of columns within a table that references a key in some other table (or sometimes, though rarely, the same table). The foreign key must always include the columns of which the types exactly match those in the referenced primary key or unique constraint.
For a table-level foreign key constraint in which you specify the columns in the table that make up the constraint, you cannot use the same column more than once.
If there is a column list in the ReferencesSpecification (a list of columns in the referenced table), it must correspond either to a unique constraint or to a primary key constraint in the referenced table. The ReferencesSpecification can omit the column list for the referenced table if that table has a declared primary key.
If there is no column list in the ReferencesSpecification and the referenced table has no primary key, a statement exception is thrown. (This means that if the referenced table has only unique keys, you must include a column list in the ReferencesSpecification.)
If the REFERENCES clause contains a CASCADE or SET NULL referential action, the primary or unique key referenced must not be deferrable.
When you insert into or update a table with an enabled foreign key constraint, Derby checks that the row does not violate the foreign key constraint by looking up the corresponding referenced key in the referenced table. If the constraint is not satisfied, Derby rejects the insert or update with a statement exception.
When you update or delete a row in a table with a referenced key (a primary or unique constraint referenced by a foreign key), Derby checks every foreign key constraint that references the key to make sure that the removal or modification of the row does not cause a constraint violation. If removal or modification of the row would cause a constraint violation, the update or delete is not permitted and Derby throws a statement exception.
If the constraint mode is IMMEDIATE (the default), Derby performs constraint checks at the time the statement is executed. If the constraint mode is DEFERRED, the checking is done later, typically at commit time. See Deferrable constraints for more information.
UNIQUE, PRIMARY KEY, and FOREIGN KEY constraints generate indexes that enforce or "back" the constraint (and are sometimes called backing indexes). PRIMARY KEY constraints generate unique indexes. FOREIGN KEY constraints generate non-unique indexes. UNIQUE constraints generate unique indexes if all the columns are non-nullable, and they generate non-unique indexes if one or more columns are nullable. Therefore, if a column or set of columns has a UNIQUE, PRIMARY KEY, or FOREIGN KEY constraint on it, you do not need to create an index on those columns for performance. Derby has already created it for you. See Indexes and constraints.
These indexes are available to the optimizer for query optimization (see CREATE INDEX statement) and have system-generated names.
You cannot drop backing indexes with a DROP INDEX statement; you must drop the constraint or the table.
A check constraint can be used to specify a wide range of rules for the contents of a table. A search condition (which is a boolean expression) is specified for a check constraint. This search condition must be satisfied for all rows in the table. The search condition is applied to each row that is modified on an INSERT or UPDATE at the time of the row modification. The entire statement is aborted if any check constraint is violated.
If a check constraint is specified as part of a columnDefinition, a column reference can only be made to the same column. Check constraints specified as part of a table definition can have column references identifying columns previously defined in the CREATE TABLE statement.
You can specify an ON DELETE clause and/or an ON UPDATE clause, followed by the appropriate action (CASCADE, RESTRICT, SET NULL, or NO ACTION) when defining foreign keys. These clauses specify whether Derby should modify corresponding foreign key values or disallow the operation, to keep foreign key relationships intact when a primary key value is updated or deleted from a table.
You specify the update and delete rule of a referential constraint when you define the referential constraint.
The update rule applies when a row of either the parent or dependent table is updated. The choices are NO ACTION and RESTRICT.
When a value in a column of the dependent table is updated, and that value is part of a foreign key, NO ACTION is the implicit update rule. NO ACTION means that if a foreign key is updated with a non-null value, the update value must match a value in the parent table's primary key when the update statement is completed. If the update does not match a value in the parent table's primary key, the statement is rejected.
The delete rule applies when a row of the parent table is deleted and that row has dependents in the dependent table of the referential constraint. If rows of the dependent table are deleted as part of a CASCADE on the parent table, the delete operation on the parent table is said to be propagated to the dependent table. If the dependent table is also a parent table, the action specified applies, in turn, to its dependents.
The choices are NO ACTION, RESTRICT, CASCADE, or SET NULL. SET NULL can be specified only if some column of the foreign key allows null values. If the delete rule is:
If ON DELETE is not specified, NO ACTION is the implicit delete rule.
Each referential constraint in which a table is a parent has its own delete rule; all applicable delete rules are used to determine the result of a delete operation. Thus, a row cannot be deleted if it has dependents in a referential constraint with a delete rule of RESTRICT or NO ACTION. Similarly, a row cannot be deleted if the deletion cascades to any of its descendants that are dependents in a referential constraint with the delete rule of RESTRICT or NO ACTION.
If a foreign key's constraint mode is DEFERRED, an insert (or update of a row that changes the foreign key) in the child table will be checked at deferred checking time, notwithstanding the ON DELETE or ON UPDATE referential action specification. If a row in the parent table is deleted (or updated so as to modify the referenced key), the behavior depends on the specification of ON DELETE or ON UPDATE. Only if NO ACTION has been specified is the checking ever deferred. If the primary table's referenced primary or unique key constraint is also deferred, any delete of a parent row can lead to a foreign key violation immediately (or at deferred checking time, if the foreign key is also deferred, as the case may be) when the last of possibly several key duplicates of the referenced key is deleted or updated.
INSERT and UPDATE statements depend on all constraints on the target table. DELETEs depend on unique, primary key, and foreign key constraints. These statements are invalidated if a constraint is added to or dropped from the target table.
-- column-level primary key constraint named OUT_TRAY_PK:
CREATE TABLE SAMP.OUT_TRAY
(
SENT TIMESTAMP,
DESTINATION CHAR(8),
SUBJECT CHAR(64) NOT NULL CONSTRAINT OUT_TRAY_PK PRIMARY KEY,
NOTE_TEXT VARCHAR(3000)
);
-- the table-level primary key definition allows you to
-- include two columns in the primary key definition:
CREATE TABLE SAMP.SCHED
(
CLASS_CODE CHAR(7) NOT NULL,
DAY SMALLINT NOT NULL,
STARTING TIME,
ENDING TIME,
PRIMARY KEY (CLASS_CODE, DAY)
);
-- Use a column-level constraint for an arithmetic check
-- Use a table-level constraint
-- to make sure that a employee's taxes does not
-- exceed the bonus
CREATE TABLE SAMP.EMP
(
EMPNO CHAR(6) NOT NULL CONSTRAINT EMP_PK PRIMARY KEY,
FIRSTNME CHAR(12) NOT NULL,
MIDINIT VARCHAR(12) NOT NULL,
LASTNAME VARCHAR(15) NOT NULL,
SALARY DECIMAL(9,2) CONSTRAINT SAL_CK CHECK (SALARY >= 10000),
BONUS DECIMAL(9,2),
TAX DECIMAL(9,2),
CONSTRAINT BONUS_CK CHECK (BONUS > TAX)
);
-- use a check constraint to allow only appropriate
-- abbreviations for the meals
CREATE TABLE FLIGHTS
(
FLIGHT_ID CHAR(6) NOT NULL ,
SEGMENT_NUMBER INTEGER NOT NULL ,
ORIG_AIRPORT CHAR(3),
DEPART_TIME TIME,
DEST_AIRPORT CHAR(3),
ARRIVE_TIME TIME,
MEAL CHAR(1) CONSTRAINT MEAL_CONSTRAINT
CHECK (MEAL IN ('B', 'L', 'D', 'S')),
PRIMARY KEY (FLIGHT_ID, SEGMENT_NUMBER)
);
-- use the same check constraint, but
-- make the MEAL_CONSTRAINT deferrable
CREATE TABLE FLIGHTS
(
FLIGHT_ID CHAR(6) NOT NULL,
SEGMENT_NUMBER INTEGER NOT NULL,
ORIG_AIRPORT CHAR(3),
DEPART_TIME TIME,
DEST_AIRPORT CHAR(3),
ARRIVE_TIME TIME,
MEAL CHAR(1) CONSTRAINT MEAL_CONSTRAINT
CHECK (MEAL IN ('B', 'L', 'D', 'S'))
DEFERRABLE INITIALLY DEFERRED,
PRIMARY KEY (FLIGHT_ID, SEGMENT_NUMBER)
);
CREATE TABLE METROPOLITAN
(
HOTEL_ID INT NOT NULL CONSTRAINT HOTELS_PK PRIMARY KEY,
HOTEL_NAME VARCHAR(40) NOT NULL,
CITY_ID INT CONSTRAINT METRO_FK REFERENCES CITIES
);
-- create a table with a table-level primary key constraint
-- and a table-level foreign key constraint
CREATE TABLE FLTAVAIL
(
FLIGHT_ID CHAR(6) NOT NULL,
SEGMENT_NUMBER INT NOT NULL,
FLIGHT_DATE DATE NOT NULL,
ECONOMY_SEATS_TAKEN INT,
BUSINESS_SEATS_TAKEN INT,
FIRSTCLASS_SEATS_TAKEN INT,
CONSTRAINT FLTAVAIL_PK PRIMARY KEY (FLIGHT_ID, SEGMENT_NUMBER),
CONSTRAINT FLTS_FK
FOREIGN KEY (FLIGHT_ID, SEGMENT_NUMBER)
REFERENCES Flights (FLIGHT_ID, SEGMENT_NUMBER)
);
-- add a unique constraint to a column
ALTER TABLE SAMP.PROJECT
ADD CONSTRAINT P_UC UNIQUE (PROJNAME);
-- create a table whose city_id column references the
-- primary key in the Cities table
-- using a column-level foreign key constraint
CREATE TABLE CONDOS
(
CONDO_ID INT NOT NULL CONSTRAINT hotels_PK PRIMARY KEY,
CONDO_NAME VARCHAR(40) NOT NULL,
CITY_ID INT CONSTRAINT city_foreign_key
REFERENCES Cities ON DELETE CASCADE ON UPDATE RESTRICT
);