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This chapter explains how to enforce the business rules associated with your database and prevent the entry of invalid information into tables by using integrity constraints. Topics include the following:
You can define integrity constraints to enforce business rules on data in your tables. Once an integrity constraint is enabled, all data in the table must conform to the rule that it specifies. If you subsequently issue a SQL statement that modifies data in the table, Oracle ensures that the resulting data satisfies the integrity constraint. Without integrity constraints, such business rules must be enforced programmatically by your application.
Enforcing rules with integrity constraints is less costly than enforcing the equivalent rules by issuing SQL statements in your application. The semantics of integrity constraints are very clearly defined, so the internal operations that Oracle performs to enforce them are optimized beneath the level of SQL statements in Oracle. Since your applications use SQL, they cannot achieve this level of optimization.
Enforcing business rules with SQL statements can be even more costly in a networked environment because the SQL statements must be transmitted over a network. In such cases, using integrity constraints eliminates the performance overhead incurred by this transmission.
To ensure that each employee in the EMP table works for a department that is listed in the DEPT table, first create a PRIMARY KEY constraint on the DEPTNO column of the DEPT table with this statement:
ALTER TABLE deptADD PRIMARY KEY (deptno)
Then create a referential integrity constraint on the DEPTNO column of the EMP table that references the primary key of the DEPT table:
ALTER TABLE empADD FOREIGN KEY (deptno) REFERENCES dept(deptno)
If you subsequently add a new employee record to the table, Oracle automatically ensures that its department number appears in the department table.
To enforce this rule without integrity constraints, your application must test each new employee record to ensure that its department number belongs to an existing department. This testing involves issuing a SELECT statement to query the DEPT table.
For best performance, define and enable integrity constraints and develop your applications to rely on them, rather than on SQL statements in your applications, to enforce business rules.
However, in some cases, you might want to enforce business rules through your application as well as through integrity constraints. Enforcing a business rule in your application might provide faster feedback to the user than an integrity constraint. For example, if your application accepts 20 values from the user and then issues an INSERT statement containing these values, you might want your user to be notified immediately after entering a value that violates a business rule.
Since integrity constraints are enforced only when a SQL statement is issued, an integrity constraint can only notify the user of a bad value after the user has entered all 20 values and the application has issued the INSERT statement. However, you can design your application to verify the integrity of each value as it is entered and notify the user immediately in the event of a bad value.
By default, all columns can contain nulls. Only define NOT NULL constraints for columns of a table that absolutely require values at all times.
For example, in the EMP table, it might not be detrimental if an employee's manager or hire date were temporarily omitted. Also, some employees might not have a commission. Therefore, these three columns would not be good candidates for NOT NULL integrity constraints. However, it might not be permitted to have a row that does not have an employee name. Therefore, this column is a good candidate for the use of a NOT NULL integrity constraint.
NOT NULL constraints are often combined with other types of integrity constraints to further restrict the values that can exist in specific columns of a table. Use the combination of NOT NULL and UNIQUE key integrity constraints to force the input of values in the UNIQUE key; this combination of data integrity rules eliminates the possibility that any new row's data will ever attempt to conflict with an existing row's data. For more information about such combinations.
Legal default values include any literal, or any expression that does not refer to a column, LEVEL, ROWNUM, or PRIOR. Default values can include the expressions SYSDATE, USER, USERENV, and UID. The datatype of the default literal or expression must match or be convertible to the column datatype.
If you do not explicitly define a default value for a column, the default for the column is implicitly set to NULL.
Only assign default values to columns that contain a typical value. For example, in the DEPT table, if most departments are located at one site, the default value for the LOC column can be set to this value (such as NEW YORK).
Defaults are also useful when you use a view to make a subset of a table's columns visible. For example, you might allow users to insert rows into a table through a view. The view is defined to show all columns pertinent to end-user operations; however, the base table might also have a column named INSERTER, not included in the definition of the view, which logs the user that originally inserts each row of the table. The column named INSERTER can record the name of the user that inserts a row by defining the column with the USER function:
. . ., inserter VARCHAR2(30) DEFAULT USER, . . .
For another example of assigning a default column value, refer to the section "Creating Tables" on page 4-3.
Each table can have one primary key. A primary key allows each row in a table to be uniquely identified and ensures that no duplicate rows exist. Use the following guidelines when selecting a primary key:
The purpose of a table's primary key is to uniquely identify each row of the table. Therefore, the column or set of columns in the primary key must contain unique values for each row.
A primary key value is only used to identify a row in the table; primary key values should never contain any data that is used for any other purpose. Therefore, primary key values should rarely need to be changed.
A PRIMARY KEY constraint, by definition, does not allow the input of any row with a null in any column that is part of the primary key.
Short primary keys are easy to type. You can use sequence numbers to easily generate numeric primary keys.
Although composite primary keys are allowed, they do not satisfy the previous recommendations. For example, composite primary key values are long and cannot be assigned by sequence numbers.
Choose unique keys carefully. In many situations, unique keys are incorrectly comprised of columns that should be part of the table's primary key (see the previous section for more information about primary keys). When deciding whether to use a UNIQUE key constraint, use the rule that a UNIQUE key constraint is only required to prevent the duplication of the key values within the rows of the table. The data in a unique key is such that it cannot be duplicated in the table.
Do not confuse the concept of a unique key with that of a primary key. Primary keys are used to identify each row of the table uniquely. Therefore, unique keys should not have the purpose of identifying rows in the table.
Some examples of good unique keys include
AREA and PHONE (the primary key is the customer number)
Whenever two tables are related by a common column (or set of columns), define a PRIMARY or UNIQUE key constraint on the column in the parent table, and define a FOREIGN KEY constraint on the column in the child table, to maintain the relationship between the two tables. Depending on this relationship, you may want to define additional integrity constraints including the foreign key, as listed in the section "Relationships Between Parent and Child Tables" on page 9-9.
Figure 9-3 shows a foreign key defined on the DEPTNO column of the EMP table. It guarantees that every value in this column must match a value in the primary key of the DEPT table (the DEPTNO column); therefore, no erroneous department numbers can exist in the DEPTNO column of the EMP table.
Foreign keys can be comprised of multiple columns. However, a composite foreign key must reference a composite primary or unique key of the exact same structure; that is, the same number of columns and datatypes. Because composite primary and unique keys are limited to 16 columns, a composite foreign key is also limited to 16 columns.
By default (that is, without any NOT NULL or CHECK clauses), and in accordance with the ANSI/ISO standard, the FOREIGN KEY constraint enforces the "match none" rule for composite foreign keys. The "full" and "partial" rules can also be enforced by using CHECK and NOT NULL constraints, as follows:
CHECK constraint that allows only all nulls or all non-nulls in the composite foreign key as follows, assuming a composite key comprised of columns A, B, and C:
CHECK ((A IS NULL AND B IS NULL AND C IS NULL) OR (A IS NOT NULL AND B IS NOT NULL AND C IS NOT NULL))
Several relationships between parent and child tables can be determined by the other types of integrity constraints defined on the foreign key in the child table.
When no other constraints are defined on the foreign key, any number of rows in the child table can reference the same parent key value. This model allows nulls in the foreign key.
This model establishes a "one-to-many" relationship between the parent and foreign keys that allows undetermined values (nulls) in the foreign key. An example of such a relationship is shown in Figure 9-3 between EMP and DEPT; each department (parent key) has many employees (foreign key), and some employees might not be in a department (nulls in the foreign key).
When nulls are not allowed in a foreign key, each row in the child table must explicitly reference a value in the parent key because nulls are not allowed in the foreign key. However, any number of rows in the child table can reference the same parent key value.
This model establishes a "one-to-many" relationship between the parent and foreign keys. However, each row in the child table must have a reference to a parent key value; the absence of a value (a null) in the foreign key is not allowed. The same example in the previous section can be used to illustrate such a relationship. However, in this case, employees must have a reference to a specific department.
When a UNIQUE constraint is defined on the foreign key, one row in the child table can reference a parent key value. This model allows nulls in the foreign key.
This model establishes a "one-to-one" relationship between the parent and foreign keys that allows undetermined values (nulls) in the foreign key. For example, assume that the EMP table had a column named MEMBERNO, referring to an employee's membership number in the company's insurance plan. Also, a table named INSURANCE has a primary key named MEMBERNO, and other columns of the table keep respective information relating to an employee's insurance policy. The MEMBERNO in the EMP table should be both a foreign key and a unique key:
EMP and INSURANCE tables (the FOREIGN KEY constraint)
UNIQUE key constraint)
When both UNIQUE and NOT NULL constraints are defined on the foreign key, only one row in the child table can reference a parent key value. Because nulls are not allowed in the foreign key, each row in the child table must explicitly reference a value in the parent key.
This model establishes a "one-to-one" relationship between the parent and foreign keys that does not allow undetermined values (nulls) in the foreign key. If you expand the previous example by adding a NOT NULL constraint on the MEMBERNO column of the EMP table, in addition to guaranteeing that each employee has a unique membership number, you also ensure that no undetermined values (nulls) are allowed in the MEMBERNO column of the EMP table.
Oracle allows a column to be referenced by multiple FOREIGN KEY constraints; effectively, there is no limit on the number of dependent keys. This situation might be present if a single column is part of two different composite foreign keys.
Oracle maximizes the concurrency control of parent keys in relation to dependent foreign key values. You can control what concurrency mechanisms are used to maintain these relationships and, depending on the situation, this can be highly beneficial. The following sections explain the possible situations and give recommendations for each.
Figure 9-4 illustrates the locking mechanisms used by Oracle when no index is defined on the foreign key and when rows are being updated or deleted in the parent table. Inserts into the parent table do not require any locks on the child table.
Notice that a share lock of the entire child table is required until the transaction containing the DELETE statement for the parent table is committed. If the foreign key specifies ON DELETE CASCADE, the DELETE statement results in a table-level share-subexclusive lock on the child table. A share lock of the entire child table is also required for an UPDATE statement on the parent table that affects any columns referenced by the child table. Share locks allow reading only; therefore, no INSERT, UPDATE, or DELETE statements can be issued on the child table until the transaction containing the UPDATE or DELETE is committed. Queries are allowed on the child table.
This situation is tolerable if updates and deletes can be avoided on the parent.
INSERT, UPDATE, and DELETE statements on the child table do not acquire any locks on the parent table; although INSERT and UPDATE statements will wait for a row-lock on the index of the parent table to clear.
Figure 9-5 illustrates the locking mechanisms used by Oracle when an index is defined on the foreign key, and new rows are inserted, updated or deleted in the child table.
Notice that no table locks of any kind are acquired on the parent table or any of its indexes as a result of the insert, update or delete. Therefore, any type of DML statement can be issued on the parent table, including inserts, updates, deletes, and queries.
This situation is preferable if there is any update or delete activity on the parent table while update activity is taking place on the child table. Inserts, updates, and deletes on the parent table do not require any locks on the child table; although updates and deletes will wait for row-level locks on the indexes of the child table to clear.
If the child table specifies ON DELETE CASCADE, deletes from the parent table may result in deletes from the child table. In this case, waiting and locking rules are the same as if you deleted from the child table yourself after performing the delete from the parent table.
Oracle does not permit declarative referential integrity constraints to be defined across nodes of a distributed database (that is, a declarative referential integrity constraint on one table cannot specify a foreign key that references a primary or unique key of a remote table). However, parent/child table relationships across nodes can be maintained using triggers. For more information about triggers that enforce referential integrity, refer to Chapter 13, "Using Database Triggers". Using triggers to maintain referential integrity requires the distributed option; for more information refer to Oracle8 Distributed Database Systems
Use CHECK constraints when you need to enforce integrity rules that can be evaluated based on logical expressions. Never use CHECK constraints when any of the other types of integrity constraints can provide the necessary checking.
Examples of appropriate CHECK constraints include the following:
CHECK constraint on the SAL column of the EMP table so that no salary value is greater than 10000
CHECK constraint on the LOC column of the DEPT table so that only the locations "BOSTON", "NEW YORK", and "DALLAS" are allowed
CHECK constraint on the SAL and COMM columns to compare the SAL and COMM values of a row and prevent the COMM value from being greater than the SAL value
A CHECK integrity constraint requires that a condition be true or unknown for every row of the table. If a statement causes the condition to evaluate to false, the statement is rolled back. The condition of a CHECK constraint has the following limitations:
SYSDATE, UID, USER, or USERENV SQL functions.
LEVEL, PRIOR, or ROWNUM;
When using CHECK constraints, consider the ANSI/ISO standard, which states that a CHECK constraint is violated only if the condition evaluates to false; true and unknown values do not violate a check condition. Therefore, make sure that a CHECK constraint that you define actually enforces the rule you need enforced.
For example, consider the following CHECK constraint:
CHECK (sal > 0 OR comm >= 0)
At first glance, this rule may be interpreted as "do not allow a row in the EMP table unless the employee's salary is greater than zero or the employee's commission is greater than or equal to zero." However, note that if a row is inserted with a null salary and a negative commission, the row does not violate the CHECK constraint because the entire check condition is evaluated as unknown. In this particular case, you can account for such violations by placing NOT NULL integrity constraints on both the SAL and COMM columns.
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Note:
If you are not sure when unknown values result in |
A single column can have multiple CHECK constraints that reference the column in its definition. There is no limit to the number of CHECK constraints that can be defined that reference a column.
According to the ANSI/ISO standard, a NOT NULL integrity constraint is an example of a CHECK integrity constraint, where the condition is
CHECK (column_name IS NOT NULL)
Therefore, NOT NULL integrity constraints for a single column can, in practice, be written in two forms: using the NOT NULL constraint or a CHECK constraint. For ease of use, you should always choose to define NOT NULL integrity constraints instead of CHECK constraints with the "IS NOT NULL" condition.
In the case where a composite key can allow only all nulls or all values, you must use a CHECK integrity constraint. For example, the following expression of a CHECK integrity constraint allows a key value in the composite key made up of columns C1 and C2 to contain either all nulls or all values:
CHECK ((c1 IS NULL AND c2 IS NULL) OR(c1 IS NOT NULL AND c2 IS NOT NULL))
Define an integrity constraint using the constraint clause of the SQL commands CREATE TABLE or ALTER TABLE. The next two sections describe how to use these commands to define integrity constraints.
The following examples of CREATE TABLE statements show the definition of several integrity constraints:
CREATE TABLE dept (deptno NUMBER(3) PRIMARY KEY, dname VARCHAR2(15), loc VARCHAR2(15), CONSTRAINT dname_ukey UNIQUE (dname, loc), CONSTRAINT loc_check1 CHECK (loc IN ('NEW YORK', 'BOSTON', 'CHICAGO')));CREATE TABLE emp (empno NUMBER(5) PRIMARY KEY, ename VARCHAR2(15) NOT NULL, job VARCHAR2(10), mgr NUMBER(5) CONSTRAINT mgr_fkey REFERENCES emp, hiredate DATE, sal NUMBER(7,2), comm NUMBER(5,2), deptno NUMBER(3) NOT NULLCONSTRAINT dept_fkey REFERENCES dept ON DELETE CASCADE);
You can also define integrity constraints using the constraint clause of the ALTER TABLE command. For example, the following examples of ALTER TABLE statements show the definition of several integrity constraints:
ALTER TABLE deptADD PRIMARY KEY (deptno);ALTER TABLE empADD CONSTRAINT dept_fkey FOREIGN KEY (deptno) REFERENCES dept MODIFY (ename VARCHAR2(15) NOT NULL);
Because data is likely to be in the table at the time an ALTER TABLE statement is issued, there are several restrictions to be aware of. Table 9-1 lists each type of constraint and the associated restrictions with the ALTER TABLE command.
* Assumes DISABLE clause not included in statement.
If you attempt to define a constraint with an ALTER TABLE statement and violate one of these restrictions, the statement is rolled back and an informative error is returned explaining the violation.
The creator of a constraint must have the ability to create tables (that is, the CREATE TABLE or CREATE ANY TABLE system privilege) or the ability to alter the table (that is, the ALTER object privilege for the table or the ALTER ANY TABLE system privilege) with the constraint. Additionally, UNIQUE key and PRIMARY KEY integrity constraints require that the owner of the table have either a quota for the tablespace that contains the associated index or the UNLIMITED TABLESPACE system privilege. FOREIGN KEY integrity constraints also require some additional privileges.
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See Also:
"Privileges Required for FOREIGN KEY Integrity Constraints" on page 9-26 for specific information. |
Assign names to NOT NULL, UNIQUE KEY, PRIMARY KEY, FOREIGN KEY, and CHECK constraints using the CONSTRAINT option of the constraint clause. This name must be unique with respect to other constraints that you own. If you do not specify a constraint name, one is assigned by Oracle.
See the previous examples of the CREATE TABLE and ALTER TABLE statements for examples of the CONSTRAINT option of the Constraint clause. Note that the name of each constraint is included with other information about the constraint in the data dictionary.
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See Also:
"Listing Integrity Constraint Definitions" on page 9-27 for examples of data dictionary views. |
By default, whenever an integrity constraint is defined in a CREATE or ALTER TABLE statement, the constraint is automatically enabled (enforced) by Oracle unless it is specifically created in a disabled state using the DISABLE clause.
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See Also:
"Enabling and Disabling Key Integrity Constraints" on page 9-22 for more information about important issues for enabling and disabling constraints. |
When defining UNIQUE key, PRIMARY KEY, and FOREIGN KEY integrity constraints, you should be aware of several important issues and prerequisites. For information about defining and managing FOREIGN KEY constraints
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See Also:
"Managing FOREIGN KEY Integrity Constraints" on page 9-25. UNIQUE key and PRIMARY KEY constraints are usually enabled by the database administrator, and the Oracle8 Administrator's Guide . |
This section explains the mechanisms and procedures for manually enabling and disabling integrity constraints.
In summary, an integrity constraint can be thought of as a statement about the data in a database. This statement is always true when the constraint is enabled; however, the statement may or may not be true when the constraint is disabled because data in violation of the integrity constraint can be in the database.
To enforce the rules defined by integrity constraints, the constraints should always be enabled; however, in certain situations, it is desirable to disable the integrity constraints of a table temporarily for performance reasons. For example:
In cases such as these, integrity constraints may be temporarily turned off to improve the performance of the operation.
If a row of a table does not adhere to an integrity constraint, this row is said to be in violation of the constraint and is known as an exception to the constraint. If any exceptions exist, the constraint cannot be enabled. The rows that violate the constraint must be either updated or deleted in order for the constraint to be enabled.
Exceptions for a specific integrity constraint can be identified while attempting to enable the constraint. This procedure is discussed in the section "Exception Reporting" on page 9-23.
When you define an integrity constraint in a CREATE TABLE or ALTER TABLE statement, you can enable the constraint by including the ENABLE clause in its definition or disable it by including the DISABLE clause in its definition. If neither the ENABLE nor the DISABLE clause is included in a constraint's definition, Oracle automatically enables the constraint.
The following CREATE TABLE and ALTER TABLE statements both define and enable integrity constraints:
CREATE TABLE emp ( empno NUMBER(5) PRIMARY KEY, . . . ); ALTER TABLE emp ADD PRIMARY KEY (empno);
An ALTER TABLE statement that defines and attempts to enable an integrity constraint may fail because rows of the table may violate the integrity constraint. In this case, the statement is rolled back and the constraint definition is not stored and not enabled. Refer to the section "Exception Reporting" on page 9-23 for more information about rows that violate integrity constraints.
The following CREATE TABLE and ALTER TABLE statements both define and disable integrity constraints:
CREATE TABLE emp (empno NUMBER(5) PRIMARY KEY DISABLE, . . . );ALTER TABLE empADD PRIMARY KEY (empno) DISABLE;
An ALTER TABLE statement that defines and disables an integrity constraints never fails. The definition of the constraint is always allowed because its rule is not enforced.
Use the ALTER TABLE command to
The following statements are examples of statements that enable disabled integrity constraints:
ALTER TABLE deptENABLE CONSTRAINT dname_ukey;ALTER TABLE deptENABLE PRIMARY KEY, ENABLE UNIQUE (dname, loc);
An ALTER TABLE statement that attempts to enable an integrity constraint fails when the rows of the table violate the integrity constraint. In this case, the statement is rolled back and the constraint is not enabled. Refer to the section "Exception Reporting" on page 9-23 for more information about rows that violate integrity constraints.
The following statements are examples of statements that disable enabled integrity constraints:
ALTER TABLE deptDISABLE CONSTRAINT dname_ukey;ALTER TABLE deptDISABLE PRIMARY KEY, DISABLE UNIQUE (dname, loc);
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Tip - Using the Data Dictionary for Reference: The example statements in the previous sections require that you have some information about a constraint to enable or disable it. For example, the first statement of each section requires that you know the constraint's name, while the second statement of each section requires that you know the unique key's column list. If you do not have such information, you can query one of the data dictionary views defined for constraints; for more information about these views, see "Listing Integrity Constraint Definitions" on page 9-27 and Oracle8 Reference. |
When enabling or disabling UNIQUE key, PRIMARY KEY, and FOREIGN KEY integrity constraints, you should be aware of several important issues and prerequisites. For more information about enabling, disabling, and managing FOREIGN KEY constraints. UNIQUE key and PRIMARY KEY constraints are usually managed by the database administrator.
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See Also:
"Managing FOREIGN KEY Integrity Constraints" on page 9-25., and the Oracle8 Administrator's Guide. |
SQL*Loader permits multiple concurrent sessions to perform a direct path load into the same table. Because each SQL*Loader session can attempt to re-enable constraints on a table after a direct path load, there is a danger that one session may attempt to re-enable a constraint before another session is finished loading data. In this case, the first session to complete the load will be unable to enable the constraint because the remaining sessions possess share locks on the table.
Because there is a danger that some constraints might not be re-enabled after a direct path load, you should check the status of the constraint after completing the load to ensure that it was enabled properly.
PRIMARY KEY and UNIQUE key constraints create indexes on a table when they are enabled, and subsequently can take a significantly long time to enable after a direct path loading session if the table is very large.
You should consider enabling these constraints manually after a load (and not specify the automatic enable feature). This allows you to manually create the required indexes in parallel to save time before enabling the constraint.
If no exceptions are present when you issue a CREATE TABLE... ENABLE... or ALTER TABLE... ENABLE... statement, the integrity constraint is enabled and all subsequent DML statements are subject to the enabled integrity constraints.
If exceptions exist when you enable a constraint, an error is returned and the integrity constraint remains disabled. When a statement is not successfully executed because integrity constraint exceptions exist, the statement is rolled back. If exceptions exist, you cannot enable the constraint until all exceptions to the constraint are either updated or deleted.
To determine which rows violate the integrity constraint, include the EXCEPTIONS option in the ENABLE clause of a CREATE TABLE or ALTER TABLE statement. The EXCEPTIONS option places the ROWID, table owner, table name, and constraint name of all exception rows into a specified table. For example, the following statement attempts to enable the primary key of the DEPT table; if exceptions exist, information is inserted into a table named EXCEPTIONS:
ALTER TABLE dept ENABLE PRIMARY KEY EXCEPTIONS INTO exceptions;
Create an appropriate exceptions report table to accept information from the EXCEPTIONS option of the ENABLE clause. Create an exception table by submitting the script UTLEXCPT.SQL. The script creates a tabled named EXCEPTIONS. You can create additional exceptions tables with different names by modifying and resubmitting the script.
If duplicate primary key values exist in the DEPT table and the name of the PRIMARY KEY constraint on DEPT is SYS_C00301, the following rows might be placed in the table EXCEPTIONS by the previous statement:
SELECT * FROM exceptions; ROWID OWNER TABLE_NAME CONSTRAINT ------------------ ------ ------------ ----------- AAAA5bAADAAAAEQAAA SCOTT DEPT SYS_C00301 AAAA5bAADAAAAEQAAB SCOTT DEPT SYS_C00301
A more informative query would be to join the rows in an exception report table and the master table to list the actual rows that violate a specific constraint. For example:
SELECT deptno, dname, loc FROM dept, exceptionsWHERE exceptions.constraint = 'SYS_C00301' AND dept.rowid = exceptions.row_id;DEPTNO DNAME LOC ---------- -------------- ------------- 10 ACCOUNTING NEW YORK 10 RESEARCH DALLAS
Rows that violate a constraint must be either updated or deleted from the table that contains the constraint. If updating exceptions, you must change the value that violates the constraint to a value consistent with the constraint or a null (if allowed). After updating or deleting a row in the master table, delete the corresponding rows for the exception in the exception report table to avoid confusion with later exception reports. The statements that update the master table and the exception report table should be in the same transaction to ensure transaction consistency.
For example, to correct the exceptions in the previous examples, the following transaction might be issued:
UPDATE dept SET deptno = 20 WHERE dname = 'RESEARCH';
DELETE FROM exceptions WHERE constraint = 'SYS_C00301';
COMMIT;
When you manage exceptions, your goal should be to eliminate all exceptions in your exception report table. After eliminating all exceptions, you must re-enable the constraint; the constraint is not automatically enabled after the exceptions are handled.
While you are correcting current exceptions for a table with the constraint disabled, other users can issue statements creating new exceptions.
You cannot alter integrity constraints. If you must alter the action defined by a given integrity constraint, drop the existing constraint and create a replacement.
Drop an integrity constraint if the rule that it enforces is no longer true or if the constraint is no longer needed. Drop an integrity constraint using the ALTER TABLE command and the DROP clause. For example, the following statements drop integrity constraints:
ALTER TABLE deptDROP UNIQUE (dname, loc);ALTER TABLE empDROP TABLE emp CASCADE CONSTRAINTS;DROP PRIMARY KEY,DROP CONSTRAINT dept_fkey;
When dropping UNIQUE key, PRIMARY KEY, and FOREIGN KEY integrity constraints, you should be aware of several important issues and prerequisites. For more information about dropping FOREIGN KEY constraints. UNIQUE key and PRIMARY KEY constraints are usually managed by the database administrator.
General information about defining, enabling, disabling, and dropping all types of integrity constraints is given in the previous sections. The following section supplements this information, focusing specifically on issues regarding FOREIGN KEY integrity constraints.
The following topics are of interest when defining FOREIGN KEY integrity constraints.
When defining referential integrity constraints, the corresponding column names of the dependent and referenced tables do not need to match. However, they must be of the same datatype.
Because foreign keys reference primary and unique keys of the parent table, and PRIMARY KEY and UNIQUE key constraints are enforced using indexes, composite foreign keys are limited to 16 columns.
If the column list is not included in the REFERENCES option when defining a FOREIGN KEY constraint (single column or composite), Oracle assumes that you intend to reference the primary key of the specified table. Alternatively, you can explicitly specify the column(s) to reference in the parent table within parentheses. Oracle automatically checks to verify that this column list references a primary or unique key of the parent table. If it does not, an informative error is returned.
To create a FOREIGN KEY constraint, the creator of the constraint must have privileged access to both the parent and the child table.
REFERENCES object privileges on the columns that constitute the parent key of the parent table.
CREATE TABLE or CREATE ANY TABLE system privilege) or the ability to alter the child table (that is, the ALTER object privilege for the child table or the ALTER ANY TABLE system privilege).
In both cases, necessary privileges cannot be obtained via a role; they must be explicitly granted to the creator of the constraint.
These restrictions allow
Oracle allows two different types of referential integrity actions to be enforced, as specified with the definition of a FOREIGN KEY constraint:
FOREIGN KEY constraints enforce the no action restriction; no option needs to be specified when defining the constraint to enforce the no action restriction. For example:
CREATE TABLE emp ( . . ., FOREIGN KEY (deptno) REFERENCES dept);
ON DELETE CASCADE option in the definition of the FOREIGN KEY constraint. For example:
CREATE TABLE emp ( . . ., FOREIGN KEY (deptno) REFERENCES dept ON DELETE CASCADE);
FOREIGN KEY integrity constraints cannot be enabled if the referenced primary or unique key's constraint is not present or not enabled.
The data dictionary contains the following views that relate to integrity constraints:
ALL_CONSTRAINTS
ALL_CONS_COLUMNS
CONSTRAINT_COLUMNS
CONSTRAINT_DEFS
USER_CONSTRAINTS
USER_CONS_COLUMNS
USER_CROSS_REFS
DBA_CONSTRAINTS
DBA_CONS_COLUMNS
DBA_CROSS_REFS
Refer to Oracle8 Reference for detailed information about each view.
Consider the following CREATE TABLE statements that define a number of integrity constraints, and the following examples:
CREATE TABLE dept (deptno NUMBER(3) PRIMARY KEY, dname VARCHAR2(15), loc VARCHAR2(15), CONSTRAINT dname_ukey UNIQUE (dname, loc), CONSTRAINT loc_check1CREATE TABLE emp (CHECK (loc IN ('NEW YORK', 'BOSTON', 'CHICAGO')));empno NUMBER(5) PRIMARY KEY, ename VARCHAR2(15) NOT NULL, job VARCHAR2(10), mgr NUMBER(5) CONSTRAINT mgr_fkeyREFERENCES emp ON DELETE CASCADE,hiredate DATE, sal NUMBER(7,2), comm NUMBER(5,2), deptno NUMBER(3) NOT NULL CONSTRAINT dept_fkey REFERENCES dept);
The following query lists all constraints defined on all tables accessible to you, the user:
SELECT constraint_name, constraint_type, table_name,r_constraint_nameFROM user_constraints;
Considering the example statements at the beginning of this section, a list similar to the one below is returned:
CONSTRAINT_NAME C TABLE_NAME R_CONSTRAINT_NAME --------------- - ----------- ------------------ SYS_C00275 P DEPT DNAME_UKEY U DEPT LOC_CHECK1 C DEPT SYS_C00278 C EMP SYS_C00279 C EMP SYS_C00280 P EMP MGR_FKEY R EMP SYS_C00280 DEPT_FKEY R EMP SYS_C00275
Notice the following:
DNAME_UKEY), while others are system specified (such as SYS_C00275).
CONSTRAINT_TYPE column. The table below summarizes the characters used for each constraint type.
| Constraint Type | Character |
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P |
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U |
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R |
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C |
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Note:
An additional constraint type is indicated by the character "V" in the |
In the previous example, several constraints are listed with a constraint type of "C". To distinguish which constraints are NOT NULL constraints and which are CHECK constraints in the EMP and DEPT tables, issue the following query:
SELECT constraint_name, search_conditionFROM user_constraints WHERE (table_name = 'DEPT' OR table_name = 'EMP') ANDconstraint_type = 'C';
Considering the example CREATE TABLE statements at the beginning of this section, a list similar to the one below is returned:
CONSTRAINT_NAME SEARCH_CONDITION --------------- ---------------------------------------- LOC_CHECK1 loc IN ('NEW YORK', 'BOSTON', 'CHICAGO') SYS_C00278 ENAME IS NOT NULL SYS_C00279 DEPTNO IS NOT NULL
Notice the following:
NOT NULL constraints are clearly identified in the SEARCH_CONDITION column.
CHECK constraints are explicitly listed in the SEARCH_CONDITION column.
The following query lists all columns that constitute the constraints defined on all tables accessible to you, the user:
SELECT constraint_name, table_name, column_name FROM user_cons_columns;
Considering the example statements at the beginning of this section, a list similar to the one below is returned:
CONSTRAINT_NAME TABLE_NAME COLUMN_NAME --------------- ----------- --------------- DEPT_FKEY EMP DEPTNO DNAME_UKEY DEPT DNAME DNAME_UKEY DEPT LOC LOC_CHECK1 DEPT LOC MGR_FKEY EMP MGR SYS_C00275 DEPT DEPTNO SYS_C00278 EMP ENAME SYS_C00279 EMP DEPTNO SYS_C00280 EMP EMPNO
