Constraints constraints of oracle data base management systems

Copyright © 2009, Oracle. All rights reserved.
Constraints

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 While creating tables in a relational database, you must ensure that
no one should enter invalid data in it. Therefore, you need to apply
certain rules and constraints for columns that specify the kind of
data to be stored.
 If the checks are not applied while defining and creating tables, the
data stored in the tables can become redundant.
 Therefore, it is important to ensure that the data stored in tables is
complete and consistent. The concept of maintaining consistency
and completeness of data is called Data Integrity.
Implementing Data Integrity

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 Data integrity:
 Is enforced to ensure that the data in a database is accurate,
consistent, and reliable.
 Is broadly classified into the following categories:
 Entity integrity
 Domain integrity
 Referential integrity
Implementing Data Integrity (Contd.)

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Entity Integrity
 Ensures that each row can be uniquely identified by an attribute
called the primary key. The primary key column contains unique
value in all the rows. In addition, this column cannot be NULL.
Domain Integrity
 Ensures that only a valid range of values is stored in a column. It
can be enforced by restricting the type of data, the range of values,
and the format of the data.
Referential Integrity
 Ensures that the values of the foreign key match the value of the
corresponding primary key.

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 Constraints:
• Enforce rules that must be followed to maintain consistency
and correctness of the data.
• Constraints prevent the deletion of a table if there are
dependencies.
• The following constraint types are valid:
– NOT NULL
– UNIQUE
– PRIMARY KEY
– FOREIGN KEY
– CHECK
Types of Constraints

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Guidelines
• You can name a constraint, or the Oracle server generates a name with
the SYS_Cn format.
• Create a constraint at either of the following times:
– At the time of creation of table
– After the table has been created
• Check the existing data if added after the creation of the table
• Define a constraint at the column or table level.
• View a constraint in the data dictionary.
• Can be created by using either of the following statements:
CREATE TABLE statement
ALTER TABLE statement

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• Syntax:
CREATE TABLE [schema.]table
(column datatype [DEFAULT expr]
[column_constraint],
...
[table_constraint][,...]);
column,...
[CONSTRAINT constraint_name] constraint_type
(column, ...),
column [CONSTRAINT constraint_name] constraint_type,
Defining Constraints
• Column-level constraint:
• Table-level constraint:
Note
NOT NULL constraints must be defined at the column level.

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CREATE TABLE Student(
Sno Number(5),
Sname Varchar2(12),
. . .
Fees Number(8,2),
CONSTRAINT sno_pk PRIMARY KEY(Sno)
);
CREATE TABLE Student(
Sno Number(5) CONSTRAINT sno_pk PRIMARY KEY,
Sname Varchar2(12),
Course Varchar2(10),
Fees Number(8,2)
);
1
2
Defining Constraints

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• Ensures that null values are not permitted for the column:
NOT NULL constraint
(No row can contain
a null value for
this column.)
Absence of NOT NULL
constraint
(Any row can contain
a null value for this
column.)
NOT NULL
constraint
…
NOT NULL Constraint

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NOT NULL Constraint
Columns Data Type Checks
EmployeeID Number(5) NOT NULL
LeaveStartDate Date NOT NULL
LeaveEndDate Date NOT NULL
LeaveReason varchar(100)
LeaveType Char(2) NOT NULL
• Consider the following EmployeeLeave table.
CREATE TABLE EmployeeLeave(
EmployeeID Number(5) NOT NULL,
LeaveStartDate Date NOT NULL.
LeaveEndDate Date NOT NULL,
LeaveReason Varchar2(15),
LeaveType Char(2) NOT NULL
);

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UNIQUE Constraint
• Ensures that every value in a column or set of columns (key) be
unique—that is, no two rows of a table can have duplicate values in a
specified column or set of columns.
• The column (or set of columns) included in the definition of the
UNIQUE key constraint is called the unique key.
• If the UNIQUE constraint comprises more than one column, that group
of columns is called a composite unique key.
• UNIQUE constraints enable the input of nulls unless you also define
NOT NULL constraints for the same columns.
• The Oracle server enforces the UNIQUE constraint by implicitly
creating a unique index on the unique key column or columns.

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EMPLOYEES
UNIQUE constraint
INSERT INTO
Not allowed:
already exists
Allowed
…
UNIQUE Constraint

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Defining UNIQUE Constraint
CREATE TABLE EmpInf(
EmpCode Number(5),
EmpName Varchar2(15) NOT NULL,
Email Varchar2(25) CONSTRAINT email_unq UNIQUE,
Salary Number(8,2)
);
EmpCode Number(5),
. . .
Salary Number(8,2),
CONSTRAINT email_unq UNIQUE(Email)
);

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Defining UNIQUE Constraint
EmpCode Number(5),
Email Varchar2(25)
CONSTRAINT email_nn NOT NULL
CONSTRAINT email_unq UNIQUE,
Salary Number(8,2)
);
• More than One Constraint

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PRIMARY KEY Constraint
• A PRIMARY KEY constraint creates a primary key for the table. Only
one primary key can be created for each table.
• The PRIMARY KEY constraint is a column or set of columns that
uniquely identifies each row in a table.
• This constraint enforces uniqueness of the column or column
combination and ensures that no column that is part of the primary key
can contain a null value.
• It is similar to the Unique key constraint except that it does not allow
NULLrow.

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DEPARTMENTS
PRIMARY KEY
INSERT INTO
Not allowed
(null value)
Not allowed
(50 already exists)
…
PRIMARY KEY Constraint

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Defining PRIMARY KEY Constraint
CREATE TABLE Department(
DeptNo Number(5) CONSTRAINT dept_pk PRIMARY KEY,
DeptName Varchar2(15) NOT NULL,
Location Varchar2(25) NOT NULL,
);
DeptNo Number(5),
CONSTRAINT dept_pk PRIMARY KEY(DeptNo)
);

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Defining PRIMARY KEY Constraint
• Composite Primary Key
Create Table EmployeeLeave(
EmployeeID Number(5),
LeaveStartDate Date,
LeaveType Char(2) NOT NULL,
CONSTRAINT comp_Key PRIMARY KEY(EmployeeID,LeaveStartDate)
);

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FOREIGN KEY Constraint
• The FOREIGN KEY (or referential integrity) constraint designates a
column or combination of columns as a foreign key and establishes a
relationship between a primary key or a unique key in the same table
or a different table.
• Removes the inconsistency in two tables when the data in one table
depends on the data in another table.
• Always refers the primary key column of another table or in the same
table.
Guidelines
• A foreign key value must match an existing value in the parent table or
be NULL.
• Foreign keys are based on data values and are purely logical, rather
than physical pointers.

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DEPARTMENTS
EMPLOYEES
FOREIGN
KEY
INSERT INTO Not allowed
(9 does not
exist)
Allowed
PRIMARY
KEY
…
…
FOREIGN KEY Constraint

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Defining FOREIGN KEY Constraint
DeptNo Number(4) CONSTRAINT dept_pk PRIMARY KEY,
);
• Create Department Table
• Column-level Foreign Key constraint:
CREATE TABLE EmpDetails(
EmpCode Number(5) CONSTRAINT emp_pk PRIMARY KEY,
Job Varchar2(12),
DeptNo Number(4) CONSTRAINT dept_fk REFERENCES Department(DeptNo),
Salary Number(8,2)
);

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• Table-level Foreign Key constraint:
EmpCode Number(5),
Job Varchar2(12),
Salary Number(8,2),
DeptNo Number(4),
CONSTRAINT emp_pk PRIMARY KEY(EmpCode),
CONSTRAINT dept_fk FOREIGN KEY(DeptNo) REFERENCES Department(DeptNo)
);

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• Self Reference
CREATE TABLE Employee(
EmpNo Number(5) CONSTRAINT emp_pk PRIMARY KEY,
Job Varchar2(12),
Salary Number(8,2),
MgrID Number(5),
DeptNo Number(4),
CONSTRAINT emp_fk FOREIGN KEY(MgrID) REFERENCES Employee,
CONSTRAINT dept_fk FOREIGN KEY(DeptNo) REFERENCES Department(DeptNo)
);

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• FOREIGN KEY: Defines the column in the child table at the
table-constraint level
• REFERENCES: Identifies the table and column in the parent
table
• ON DELETE CASCADE: Deletes the dependent rows in the
child table when a row in the parent table is deleted
• ON DELETE SET NULL: Converts dependent foreign key
values to null
FOREIGN KEY Constraint Keywords

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Defining FOREIGN KEY Constraint with Keyword
CREATE TABLE BatchDetails(
BatchCode Varchar2(6) CONSTRAINT batch_pk PRIMARY KEY,
CrsName Varchar2(15) NOT NULL,
Duration Number(3),
Faculty Varchar2(5),
CONSTRAINT batch_fk FOREIGN KEY(Faculty) REFERENCES Faculty(F_Code)
ON DELETE SET NULL
);
• BatchDetails Table
• Faculty Table
CREATE TABLE Faculty(
F_Code Varchar2(5) CONSTRAINT fac_pk PRIMARY KEY,
F_Name Varchar2(15) NOT NULL,
Exp Number(3),
);

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EmployeeID Number(5) CONSTRAINT emp_pk PRIMARY KEY,
Salary Number(8,2)
);
• EmpInf Table
• EmployeeLeave Table
LeaveType Char(2) NOT NULL,
CONSTRAINT comp_Key PRIMARY KEY(EmployeeID,LeaveStartDate),
CONSTRAINT empid_fk FOREIGN KEY(EmployeeID) REFERENCES EmpDetails(EmployeeID)
ON DELETE CASCADE
);
Defining FOREIGN KEY Constraint with Keyword

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• Defines a condition that each row must satisfy
• The following expressions are not allowed:
– References to CURRVAL, NEXTVAL, LEVEL, and ROWNUM pseudo
columns
– Calls to SYSDATE, UID, USER, and USERENV functions
– Queries that refer to other values in other rows
..., salary NUMBER(2)
CONSTRAINT emp_salary_min
CHECK (salary > 0),...
CHECK Constraint
• A single column can have multiple CHECK constraints that refer to the
column in its definition.
• There is no limit to the number of CHECK constraints that you can define
on a column.

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Defining CHECK Constraint
• Column-level Check constraint:
Create Table EMPL(
EmpCode Number(5),
EmpName Varchar2(12),
Job Varchar2(15),
Bsal Number(6) CONSTRAINT sal_chk CHECK(Bsal>=6000)
);
• Table-level Check constraint:
Create Table Project(
ProjCode Number(5),
Description Varchar2(50),
StartDate Date,
EndDate Date,
CONSTRAINT chk_date CHECK(StartDate<=EndDate)
);

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Defining CHECK Constraint Using IN Operator
LeaveType char(2) CONSTRAINT leave_chk CHECK( LeaveType IN('CL','SL','PL') ),
CONSTRAINT comp_key PRIMARY KEY(EmployeeID,LeaveStartDate),
CONSTRAINT empid_fk FOREIGN KEY(EmployeeID) REFERENCES EmpInf(EmployeeID)
);

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Defining CHECK Constraint Using BETWEEN Operator
Create Table EMPL(
EmpCode Number(5),
Job Varchar2(15),
Bsal Number(6) CONSTRAINT sal_chk CHECK(Bsal>=6000 and Bsal<=15000)
(OR)
Bsal Number(6) CONSTRAINT sal_chk CHECK(Bsal BETWEEN 6000 AND 15000)
);

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Constraints Example
LeaveStartDate Date DEFAULT SYSDATE,
LeaveEndDate Date,
LeaveType Char(2) DEFAULT 'CL',
CONSTRAINT comp_key PRIMARY KEY(EmployeeID,LeaveStartDate),
CONSTRAINT empid_fk FOREIGN KEY(EmployeeID) REFERENCES EmpInf(EmployeeID),
CONSTRAINT leave_chk CHECK(LeaveType IN('CL','SL','PL')),
CONSTRAINT date_chk CHECK(LeaveStartDate<=LeaveEndDate)
);

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Managing Constraints
 Use the ALTER TABLE Statement to:
• Add or Drop a constraint, but not Modify its structure
• Enable or Disable constraints
• Add a NOT NULL constraint by using the MODIFY clause
Adding a Constraint:
ALTER TABLE tablename
ADD [ CONSTRAINT constraint_name ] constraint_type( column_name) ;

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Managing Constraints (Contd.)
• Adding PRIMARY KEY Constraint
• EmpDetails Table
Create Table EmpDetails(
EmpCode Number(5),
Job Varchar2(15),
Bsal Number(6)
DeptNo Number(5)
);
ALTER TABLE EmplDetails ADD CONSTRAINT empno_pk PRIMARY KEY(Empno);
• Adding FOREIGN KEY Constraint
ALTER TABLE EmplDetails ADD CONSTRAINT deptno_fk
FOREIGN KEY(DeptNo) REFERENCES Department(DeptNo)

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• Adding CHECK Constraint
• EmpDetails Table
Create Table EmpDetails(
EmpCode Number(5),
Job Varchar2(15),
Bsal Number(6)
DeptNo Number(5)
);
ALTER TABLE EmpDetails ADD CONSTRAINT bsal_chk CHECK(Bsal>=8000)
• Adding NOT NULL Constraint by using MODIFY Clause
ALTER TABLE EmpDetails MODIFY EmpName NOT NULL

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Dropping a Constraint
DROP PRIMARY KEY | UNIQUE ( column_name )
CONSTRAINT constraint_name [CASCADE];
• Dropping bsal_chk Constraint
To drop a constraint, you can identify the constraint name from the
USER_CONSTRAINTS and USER_CONS_COLUMNS data dictionary views.
SELECT CONSTRAINT_NAME, TABLE_NAME FROM USER_CONSTRAINTS;
ALTER TABLE EmpDetails DROP CONSTRAINT bsal_chk ;

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• Dropping Dependent Constraints
ALTER TABLE EmpDetails DROP CONSTRAINT deptno_fk ;
ALTER TABLE Department DROP PRIMARY KEY ;
(OR)
ALTER TABLE Department DROP CONSTRAINT deptno_pk ;
• Using CASCADE Option
The CASCADE option of the DROP clause causes any dependent
Constraints also to be dropped.
ALTER TABLE Department DROP PRIMARY KEY CASCADE ;
(OR)
ALTER TABLE Department DROP CONSTRAINT deptno_pk CASCADE ;

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Disabling Constraints
• Execute the DISABLE clause of the ALTER TABLE statement to deactivate
an integrity constraint.
• Apply the CASCADE option to disable dependent integrity constraints.
DISABLE CONSTRAINT constraint_name [ CASCADE ];
• Disabling a Constraint
ALTER TABLE EmpDetails DISABLE CONSTRAINT bsal_chk ;
• Using CASCADE option
ALTER TABLE Department DISABLE CONSTRAINT deptno_pk CASCADE ;

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Enabling Constraints
• Activate an integrity constraint currently disabled in the table.
• A UNIQUE or PRIMARY KEY index is automatically created when you enable
ENABLE CONSTRAINT constraint_name ;
• Enabling a Constraint
ALTER TABLE EmpDetails ENABLE CONSTRAINT bsal_chk ;
Guidelines
• If you enable a constraint, that constraint applies to all the data in the table. All the
data in the table must fit the constraint.
• Enabling a primary key constraint that was disabled with the CASCADE option does
not enable any foreign keys that are dependent upon the primary key.

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Cascading Constraints
• The CASCADE CONSTRAINTS clause is used along with the
DROP TABLE and DROP COLUMN clause.
• The CASCADE CONSTRAINTS clause drops all referential
integrity constraints that refer to the primary and unique keys
defined on the dropped columns.
• The CASCADE CONSTRAINTS clause also drops all multicolumn
constraints defined on the dropped columns.

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Create Table Project(
ProjCode Number(5) CONSTRAINT proj_pk PRIMARY KEY,
Description Varchar2(50),
StartDate Date,
EndDate Date,
CONSTRAINT chk_date CHECK(StartDate<=EndDate)
);
• Consider Project Table
ALTER TABLE Project DROP (ProjCode) ; (Assume ProjCode has Dependencies)
ALTER TABLE Project DROP (StartDate) ;
• An error is returned for the following statements:
• Reason:
o ProjCode Column has Dependencies
o StartDate Column is referenced by multicolum constraint chk_date
o Project Table is a Parent Table
DROP TABLE Project;

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ALTER TABLE Project DROP COLUMN ProjCode CASCADE CONSTRAINTS ;
Dropping Constrained Column
Dropping Parent Table
DROP TABLE Project CASCADE CONSTRAINTS ;
ALTER TABLE Project DROP COLUMN StartDate CASCADE CONSTRAINTS ;
ALTER TABLE Project DROP (ProjCode,StartDate) CASCADE CONSTRAINTS ;
• If all columns referenced by the constraints defined on the dropped columns, then
CASCADE CONSTRAINTS is not required.
• For example consider Employee Table, assuming that no other referential constraints
from other tables refer to column EmpNo,
ALTER TABLE Employee DROP (EmpNo, MgrID) ;

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Viewing Constraints
• After creating a table, you can confirm its existence by issuing a DESCRIBE
command. The only constraint that you can verify is the NOT NULL constraint.
To view all constraints on your table, query the USER_CONSTRAINTS table.
SELECT Constraint_Name, Constraint_Type, Search_Condition, Table_Name
FROM User_Constraints;
Note:
 Constraints that are not named by the table owner receive the system-
assigned constraint name.
 In constraint type, C stands for CHECK, P for PRIMARY KEY, R for referential
integrity, and U for UNIQUE key.
 Notice that the NOT NULL constraint is really a CHECK constraint.

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Viewing Constraints (Contd.)
SELECT Constraint_Name, Column_Name
FROM User_Cons_Columns
WHERE Table_Name = ‘EMPLOYEE’ ;
• Viewing the columns associated with the constraint names.

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UPDATE employees
SET department_id = 55
WHERE department_id = 110;
Violating Constraints

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Violating Constraints
You cannot delete a row that contains a primary key that is
used as a foreign key in another table.
DELETE FROM departments
WHERE department_id = 60;

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