CSE 880
Database Systems
Database Constraints
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Introduction
1. CON ST RAIN T S: Constraints are general restrictions on the data in the database.
2. Three Important Components
(a) Event: Execution of insert, delete and update
function on a table.
(b) Condition: Restrictions on the data in the database
defined by the database system or user-procedure.
(c) Action: When the Condition is satisfied, certain
action is taken by the database system or by userprocedure.
3. When an event occurs, condition is checked and if
the condition is true the action is executed.
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Built-in Constraints, Assertions and
Triggers
1. BU ILT IN CON ST RAIN T : Both the condition
and the action are system defined and is created
at the time the table is created using CREAT E
T ABLE command. Example:
CREATE TABLE Student(
Sid NUMBER (5) CONSTRAINT
PK_Student PRIMARY KEY,
Sname VARCHAR2 (30),
------- );
2. ASSERT ION : Here the condition is user defined (not
built-in) and the action is system defined.
Example: A student must take at least two courses: For
example, will not allow a student to delete the last second
course.
CREATE ASSERTION EnrollmentControl
CHECK (Exists( Select count(*)
From StudentCourse
Group by sno
Having count(cno)<2))
Assertion is invoked as a result of insert, delete or update events. CHECK provides the condition and when the
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condition is true violation occurs and the action is taken
by the system which is to prevent the event from taking
place.
3. T RIGGERS: User defines the condition and the action.
Example: Enter the inserted student record into the table EastLansingStudents if the inserted student’s address
is East Lansing.
CREATE TRIGGER InsertStudent
AFTER INSERT ON Student
FOR EACH ROW
WHEN (NEW.Saddr="East Lansing")
BEGIN
INSERT INTO TABLE
EastLansingStudents(Sid, Sname, Saddr, Sdno)
(NEW.Sid, NEW.Sname, NEW.Saddr, NEW.Sdno)
END
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Constraints in Oracle
1. P RIM ARY KEY : A single or a set of columns of
a table with unique or mandatory value. A table can
be defined without a primary key.
Example:
CREATE TABLE Student(
Sid NUMBER (5) CONSTRAINT PK_Student PRIMARY KEY,
Sname VARCHAR2 (30),
Saddr VARCHAR2 (30),
Sdno NUMBER(3),
LisenceNo VARCHAR2(10));
2. Attribute level constraint versus table level constraint:
For concatenated keys the constraint has to be defined for more than one attributes. Therefore, it cannot be defined on each attribute separately, requiring
a table level constraint definition.
example: Concatenated Primary key
CONSTRAINT P K StudentCourse PRIMARY KEY
(sid, cid)
Attribute values of the concatenated key can be NULL
but at least one of the attribute values must be NOT
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NULL. Any of the columns of the concatenated key
can be made NOT NULL explicitly by declaring NOT
NULL against that attribute definition.
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3. U N IQU E: All values in a column or columns must
be unique (allows NULL)
Example:
CREATE TABLE Student(
Sid NUMBER (5) CONSTRAINT PK_Student PRIMARY KEY,
Sname VARCHAR2 (30),
Saddr VARCHAR2 (30),
Sdno NUMBER(3),
LisenceNo VARCHAR2(10) UNIQUE);
4. N OT N U LL: Guarantees that a column has a value
(i.e., value cannot be NULL).
Example:
CREATE TABLE Student(
Sid NUMBER (5) CONSTRAINT PK_Student PRIMARY KEY,
Sname VARCHAR2 (30) NOT NULL,
Saddr VARCHAR2 (30),
Sdno NUMBER(3) NOT NULL,
LisenceNo VARCHAR2(10) UNIQUE);
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5. CHECK: Defines a discrete list of values that a
column can have.
Example:
CREATE TABLE Student(
Sid NUMBER (5) CONSTRAINT PK_Student PRIMARY KEY,
Sname VARCHAR2 (30) NOT NULL,
Saddr VARCHAR2 (30),
Sdno NUMBER(4) NOT NULL CONSTRAINT CK_Sdno
CHECK (Sdno BETWEEN 0 AND 700),
LisenceNo VARCHAR2(10) UNIQUE);
6. F OREIGN KEY : Implements referential integrity
which constraints the values of a column to the current values that exist in another column in the same
or different table.
Example:
CREATE TABLE Student(
Sid NUMBER (5) CONSTRAINT PK_Student PRIMARY KEY,
- - Sdno NUMBER(4) NOT NULL CONSTRAINT CK_Sdno CHECK
(Sdno BETWEEN 0 AND 700)
CONSTRAINT FOREIGN KEY(Sdno)
REFERENCES Department(Dno),
LisenceNo VARCHAR2(10) UNIQUE);
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(a) Parent Table: Department
Child Table: Student
(b) Child Table insert restrict
Child Table update restrict
Parent Table update restrict
Parent Table delete restrict
Parent Table insert and Child Table delete not
restricted.
(c) NOT NULL in the foreign key enforces that the
student must have a department tuple in the Department table.
(d) Delete Cascade:
i. version1: no parent delete when there is a
child.
Example on previous page.
ii. version2: parent delete as well as all its children delete recursively.
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Example of version2:
CREATE TABLE Student(
Sid NUMBER (5) CONSTRAINT PK_Student
PRIMARY KEY,
Sname VARCHAR2 (30) NOT NULL,
- - - CONSTRAINT FK_Student_Sdno FOREIGN KEY
(Sdno) REFERENCES Department(Dno)
ON DELETE CASCADE);
• All descendant rows at all levels are deleted
when parent is deleted.
• Lookup table should not have delete cascade
because of many references to it.
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Analyzing Constraints Through Data
Dictionary
1. Each Oracle Database has a data dictionary with the
following information.
• Table definitions
• Column names and data types
• Sequences
• View definitions
• database links
• Stored procedure
• Other object descriptive information
2. Querying data dictionary information through views
3. View names:
• U SER : Views of those objects created by the
user.
Example: U SER T ABLES, U SER T AB COLU M N S,
U SER CON S COLU M N S, U SER CON ST RAIN T S,
U SER T RIGGERS, U SER T RIGGERS COLS.
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• ALL : Views of objects created by the user as
well as others the user has access permission.
• DBS : Views on all objects
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Accessing Views on Data Dictionary
1. U SER T ABLES :
This view has information about table names, tablespace names, etc.
Example: select all table names for those tables that
have been created in your current schema.
SELECT TABLE_NAME
FROM USER_TABLES
2. U SER T AB COLU M N S : This view contains information such as table names, column names, data
types, etc.
Example: Get table names for those tables with column like Sname.
SELECT TABLE_NAME
FROM USER_TAB_COLUMNS
WHERE COLUMN_NAME LIKE ’%Sname%’
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Accessing Views on Constraints
1. U SER CON S COLU M N S : this view has columns
such as owner of the table, constraint name, table
name, column name, sequence position.
View U SER CON S COLU M N S
OWNER_TABLE TABLE_NAME COLUMN_NAME CONSTRAINT_NAME
Admin
Student
Sid
PK_Student
Admin
Student
Sdno
FK_Student_Sdno
Admin
Department Dno
PK_Department
Example: Get all constraint names for table Student
on column Sdno.
SELECT TABLE_NAME, COLUMN_NAME, CONSTRAINT_NAME
FROM USER_CONS_COLUMNS
WHERE TABLE_NAME="Student" AND
COLUMN_NAME="Sdno"
2. U SER CON ST RAIN T S : This view has columns
such as owner of the table, constraint name, constraint type (one of five), table name, search condition (for CHECK), delete cascade, owner of parent
table, primary or unique key in the parent table.
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R OW N ER:owner of the parent table. R stands for
Relative.
R CON ST RAIN T N AM E: name of Parent table PRIMARY KEY Constraint.
View U SER CON ST RAIN T S
OWNER
Admin
Admin
Admin
Admin
CONSTRAINT_NAME PK_Student FK_Student_Sdno PK_Department CK_Sdno
CONSTRAINT_TYPE
’P’
’F’
’P’
’C’
TABLE_NAME
Student
Student
Department Student
SEARCH_CONDITION
NULL
NULL
NULL Sdno BETWEEN 0 AND 700
R_OWNER
NULL
Admin
NULL
NULL
R_CONSTRAINT_NAME
NULL
PK_Department NULL
NULL
DELETE_RULE
NULL
CASCADE
NULL
NULL
Example: Get search conditions for all constraints
on Student table.
SELECT SEARCH_CONDITION
FROM USER_CONSTRAINTS
WHERE CONSTRAINT_NAME=’CK%’ AND
TABLE_NAME=’Student’
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Example: Given a table name, get it’s foreign key
constraint name, parent table and it’s primary key
constraint name.
SELECT A.CONSTRAINT_NAME Foreign_Key
B.TABLE_NAME Parent_Table
A.R_CONSTRAINT_NAME Parent_key
A.DELETE_RULE Delete_Rule
FROM USER_CONSTRAINTS A, USER_CONSTRAINTS B
WHERE A.TABLE_NAME="Student" AND
A.R_CONSTRAINT_NAME=B.CONSTRAINT_NAME
Result:
Foreign_key
Parent_Table Parent_key Delete_Rule
FK_Student_Sdno Department
PK_Department CASCADE
Example: Given a table name get all it’s foreign keys,
parent table names and their primary keys, and apply
this recursively using delete cascade (useful, say to
find out which triggers will affect a table).
This will require both SQL and stored programming.
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