SQL PRIMARY KEY ConstraintThe PRIMARY KEY constraint uniquely
identifies each record in a database table.Primary keys must
contain unique values.A primary key column cannot contain NULL
values.Each table should have a primary key, and each table can
have only ONE primary key.SQL PRIMARY KEY Constraint on CREATE
TABLEThe following SQL creates a PRIMARY KEY on the "P_Id" column
when the "Persons" table is created:MySQL:CREATE TABLE Persons(P_Id
int NOT NULL,LastName varchar(255) NOT NULL,FirstName
varchar(255),Address varchar(255),City varchar(255),PRIMARY KEY
(P_Id))SQL Server / Oracle / MS Access:CREATE TABLE Persons(P_Id
int NOT NULL PRIMARY KEY,LastName varchar(255) NOT NULL,FirstName
varchar(255),Address varchar(255),City varchar(255))To allow naming
of a PRIMARY KEY constraint, and for defining a PRIMARY KEY
constraint on multiple columns, use the following SQL syntax:MySQL
/ SQL Server / Oracle / MS Access:CREATE TABLE Persons(P_Id int NOT
NULL,LastName varchar(255) NOT NULL,FirstName varchar(255),Address
varchar(255),City varchar(255),CONSTRAINT pk_PersonID PRIMARY KEY
(P_Id,LastName))Note: In the example above there is only ONE
PRIMARY KEY (pk_PersonID). However, the value of the pk_PersonID is
made up of two columns (P_Id and LastName).SQL PRIMARY KEY
Constraint on ALTER TABLETo create a PRIMARY KEY constraint on the
"P_Id" column when the table is already created, use the following
SQL:MySQL / SQL Server / Oracle / MS Access:ALTER TABLE PersonsADD
PRIMARY KEY (P_Id)To allow naming of a PRIMARY KEY constraint, and
for defining a PRIMARY KEY constraint on multiple columns, use the
following SQL syntax:MySQL / SQL Server / Oracle / MS Access:ALTER
TABLE PersonsADD CONSTRAINT pk_PersonID PRIMARY KEY
(P_Id,LastName)Note: If you use the ALTER TABLE statement to add a
primary key, the primary key column(s) must already have been
declared to not contain NULL values (when the table was first
created).To DROP a PRIMARY KEY ConstraintTo drop a PRIMARY KEY
constraint, use the following SQL:MySQL:ALTER TABLE PersonsDROP
PRIMARY KEYSQL Server / Oracle / MS Access:ALTER TABLE PersonsDROP
CONSTRAINT
pk_PersonID===================================================================================================foriegn
key:CREATE TABLE Orders(O_Id int NOT NULL,OrderNo int NOT NULL,P_Id
int,PRIMARY KEY (O_Id),FOREIGN KEY (P_Id) REFERENCES
Persons(P_Id))SQL Server / Oracle / MS Access:CREATE TABLE
Orders(O_Id int NOT NULL PRIMARY KEY,OrderNo int NOT NULL,P_Id int
FOREIGN KEY REFERENCES Persons(P_Id))To allow naming of a FOREIGN
KEY constraint, and for defining a FOREIGN KEY constraint on
multiple columns, use the following SQL syntax:MySQL / SQL Server /
Oracle / MS Access:CREATE TABLE Orders(O_Id int NOT NULL,OrderNo
int NOT NULL,P_Id int,PRIMARY KEY (O_Id),CONSTRAINT fk_PerOrders
FOREIGN KEY (P_Id)REFERENCES Persons(P_Id))SQL FOREIGN KEY
Constraint on ALTER TABLETo create a FOREIGN KEY constraint on the
"P_Id" column when the "Orders" table is already created, use the
following SQL:MySQL / SQL Server / Oracle / MS Access:ALTER TABLE
OrdersADD FOREIGN KEY (P_Id)REFERENCES Persons(P_Id)To allow naming
of a FOREIGN KEY constraint, and for defining a FOREIGN KEY
constraint on multiple columns, use the following SQL syntax:MySQL
/ SQL Server / Oracle / MS Access:ALTER TABLE OrdersADD CONSTRAINT
fk_PerOrdersFOREIGN KEY (P_Id)REFERENCES Persons(P_Id)To DROP a
FOREIGN KEY ConstraintTo drop a FOREIGN KEY constraint, use the
following SQL:MySQL:ALTER TABLE OrdersDROP FOREIGN KEY
fk_PerOrdersSQL Server / Oracle / MS Access:ALTER TABLE OrdersDROP
CONSTRAINT
fk_PerOrders===========================================================================================Constraints
are used to limit the type of data that can go into a table. two
basic types of constraints are there 1.column level2.table levelthe
difference is that column level constraints are applyonly to one
column where as table level constraints areapply to whole table
Constraints are declare at the time of table creation usingCREATE
TABLE command there are 6 constraints used in oracle1.NOT
NULL2.UNIQUE3.PRIMARY
KEY4.CHECK5.DEFAULT6.REFERENCES=========================================================================================A
Constraint is a property that we can assign to column in table. By
assigning constraint property on column we can prevent the users to
enter inconsistent of data in columns. We can assign these
Constraint properties during the time of creation of table (By
Using CREATE TABLE statement) or during the time of changing the
existing table structure (By Using ALTER TABLE statement).In SQL we
have different types of constraints are available those are1.
Primary Key Constraint2. Unique Key Constraint3. Foreign Key
Constraint4. Not Null Constraint5. Check ConstraintNow I will
explain about each constraint clearlySQL Primary Key
Constraint:Primary key constraint is used to uniquely identify each
record in database table. It wont allow repetition or duplication
of data. Each table is having only one primary key constraint and
it contains only unique values. Primary key constraint doesnt
accept null values.Example of creating Primary Key constraint
during the time of CREATE TABLECreate Table SampleUserDetail(UserID
integer PRIMARY KEY,UserName varchar(50),FirstName
varchar(50),LastName varchar(50))Example of creating Primary Key
constraint during the time of ALTER TABLEALTER TABLE
SampleUserDetail ADD PRIMARY KEY (UserID)To Drop Primary Key
constraint on table use the below statementALTER TABLE
SampleUserDetail DROP Constraint UserIDSQL Unique Key
ConstraintUnique key constraint is same as Primary key Constraint
it doesnt allow duplication or repetition of data in column and we
can uniquely identify records in table. The main difference is
Primary Key constraint wont allow null values but unique key
constraint allows null values. We have a chance to define only one
primary key on table but we can define many unique key constraints
on table.Example of creating Unique Key constraint during the time
of table creationCreate Table SampleUserDetail(UserID integer
,UserName varchar(50),FirstName varchar(50),LastName
varchar(50)CONSTRAINT us_UserId UNIQUE (UserID))Example of creating
Unique Key constraint during the time of ALTER TABLEALTER TABLE
SampleUserDetail ADD CONSTRAINT us_UserId UNIQUE (UserID)To Drop
Unique Key constraint on table use the below statementALTER TABLE
SampleUserDetail DROP Constraint us_UserIdSQL Foreign Key
ConstraintA Foreign key in one table point to primary key in
another table. The foreign key constraint is used to prevent the
actions that would destroy the links between two tables.Example of
Foreign key constraintCreate one table with primary key and give
name as UserDetailsUserIDUserNameFirstNameLastName
1SureshDasariSureshDasari 2PrasanthiDonthiPrasanthiDonthi
3MaheshDasariMaheshDasariAfter create another table with Foreign
Key and give name as SalaryDetailsSalIDSalaryUserID 1100001 2200002
3300003The column UserID is a primary key in UserDetails tableThe
column SalID is a foreign key in SalaryDetails tablesIf you observe
above two tables UserID in UserDetails table points to UserID in
SalaryDetailsExample of creating Foreign Key constraint during the
time of table creationCreate Table SalaryDetails(SalaryID integer
,Salary integer,UserID varchar(50),PRIMARY KEY
(SalaryID),CONSTRAINT fk_SalaryID FOREIGN KEY(UserID)REFERENCES
UserDetails(UserID))Example of creating Foreign Key constraint
during the time of ALTER TABLEALTER TABLE SalaryDetails ADD
CONSTRAINT fk_SalaryID FOREIGN KEY (UserID) REFERENCES
UserDetails(UserID)To Drop Foreign Key constraint on table use the
below statementALTER TABLE SampleUserDetail DROP Constraint
fk_SalaryIDSQL Not NULL Constraint:If we set Not Null constraint
property on any column in table that column wont accept NULL or
Empty values. If you want enforce any column not to accept NULL or
empty value just set Not NULL Constraint property for that
particular columnExample of creating NOT NULL constraint during the
time of table creationCreate Table SampleUserDetail(UserID integer
NOT NULL,UserName varchar(50) NOT NULL,FirstName
varchar(50),LastName varchar(50))SQL Check Constraint:The check
constraint is used to limit the value range that can be placed in a
column. If we set Check constraint property on particular column
the values in particular column must satisfy condition set by check
constraint. Example of creating Check constraint during the time of
CREATE TABLECreate Table SampleUserDetail(UserID integer NOT NULL
CHECK(UserID > 0),UserName varchar(50),FirstName
varchar(50),LastName varchar(50))Example of creating Check
constraint during the time of ALTER TABLEALTER TABLE
SampleUserDetails ADD CONSTRAINT chk_UserID CHECK(UserID > 0)To
Drop Check constraint on table use the below statementALTER TABLE
SampleUserDetails DROP CONSTRAINT chk_UserID
