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3. Keys and Constraints

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§ Key are used for identifying unique rows from table. It also establishes relationship among tables.
§ Constraint are used to specify rules for the data in a table.
§ Constraints can be specified when the table is created (inside the CREATE TABLE statement) or after the table is created (inside the ALTER TABLE statement).
Identity
§ Its purpose is to allow SQL Server to automatically insert numerical primary key values to a  table as new data is inserted.

CREATE TABLE TABLE_NAME
( PRIMARY_KEY_COLUMN INT PRIMARY KEY IDENTITY { [Initial_Value],[Interval]},

Where [Initial_Value] is the first value of the primary key, and [Interval] is the interval between two consecutive identity values. If no [Initial_Value] or [Interval] is specified, the default for both is 1. In other words, the first row would be 1, and subsequent rows would get a value that is 1 larger than the previous row.

Primary Key

§ A primary key is a column (or columns) in a table that uniquely identifies the rows in that table.
§ Nulls are not allowed in primary key columns.
§ It is applicable for single column in a table.
Which column should you pick?

§ Choose a column as a primary key is to fetch dataset quickly and it allocate a unique number to each row if it is an identity.
§ In a table of employees, any column like First Name is a poor choice since you cannot control employee's first names and it may be duplicate.
§ Often there is only one choice for the primary key, which is its ID.
§ CREATE TABLE Persons
(
P_IdintNOTNULL,
LastNamevarchar(255)NOTNULL,
FirstNamevarchar(255),
Address varchar(255),
City varchar(255),
PRIMARY KEY (P_Id)
)

Alter
ALTERTABLE Persons
ADDPRIMARYKEY (P_Id)

Drop
ALTERTABLE Persons
DROPPRIMARYKEY

For Constraints
ALTERTABLE Persons
ADDCONSTRAINTpk_PersonIDPRIMARYKEY (P_Id)

Foreign Key

§ Foreign keys are columns that point to primary key columns.
§ For example, OrderNo is the primary key of the table ORDERS and CustomerNo is a foreign key that points to the primary key in the CUSTOMERS table.
CREATETABLE Orders
(
O_IdintNOTNULL,
OrderNointNOTNULL,
P_Idint,
PRIMARYKEY (O_Id),
FOREIGNKEY (P_Id)REFERENCES Persons(P_Id)
)

Alter
ALTERTABLE Orders
ADDFOREIGNKEY (P_Id)
REFERENCES Persons(P_Id)

Drop
ALTERTABLE Orders
DROPFOREIGNKEYfk_PerOrders

For Constraints
ALTERTABLE Orders
ADDCONSTRAINTfk_PerOrders
FOREIGNKEY (P_Id)
REFERENCES Persons(P_Id)

Unique Key

§ Unique key uniquely identifies each record in a database table or ensures that all values in a column are different.
§ Note that you can have many UNIQUE key per table, but only one PRIMARY KEY per table.

CREATETABLE Persons
(
P_IdintNOTNULLUNIQUE,
LastNamevarchar(255)NOTNULL,
FirstNamevarchar(255),
Addressvarchar(255),
City varchar(255)
)

Alter
ALTERTABLE Persons
ADDUNIQUE (P_Id)

Drop
ALTERTABLE Persons
DROPINDEXuc_PersonID

For Constraints
ALTERTABLE Persons
ADDCONSTRAINTuc_PersonIDUNIQUE (P_Id,LastName)

Not Null

NOT NULL constraint enforces a column to NOT accept NULL values.This means that you cannot insert a new record, or update a record without adding a value to this field.
CREATETABLEPersonsNotNull
(
P_IdintNOTNULL,
LastNamevarchar(255)NOTNULL,
FirstNamevarchar(255),
Addressvarchar(255),
City varchar(255)
)

Check Constraints

The CHECK constraint ensures that all values in a column satisfy certain conditions.It is used to limit the value range that can be placed in a column.
CREATETABLE Persons
(
P_IdintNOTNULL,
LastNamevarchar(255)NOTNULL,
FirstNamevarchar(255),
Addressvarchar(255),
City varchar(255),
CHECK (P_Id>0)
)

Alter
ALTERTABLE Persons
ADDCHECK (P_Id>0)

Or
ALTERTABLE Persons
ADDCONSTRAINTchk_PersonCHECK (P_Id>0 AND City='Sandnes')

Drop
ALTERTABLE Persons
DROPCONSTRAINTchk_Person
Or
ALTERTABLE Persons
DROPCHECKchk_Person

Default Constraints

Provides a default value for a column when none is specified. The DEFAULT constraint is used to insert a default value into a column, if no other value is specified.

CREATETABLE Persons
(
P_IdintNOTNULL,
LastNamevarchar(255)NOTNULL,
FirstNamevarchar(255),
Addressvarchar(255),
City varchar(255)DEFAULT'Sandnes'
)
Alter
ALTERTABLE Persons
ALTER City SETDEFAULT'SANDNES'

Drop
ALTERTABLE Persons
ALTER City DROPDEFAULT


Difference Between Primary Key and Unique Key


               PRIMARY KEY
             UNIQUE KEY
It doesn’t allow null
Allow Null value, but only one Null value.
By default it adds a clustered index
By default it adds unique non-clustered index
A table can have only one primary key column
A table can have more than one unique key















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