Primary key, Foreign Key and Default constraint are the 3 main constraints that need to be considered while creating tables or even after that. It seems very easy to apply these constraints but still we have some confusions and problems while implementing it. So I tried to write about these constraints that can be created or added at different levels and in different ways or methods.
Primary Key Constraint: Primary Keys constraints prevents duplicate values for columns and provides unique identifier to each column, as well it creates clustered index on the columns.
1) Create Table Statement to create Primary Key
a. Column Level
USE AdventureWorks2008
GO
CREATE TABLE Products
(
ProductID INT CONSTRAINT pk_products_pid PRIMARY KEY,
ProductName VARCHAR(25)
);
GO
b. Table Level
CREATE TABLE Products
(
ProductID INT,
ProductName VARCHAR(25)
CONSTRAINT pk_products_pid PRIMARY KEY(ProductID)
);
GO
2) Alter Table Statement to create Primary Key
ALTER TABLE Products
ADD CONSTRAINT pk_products_pid PRIMARY KEY(ProductID)
GO
3) Alter Statement to Drop Primary key
ALTER TABLE Products
DROP CONSTRAINT pk_products_pid;
GO
Foreign Key Constraint: When a FOREIGN KEY constraint is added to an existing column or columns in the table SQL Server, by default checks the existing data in the columns to ensure that all values, except NULL, exist in the column(s) of the referenced PRIMARY KEY or UNIQUE constraint.
1) Create Table Statement to create Foreign Key
a. Column Level
USE AdventureWorks2008
GO
CREATE TABLE ProductSales
(
SalesID INT CONSTRAINT pk_productSales_sid PRIMARY KEY,
ProductID INT CONSTRAINT fk_productSales_pid FOREIGN KEY REFERENCES Products(ProductID),
SalesPerson VARCHAR(25)
);
GO
b. Table Level
CREATE TABLE ProductSales
(
SalesID INT,
ProductID INT,
SalesPerson VARCHAR(25)
CONSTRAINT pk_productSales_sid PRIMARY KEY(SalesID),
CONSTRAINT fk_productSales_pid FOREIGN KEY(ProductID)REFERENCES Products(ProductID)
);
GO
1) Alter Table Statement to create Foreign Key
ALTER TABLE ProductSales
ADD CONSTRAINT fk_productSales_pid FOREIGN KEY(ProductID)REFERENCES Products(ProductID)
GO
2) Alter Table Statement to Drop Foreign Key
ALTER TABLE ProductSales
DROP CONSTRAINT fk_productSales_pid;
GO
Default Constraint: Default constraint when created on some column will have the default data which is given in the constraint when no records or data is inserted in that column.
1) Create Table Statement to create Default Constraint
a. Column Level
USE AdventureWorks2008
GO
CREATE TABLE Customer
(
CustomerID INT CONSTRAINT pk_customer_cid PRIMARY KEY,
CustomerName VARCHAR(30),
CustomerAddress VARCHAR(50) CONSTRAINT df_customer_Add DEFAULT 'UNKNOWN'
);
GO
b. Table Level : Not applicable for Default Constraint
2) Alter Table Statement to Add Default Constraint
ALTER TABLE Customer
ADD CONSTRAINT df_customer_Add DEFAULT 'UNKNOWN' FOR CustomerAddress
AGO
3) Alter Table to Drop Default Constraint
ALTER TABLE Customer
DROP CONSTRAINT df_customer_Add
GO
Reference : Pinal Dave (http://blog.SQLAuthority.com)
i see the different between column level & table level in terms of creating them, but what’s the significance by having a foreign key defined at column level ?
Hi! Pinal ,
Could you please tell me, what is difference between defining the constraint on column level or Table level?
Hi!
How can we add Unique constraint through Design mode? I do not want Code or query.
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Sir,
I am newbiee…not clear on CREATE TABLE.
CREATE TABLE dbo.Project (
ProjectID VARCHAR(10) NOT NULL,
ProjectName VARCHAR (50) NOT NULL,
ProjectDesc VARCHAR (50) SPARSE NULL
CONSTRAINT pk_project_pid PRIMARY KEY (ProjectID);
GO
ALTER TABLE dbo.Project
DROP CONSTRAINT pk_project_pid;
GO
ALTER TABLE dbo.Project
ADD CONSTRAINT pk_project_pid PRIMARY KEY (ProjectID);
go
Questions:
1. It’s necessary to DROP CONTRAINT pk_project_pid?
2. It’s a good practice to create constraint in Table level?
Many thanks for your kind help.
Edwin
hi…..,
can u notify d difference between primaryn foreign key constraints cn anyone xplain me in detail tanx fr ur kind response…….
hi satish,
suppose we have two tables like employee and department emptable have few columns like eno,ename,sal,deptno and same like dept table is also having same columns like deptno,dname so we want establish the relation between emp table and dept table there is a need to create primary and foreign key relationship on these two tables.make sure that these two tables same the same columns. in my example I have taken one deptno column in emp table and same deptno in dept table also then only this relation is established.
only one primary key is allowed per table and if we created any primary on any particular that that column accepts only unique values and it should not accept null values and it automatically creates clustered index on that column .
these constraints are very valuable in any database without these there is a chance to mislead the data
My primary key table, PrimaryLiterals stores two different types of literals – race and ethnic. In another table, I have two different columns that refer back to the key, PrimaryLiteralID, of primary key table – RaceLiteralID and EthnicLiteralID. Is it possible to create two foreign key constraints both referencing the PrimaryLiteralID or do two separate tables need to be created?
Hello Diane,
A primary key can be referenced by multiple foreign keys.
Kind Regards,
Pinal Dave
hi,
i m using sql server 2008 and for that i have one table named tabdealermaster in that i m using one primary key as DealerId .and in same table i m using that field as foreign key as dealer_ParentId .is it possible? m i right?
plz tell me. its urgent
yes , it is….but not in all cases…
just want to thanks you for your priceless work
how can i use default primary key such as (0 ,1,2,3,4,5,6 ……)
for the data
how to add foreign key when we have more than two tables
Hello Ashwini,
Your question is not clear to me. I think you want to refer a primary key in multiple tables. You can do that as a primary key can be referenced from multiple tables.
Regards,
Pinal Dave
This is the good question….i will give u answr latter….
Hello sir,
i’m creating a new Foreign key in Factresellersales table
using PK in Dimtime table and timekey(PK)
the below statement gives an error I cannot understand the reason why? please guide me the correct procedure…I’m new to SQL server
alter table dbo.FactResellerSales
ADD constraint fk_Timekey
foreign key (TimeKey) References DimTime(TimeKey)
Go
the error I get is ,
Foreign key ‘fk_Timekey’ references invalid column ‘TimeKey’ in referencing table ‘FactResellerSales’.
@Prathyusha
Make sure DimTime.TimeKey is a valid COLUMN, has a UNIQUE INDEX, and FactResellerSales.TimeKey is the same data type.
Hello Prathyusha,
Even the syntax of your statment is correct but following statement should also complete:
alter table dbo.FactResellerSales
ADD FOREIGN KEY (TimeKey) References DimTime(TimeKey)
Go
Let us know if you still get the error.
Regards,
Pinal Dave
Thank for ur Wonderful Support…
Ur posts r really helpful..
thanx..
Great Article thanks !!
Sir,
Sir i am firstly going to make table using normal forms so please you have help me to create table with discretion.
{custid,firstname,middlename,Lastname,birthofdate,accountcreationdate,Address,type(creadit or cash),mobilenumber(multivalue),emailid(multivalue).
Sir,
I want how to learn code in c# .net,please specify if there is any book,site……………………
Sir,
What is the difference between table level and column level constraints?
Actually, when you create a column level constraint and generate the script of the same table, it shows as table level.
Thanks in Advance
ok i have understand
Sir,
is it possable to create a foreign key on unique key colum?
i have table which has a unique key (organization_code,product_barcode) and i want to add a foreign key from another table which has these two column s in this table please help me
Preeti
CREATE TABLE Software_Manuals (
item_no varchar(40)PRIMARY KEY,
title varchar(100),
descript varchar(100),
vers varchar(30),
manufacturer varchar(50)
);
SELECT *
From Software_Manuals
INSERT INTO Software_Manuals
VALUES (100,’User guide’,'Adout Hardware’,2009,’Samsung’);
INSERT INTO Software_Manuals
VALUES (101,’Software testing’,'About Software’,2008,’Samsung’);
INSERT INTO Software_Manuals
VALUES (102,’Protected mode’,'Security’,2010,’Kaspaskey’);
INSERT INTO Software_Manuals
VALUES (103,’Macintosh’,'MAC address’,2007,’LG’);
INSERT INTO Software_Manuals
VALUES (104,’Educational software’,'About Education’,2011,’Joeant’);
CREATE TABLE Copy_SW_Manual (
access_no integer PRIMARY KEY,
item_no varchar(40)REFERENCES Software_Manuals
);
SELECT *
FROM Copy_SW_Manual
INSERT INTO Copy_SW_Manual
VALUES(100,0001);
INSERT INTO Copy_SW_Manual
VALUES(101,0002);
INSERT INTO Copy_SW_Manual
VALUES(102,0003);
INSERT INTO Copy_SW_Manual
VALUES(103,0004);
INSERT INTO Copy_SW_Manual
VALUES(104,0005);
CREATE VIEW VW_SW_MANUAL (title,manufacturer,vers,access_no)
AS
SELECT SM.title,SM.manufacturer,SM.vers,CSM.access_no
FROM Software_Manuals SM, Copy_SW_Manual CSM
WHERE SM.item_no=CSM.item_no
Sir,
this is argent. i can’t understand these questions.
1) if the inserted row contains an access that exists in the table, then update the version, title & manufacture columns of Software_manuals table with the inserted data
2) Else,if the title,manufacture and version exists in the Software-Manuals table,insert a new row to Copy_SW_Manual table with its item information referncing the existing row of the Software_Manuals table
3)Else, insert rows to Software_Manuals and Copy_SW_Manual tables to reflect the new information
Please send me the Answers of these questions pls………..sir
Thankyou..!
hi chaturi,
you didn’t mention your doubt clearly but you created a view using both of those table I will tell you something about view.
a view is nothing but a query or output of a query suppose we have to retrieve a data using query we can get the data successfully but again you want that data so you have to write the same query again n again so to over come these type of problems a views coming into picture I mean you can create view within that view you write your required query view are stored in database. so we want to retrieve the data again no need to write a query simply call a view with select statement,some important about view is view cant store any data just its stores the query.
we want to retrieve the data again you can run view in back ground the query is going to the original table and retrives our required data .view with in a view is also possible .
Thanks,
Avinash Reddy Munnangi
I’ve a problem in sql server just could you tel me ?
there are four tables in my database and there are two primary keys in two different table I need to filter data s by using those primary keys when I prepare reports, but in the beginning I’ve given only 1 primary key as foreign key to other tables
My question is can I insert in this stage the other particular primary key as foreign key to other tables?
its very argent ! I know you can help me
Hello Mr. Stephen,
You can insert the other primary key as foreign key to other tables provided you have the data in the tables that do not conflict with the combination keys (existing primary key and the new foreign key). In that case, you will have to make the corrections in the data already existing in the table and then insert the keys.
Also, after making changes in the design of the table, do not forget to refresh the data in reports.
Kind Regards,
Pinal Dave
sir
please tell me how can i create table in sql server.what process of start sql and then reached the table sheet.and also create contrant primary key foreign key .
Read about “CREATE TABLE” in SQL Server help file
first you have to create one table then –.> go to stored procedure –>
and write code like a
create procedure [procedure_name]
(
declare variables
[while u declare a variable start with ' @ ' and end with ,]
example
@name nvarchar(50)
Hello Mr.Pinal Dave,
I read your post regarding Constraints in Sql.Its very useful & Easy to understand.
As well,Short and Sweet.
Thanks a lot..
Regards
Anbu with Love
I am a newbee to SQL and I am working on a metrics table. I have the SQL 2008 Express on my computer.
The name of the database is PlantMetrics and the name of the table is dbo.PerformanceIndexes.
I have three columns containing:
PlantName
Month
Year
The combination of values in these three columns is unique. When I post the metrics for each plant for each month, the values of PlantName, Month and Year will repeat from record to record.
Can I denote these three columns as primary keys? If so how?
Or do I leave the table without primary key constraint?
I want to add other tables to the database with details of other characteristics for the plants and relate them to this table.
The other tables will have unique values for the PlantName etc.
Thanx for your Example Posts.They are really really helpful
hi sir,
i am vinoth we are using sql server 2008 i want to display the content table for daily updates….
hi sir,
i am vinoth. we are using for sql server 2008. i want display
recorded data… for example when i entered in office using
my id card that time..the product software recorded…
already recorded and new record data i want to display
screen with using sql server 2008.
which command we have to use…. please give me solution..
Thanks & Regards,
vinoth.A
If you have datetime column with default getdate(), you can find it
can you please tell me about logins and users n sql server?
Hi Pinal Sir,
Thanks for your tutorial about adding primary key and foreign key using alter statement at column level and table level.
Anshu
Hi Pinal Sir,
In the scenario of foreign key and primary key both are providing the same feature but what is the actual difference between column level and table level ?
hi i want create a login form in master page and relavented tables in database
CREATE table Customers
(CustomersID char(5) not null,
Address varchar (60) null,
City char(15) null,
Phone char (24)null,
Fax char (24) null,
constraint pk_Customers primary key (CustomerID)
)
CREATE table Orders
(OrdersID int,
CustomerID char not null,
Orderdate datetime null,
Shippeddate datetime null,
Quantity int null,
constraint pk_Orders primary key (OrdersID, CustomerID)
)
i am now satisfy with your answer
thanks
Hello
Pinal
Your posts are really very helpful
thank you
Hi Pinal
I create salespeople table
create table salespeople(snum int CONSTRAINT pk_snum PRIMARY KEY,sname varchar(50),city varchar(50),comm decimal(12))
i inserted data like this
insert into salespeople values(1002,’bama’,'bangalore’,.18)
insert into salespeople values(1003,’kama’,'Bobbili’,.15)
insert into salespeople values(1004,’anjuri’,'vizag’,.14)
———————-
then i create customer table
create table customer(cnum int INT CONSTRAINT pk_productSales_sid PRIMARY KEY,
Cname varchar(50),
city varchar(50),rating int ,
snum int CONSTRAINT fk_snum_pid FOREIGN KEY REFERENCES salespeople(snum))
inser values
insert into customer values(200,’jayanth’,'bangalore’,100,1004)
insert into customer values(2005,’bhagyasri’,'guntur’,200,1006)
—————-
i create order table
create table orders(onum int CONSTRAINT pk_productSales_sid PRIMARY KEY,
amount decimal(10),Orderdate datetime,
snum int CONSTRAINT fk_snum2_pid FOREIGN KEY REFERENCES salespeople(snum),
cnum int CONSTRAINT fk_cnum_pid FOREIGN KEY REFERENCES customer(cnum))
i got the error
Msg 2714, Level 16, State 4, Line 1
There is already an object named ‘fk_snum_pid’ in the database.
Msg 1750, Level 16, State 0, Line 1
Could not create constraint. See previous errors.
How to solve those errors
plz send details urgent pinla
Hi Pinal Sir,
I hava a problem in creating foreign key in my sql table.im using sql 2008.
Can i set two foreign keys in two child tables with references to one unique key in main table .
pleas help me.
try to chage your main table unique key bcoz unique key access from the null values so try to use primary key then u il set the foreign key ur table
i am now satisfy with ur answer
thanks
dear sir,
i have a table called student_ id with 3 column id(pkey),name and course.
i wants to access id (start with 3 and end with 8,means 5 values)
hw can this is possible with asp.net application
can anyone help me!!!
when ever i create identity primary key tat error showing all the times
“Must declare the scalar variable “@”ID”
HAI,
I want learn SQL more.Is any way to understand SQL easely.
REGARDS
Dinesh
Can’t we create a default constraint at table level ???
Hi Mr. Pinal,
One table having one cluster Index in the case of one primary key. If more than one primary keys are there in a table, then how many clustered Indexes are created for table?
Please reply me……
Thanks & Regards
Madhu….
Note that there cannot be more than one primary key in a table
Hi madhivanan,
we can create more than one primary key we can call it as composite key..
It is a single primary key combined on many columns. You cannot create primary key on each column seperately.
Hi, greeting from Indonesia. I’ve been looking for script of creating a foreign key related to the primary key from the column level and your explanation was great. Thank you very much! God bless
Greetings Pinal – Always find your answers helpful and spot on. At the moment I’m stuck in SQL/Server hell and can’t find a solution. I have a table that I can’t drop, can’t truncate, can’t do anything with because of this message “it is being used for foreign key constraint enforcement.” I have dropped ALL of the constraints on the table, and still can’t get around this error. There is a unique index that is being referenced that can’t be dropped either because of the same error response. I’m stuck in a circular hell. How do I get out of hell with this table?
Your posts are really usefull
Thanks for your post.
Thank you, always seem to forget the exact syntax on many of those.
how to insert a manual int value in primary key , sql server 2008
You can do it in INSERT and UPDATE statements
Hi sir,
My question is-
In sql server authentication mode which is more secure authentication? (Windows or Sql server)
window authentication mode used when you use your application with your system and sql server authentication is used when multiple application used a centrally database .
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How do I fetch MAC address in SQL server to implement in triggers.
Dave, thanks for you feedback, because this examples are very good and help me so much.
Thanks,
JCSL.
Great sir, thanks…
How to generate a default Primary Key
Background: There is a “Column Properties–Identity Specification” option with “Identy Increment” and “Identity Seed” in sqlsrv.
Q: Is it possible to assign a default value for new records like “getYear() + get_SQLSRV_Global_Sequence_Number()” feature/trigger available?
1) Example:
a) Syntax
(SELECT CONVERT( NUMERIC(4), (SELECT CONVERT(VARCHAR(4), GETDATE(), 112) AS [YYYY]))) + <SOME_GLOBAL_UNIQUE_SEED_FUNCTION()
2) SQL example
SELECT (SELECT CONVERT( VARCHAR(4), (SELECT CONVERT(VARCHAR(4), GETDATE(), 112) AS [YYYY]))) PK_PART1_YEAR, 1000000 as PK_PART2_SQLSRV_SEQUENCE
3) Expected result
Assigned primary key value: YEAR+CONCAT
1) First value this year: 20121000001
2) Second value this year: 20121000002
3) N-value for year 2050: 20501000002
Please refer or provide an sqlsrv example if possible…we would like to have a standard approach(dba’s, C# developers) regarding primary key assignment…
-kurt
Forgot to mention that
a) external systems (PRODUCERS) post typically + + in a Varchar(50) column
…and that
b) i’m using an additional “uniqueidentifier” and “(newid())” in sqlsrv db for internal integrity.
Hello sir,
How to create recursive query in SQL Server?
ur post was very nice and useful for me
Hello sir?
now i save the data in one table , that values also create a new columns into another table , if any possible to do automatically create a new columns
sir,
Am Jyothi. Is it posible to create a foreign with out primary and it is a unique key.
Hello Sir,
Can we drop foreign key which is not a constraint.. How to drop foreign key …
Use ALTER TABLE and DROP CONSTRAINT statements
Thanks for the reply.. I tried with this, its giving error field is not constraint..
How to drop that foreign key now…
You should find the key name and use it
Drop constraint contrain_name
How to find the constraint name..
How to find the constraint name…
exec sp_fkeys ‘table_name’
Thank You Sir….
sp_helpconstraint ‘table_name’
Hello,
If i run sequence i am getting error “Unknown object type ‘SEQUENCE’ used in a CREATE, DROP, or ALTER statement.” how to solve this problem
Note that Sequence is available from version 2012 onwards only
Hello,
How to store URL in table
I want to store URL in my tables in 2008, I will be extracting the same data for report in excel, if i click on the URL it should take me to particular site, I hope to get solution for this.. Thank You in advance…..
Hi,
I am Ramesh from Chennai, how many tables allowed in sql server 2008.
It depends on the harddisk volume. Read about Maximum capacity specifications in SQL Server help file.
Database objects include objects such as tables, views, stored procedures, user-defined functions, triggers, rules, defaults, and constraints. The sum of the number of all objects in a database cannot exceed 2,147,483,647.
Awesome help as always. Why I don’t come here first every time, I have no idea.
pinal sir you r the genius hats off to your site really!
actually i had problem with default with alter you just solved it thanks sir again i say yu the best in the world for sql
Hi Sir.
How to add the default value for exsiting column
hi sir,
I need to copy the value of a primary key into another column of
another table which is a foreign key how will i do it in sql server 2008 r2
awsm
Hi sir ,
There are two table like User and comment and columns like
(Userid,Conversation,click abuse ,commentid and category) for user table and (comment id,Comments,userid).when i insert some data through asp.net website i want to update these data of two tables in database.plz what is the code for sql database
can we give reference of foreign key to primary key of table which is present in other schema table
Hey
can any one guide me on !!
How to Drop bunch of tables from database ??
drop table tablename
How to use triggers
HI I have a table test with primary and foreign key.and i created view with that table as vwtest.
i can insert into the vwtest as it contain single table.
how can i get primary,foreign keys into the views vwtest from the main table test
I have what appears to be a painstakingly manual task. I need to find all the foreign keys in a DB however they are all logical. There are no constraints in the DB. Can you reference a set of thechniques to facilitate this?
Thank you
What did you mean by logical? If they have foreign key constraints, it is easy to fine them
hello guys, i really need your help, i’m using sql server 2008 and in creating tables when i put foreign key the insert not accept but when there no foreign key the insert accept directly. what can i do to create that foreign key??there is another way to join tables in report without using foreign key??? please need your help i’m a beginner..thx
hey friends…tell me how to use a foreign key between 2 tables in a single database when one table have primary key..??
Hi
I am making a billling software and i can’t understand that how can i make data base for billing my structure is like this
INVOICE NO,
SLNO PRODUCTS PRICE QUANTITY TOTAL
1
2
3
4
5
6
7
8
9
10
GRANDTOTAL
Problem is that if i sell one products then it working but if i sell 10item at a time
i can,t understand how to create table for it please help me
VIVEK KUMAR
WHAT IS THE DIFFERENCE OF GIVING PRIMARY KEY AND FOREIGN KEY CONSTRAINT AT COLUMN LEVEL AND TABLE LEVEL….WE CAN PROVIDE DIRECTLY AT A TABLE LEVEL ITSELF RIGHT
helo sir,
how to insert values in a table that contains a foreign key
HI
CAN YOU DEFINE SCHEMAS AND DEFAULT CONSTRAINT