Tag Archives: Primary Key Constraint

How to get all the Tables with or without Primary Key Constraint in Sql Server?

Scripts, Sql Server, Tips/Tricks, , , , ,

We can write a query like below to get all the Tables with no Primary key constraint:

SELECT T.name 'Table without Primary Key'
FROM SYS.Tables T
WHERE OBJECTPROPERTY(object_id,'TableHasPrimaryKey') = 0
      AND type = 'U'

We can write a query like below to get all the Tables with Primary key constraint:

SELECT T.name 'Table with Primary Key'
FROM SYS.Tables T
WHERE OBJECTPROPERTY(object_id,'TableHasPrimaryKey') = 1
      AND type = 'U'

Let us understand this with example:

CREATE DATABASE SqlHintsDemoDB
GO
USE SqlHintsDemoDB
GO
/*Let us create Customers table with Primary Key.*/
CREATE TABLE dbo.Customers (
	CustomerId int IDENTITY (1, 1) 
              PRIMARY KEY CLUSTERED NOT NULL ,
	FirstName Varchar(50),
	LastName Varchar(50))
GO

/*Let us create Orders Table without any primary key.*/
CREATE TABLE dbo.Orders (
	OrderId int IDENTITY (1, 1) NOT NULL ,
	CustomerId int NOT NULL ,
	CreationDT DATETIME NOT NULL)
GO

Now let us run the queries to get the list of all Tables with or without Primary Keys and verify the result:
List_All_Tables_With_And_Without_Primary_Keys

Difference Between Primary Key and Unique Key In Sql Server

CONSTRAINTS, Differences, Sql Server, , , , , , , , ,

Both PRIMARY KEY and UNIQUE KEY enforces the Uniqueness of the values (i.e. avoids duplicate values) on the column[s] on which it is defined.  Also these key’s can Uniquely identify each row in database table.

[ALSO READ] Difference Between Sql Server VARCHAR and NVARCHAR Data Type

Below table lists out the major difference between PRIMARY KEY and UNIQUE KEY:

PRIMARY KEY UNIQUE KEY
NULL It doesn’t allow Null values.
Because of this we refer
PRIMARY KEY = UNIQUE KEY + Not Null CONSTRAINT		 Allows Null value. But only one Null value.
INDEX By default it adds a clustered index By default it adds a UNIQUE non-clustered index
LIMIT A table can have only one PRIMARY KEY Column[s] A table can have more than one UNIQUE Key Column[s]
CREATE SYNTAX Below is the sample example for defining a single column as a PRIMARY KEY column while creating a table:CREATE TABLE dbo.Customer
(
Id INT NOT NULL PRIMARY KEY,
FirstName VARCHAR(100),
LastName VARCHAR(100),
City VARCHAR(50)
)

Below is the Sample example for defining multiple columns as PRIMARY KEY. It also shows how we can give name for the PRIMARY KEY:

CREATE TABLE dbo.Customer
(
Id INT NOT NULL,
FirstName VARCHAR(100) NOT NULL,
LastName VARCHAR(100),
City VARCHAR(50),
CONSTRAINT PK_CUSTOMER PRIMARY KEY (Id,FirstName)
)

 Below is the sample example for defining a single column as a UNIQUE KEY column while creating a table:CREATE TABLE dbo.Customer
(
Id INT NOT NULL UNIQUE,
FirstName VARCHAR(100),
LastName VARCHAR(100),
City VARCHAR(50)
)

Below is the Sample example for defining multiple columns as UNIQUE KEY. It also shows how we can give name for the UNIQUE KEY:

CREATE TABLE dbo.Customer
(
Id INT NOT NULL,
FirstName VARCHAR(100) NOT NULL,
LastName VARCHAR(100),
City VARCHAR(50),
CONSTRAINT UK_CUSTOMER UNIQUE (Id,FirstName)
)
ALTER SYNTAX Below is the Syntax for adding PRIMARY KEY CONSTRAINT on a column when the table is already created and doesn’t have any primary key:ALTER TABLE dbo.Customer
ADD CONSTRAINT PK_CUSTOMER PRIMARY KEY (Id)		 Below is the Syntax for adding UNIQUE KEY CONSTRAINT on a column when the table is already created:ALTER TABLE dbo.Customer
ADD CONSTRAINT UK_CUSTOMER UNIQUE (Id)
DROP SYNTAX Below is the Syntax for dropping a PRIMARY KEY:ALTER TABLE dbo.Customer
DROP CONSTRAINT PK_CUSTOMER		 Below is the Syntax for dropping a UNIQUE KEY:ALTER TABLE dbo.Customer
DROP CONSTRAINT UK_CUSTOMER

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