Object-relational mapping is a programming technique for converting data
between incompatible type systems in relational databases and
object-oriented programming languages. This creates, in effect, a
"virtual object database" that can be used from within the programming
language.
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. 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 IN...
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