http://www.amitavroy.com/justread/content/articles/code-igniter-jquery-graphs-php-data
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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