Database keys

Database keys

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1) PRIMARY KEY:-

A primary key is a field that uniquely identifies each record in a table. As it uniquely identify each entity, it cannot contain null value and duplicate value.
eg:-Consider the customer table, which has field :customer_number, customer_socialsecurity_number, and customer_address.here customer_number of each entity in customer table is distinct so customer-number can be a primary key of customer-table.

2) CANDIDATE KEY:-

A nominee's for primary key field are know as candidate key.
eg:-From above example of customer table, customer_socialsecurity_number is candidate key as it has all characteristics of primary key.

3) ALTERNATE KEY:-

A candidate key that is not the primary key is called an Alternate key.
eg:- In above example, customer_socialsecurity_number is a candidate key but not a primary key so it can be considered as alternate key.

4) COMPOSITE KEY:-

Creating more than one primary key are jointly known as composite key.
eg:-In above example, if customer_number and customer_socialsecurity_number are made primary key than they will be jointly known as composite key. In addition the combination of customer_number and customer_socialsecurity_number is unique.

5) FOREIGN KEY:-

Foreign key is a primary key of master table, which is reference in the current table, so it is known as foreign key in the current table. A foreign key is one or more columns whose value must exist in the primary key of another table.
eg:-Consider two tables emp(contains employees description) and emp_edu(contains details of employee's education), so emp_id which is primary key in emp table will be referred as foreign key in emp_edu table.