A Key Field or Primary Key is a field within a database which enables every record to be uniquely identified.
For example, at school you probably have a student or office number. This uniquely identifies you from every other student in the school. If you have a bank or savings account, you will have a bank account number which is unique and different from every other bank account number. This ensures that your money won't get muddled up with anyone elses and that you can easily keep track of deposits and withdrawals of cash.
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Click on this link: Primary Key