Database keys and datatypes

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Explain about Data Types INT1, INT2, INT3,

INT4 & NUMC

INT1: 1-byte integer between 0 and 255. The length is set to 3 places for this data type.

INT2: 2-byte integer between -32767 and 32767. Fields of this type should only be used for length fields. These long

fields are positioned immediately in front of a long field (type LCHR, LRAW). With INSERT or UPDATE on the long

field, the database interface enters the length which was actually used in the length field. The length is set to 5 places

for this data type.

INT4: 4-byte integer between -2147483647 bis 2147483647. The length is set to 10 places for this data type.

NUMC: Long character field in which only numbers can be entered. The length of this field is limited to a maximum of

255 places.

Candidate key

The best way to define candidate keys is with an example. For example, a bank’s database is being designed. To

uniquely define each customer’s account, a combination of the customer’s ID or social security number (SSN) and a

sequential number for each of his or her accounts can be used. So, Mr. Andrew Smith’s checking account can be

numbered 223344-1, and his savings account 223344-2. A candidate key has just been created.

This can raise problems. What if the government makes a mistake and issues the same SSN to more than one

person, and both now want to open an account with the bank?

Because of such potential pitfalls, a frequently used option is to create your own candidate key. In this case, the

bank’s database can issue unique account numbers that are guaranteed to prevent the problem just highlighted. For

good measure, these account numbers can have some built-in logic. For example checking accounts can begin with

a ‘C,’ followed by the year and month of creation, and within that month, a sequential number. So Andrew Smith’s

checking account can now be C-200805-22. Even without referring elsewhere, a teller can identify that this was the

22nd checking account created in May 2008. Savings accounts follow the same logic, but with an ‘S’ instead of ’C.’

Note that it was possible to uniquely identify each account using the aforementioned SSNs and a sequential number

(assuming no government mess-up, in which the same number is issued to two people). So, this is a candidate key

that can potentially be used to identify records. However, a much better way of doing the same thing has just been

demonstrated - creating a candidate key. In fact, if the chosen candidate key is so good that it can certainly uniquely

identify each and every record, then it should be used as the primary key. All databases allow the definition of one,

and only one, primary key per table.

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Explain about Data Types INT1, INT2, INT3, INT4 & NUMC INT1: 1-byte integer between 0 and 255. The length is set to 3 places for this data type. INT2: 2-byte integer between -32767 and 32767. Fields of this type should only be used for length fields. These long fields are positioned immediately in front of a long field (type LCHR, LRAW). With INSERT or UPDATE on the long field, the database interface enters the length which was actually used in the length field. The length is set to 5 places for this data type. INT4: 4-byte integer between -2147483647 bis 2147483647. The length is set to 10 pl ...
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