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ECommerce >>
Database Concepts
Key Fields
A key field uniquely identifies
an occurrence of a record. An example of this would be a customer reference
number in a customer table. Since no two customers should have the same
customer reference number, this field can be called a simple key.
A compound key is formed when
you need to combine two or more fields in a table to uniquely identify
individual records. Sometimes no one field can uniquely identify every
record, but combining two gives a unique reference. For example in a stock
system it is not impossible that two different suppliers may use the same
product code for different products. Combining the product code with the
supplier reference number could then be used to give a compound key. In a
compound key, the fields, which make up the compound key, are usually also
simple keys in there own right in other tables. If one of the fields that
make up the key is not a simple key, the combination of fields is usually
called a composite key rather than a compound key.
In database systems such as MS
Access, fields can be set as Primary Keys. A primary key must be the simple
key field of a table and MS Access will enforce this. In MS Access this then
enforces the uniqueness of the Primary Key field and will not allow
duplicates fields to be entered. As well as this the table is automatically
indexed on the Primary Key in Access, which is needed to allow the system to
quickly check whether an entry is unique or not, before validating it or
blocking it. The table can also be ordered on the Primary Key field by using
this index. |
