i5/OS has a database which stores data in files. These are called physical files.

The Physical file stores the actual data.

The data is written to a physical file in arrival sequence. This data can be accessed by RRN (relative record number) where the first entry on the file (also known as a table) is relative record number 1. ie: the first record in the file. The next record is RRN=2 then RRN3 and so on.

If you want to access the data using a sequence of something on the file (for example CUSTOMER sequence) then you would design and create a logical view of this physical file specifying that you want it sequenced by customer.

The Logical File is a sequenced VIEW of the data in the Phsyical FIle.

Logicals can be used for updating, creating and deleting data within the physical that it is scoped over.

Join Logicals - are an advanced form of logical file selecting and sequencing data from more than one physical.

Wait - theres more

You can also add a KEY (the keyed sequence field used in a logical) to a physical file itself.

So a non-keyed FIFO Physical file definition might look like this

This same Phsyical file when defined to have a key sequence of the Project Code (PRJCDE) might look like this:

A logical over the same File sorting by Current Library (CURLIB) would look like this: