Useful Functions in DDS:

Data description specifications (DDS) provide a powerful and convenient way to describe data attributes in file descriptions external to the application program that processes the data. People are always uncovering ways, however, for DDS do more than you thought was possible.
1. Resizing a field:
Have you ever been in the position where you wanted to rename a field or change the size of it for programming purposes? For example suppose you wanted to read a file that had packed fields sized 17,2 into a program that had a field size of 8,0. You can do this easily enough in your DDS file definition by building a logical file with the field a different size. The field will automatically be truncated and resized into the 8,0 field unless the field is too large fit. Then that will result in an error.
Physical File: TEST1

A R GLRXX
*
A TSTN01 17P 2 TEXT('PERIOD 1 AMOUNT')
A TSTN02 17P 2 TEXT('PERIOD 2 AMOUNT')
A TSTN03 17P 2 TEXT('PERIOD 3 AMOUNT')
A TSTN04 17P 2 TEXT('PERIOD 4 AMOUNT')

Logical File: TEST2

A R GLRXX PFILE(TEST1)
*
A TSTN01 8P 0
A TSTN02 8P 0
A TSTN03 8P 0
A TSTN04 8P 0
2. RENAME function:
If you want to rename the field for RPG or CLP programming purposes, just create the field in DDS with the new name and use the RENAME function on the old field. The field can then be resized at the same time. Using the same physical file TEST1.
Logical File: TEST3

A R GLRBX PFILE(TEST1)
*
A TSTN05 8P 0 RENAME(TSTN01)
A TSTN06 8P 0 RENAME(TSTN02)
A TSTN07 8P 0 RENAME(TSTN03)
A TSTN08 8P 0 RENAME(TSTN04)
3. Creating a key field for a joined logical using the SST function:
Another neat trick if you are building joined logical files is to use partial keys or a sub-stringed field for the join. Example: Say the secondary file has a field (CSTCTR) and you want to join it to your primary file but the key field to make the join execute doesn't exist in the primary file. The key portion is embedded within a field in the primary file (CTRACC). Use the SST function on the field containing the key data and extract what will be needed for the join (XXCC). The XXCC field is then used in the join to the secondary file CTRXRFP. The "I" field in the definition represents that it is used for input only.
R GLRXX JFILE(GLPCOM CTRXRFP)
J JOIN(1 2)
JFLD(XXCC CSTCTR)
WDCO RENAME(BXCO)
WDCOCT I SST(CTRACC 1 10)
WDEXP I SST(CTREXP 12 11)
WDACCT RENAME(CTRACC)
XXCC I SST(CTRACC 5 6)
4. Concatenating fields using the CONCAT function:
Another trick for building a field that doesn't exist in your logical file is to use the CONCAT function. Example: You want to create a field FSTLST (first and last name) from 2 fields FIRST and LAST. This can be done as follows:
FIRST R
LAST R
FSTLST CONCAT(FIRST LAST)
5. Using the RANGE function:
In your logical file you may want to select a range of records rather than using the select function to select individual records. Example: You want only the records in your logical file where the selected field is in the range 100 and 900. This can be done as follows:
S XXPG# RANGE('100' '900')
You can also use the RANGE function on multiple ranges.
6. Using the VALUES function:
In your logical file you may want to select specific records that have certain values by using the VALUES function. Example: You want only the records in your logical file where the selected field has the values 'O', 'P', and 'E'. This can be done as follows:
S RPTCTR VALUES('O ' 'P ' 'E ')