READ & CHAIN, These are one of the most used (& useful) Opcodes by any RPG developer. These Opcodes are used to read a record from file.
So, What's the difference between CHAIN & READ?
CHAIN operation retrieves a record based on the Key specified. It's more like Retrieving Random record from a Database file based on the Key fields.
READ operation reads the record currently pointed to from a Database file. There are multiple Opcodes that start with READ and all are used to read a record but with slight difference. We will see more about different Opcodes and How they are different from each other (and CHAIN) in another article.
Few differences to note.
- CHAIN requires Key fields to read a record where as READ would read the record currently pointed to (SETLL or SETGT are used to point a Record).
- If there are multiple records with the same Key data, CHAIN would return the same record every time. READE can be used to read all the records with the specified Key data.
- No need of setting up pointer before reading a Record. CHAIN can directly fetch a record based on the Key fields. Unlike READE (or READPE) which would require setting up the pointer to a specific Record using SETLL/SETGT.
- CHAIN is better choice when the Key data is unique and if only one record is expected from the file for the specified Key data.
We will see more about CHAIN in this article.
More about CHAIN
CHAIN (Search Arguments/Key Data) FILENAME ;
Result can be directly moved to Data Structure, For this Data Structure name needs to be specified after the FILENAME in the Syntax.
CHAIN (Search Arguments/Key Data) FILENAME DATA_STRUCTURE_NAME ;
CHAIN can be used to read a record by using RRN (Relative Record Number), In this case, RRN needs to be passed in Search Arguments. This is Applicable for reading the data from Subfile Data format as well.
Key List (KLIST) can be defined and used as Search Arguments rather than specifying the individual fields on CHAIN in Fixed-format RPGLE.
Alternatively, %KDS can be used to fetch the Key list from a Data Structure in Free Format RPGLE.
Extenders like 'N' and/or 'E' can be used along with CHAIN.
'N' - Extender N can be used to specify the Record is retrieved for Read Only and Not to lock the record for Update. This Extender cannot be used if the File is not Opened for UPDATE operation.
'E' - Extender E can be used to capture any errors. Using %ERROR would catch the error avoiding any Program crash/failure.
CHAIN(N E) (Search Arguments/Key Data) FILENAME ;
IF %ERROR() ;
// Error Handling
%FOUND() is used to verify if the Record (with Search Arguments) is found in the file or not.
IF %FOUND() ;
// Program Logic
Let's consider, we have a file 'FILENAME1' with 3 Key fields.
- Retrieve the Record with specifying the Key fields directly on the CHAIN.
CHAIN (KEYFLD1 : KEYFLD2 : KEYFLD3) FILENAME1 ;
- It isn't necessary to use all the Key fields on CHAIN.
CHAIN (KEYFLD1 : KEYFLD2) FILENAME1 ;
- Instead, Define a Key List and use the Key List directly on CHAIN using %KDS
Dcl-DS FileKeys LikeRec(FILEREC : *KEY) ; // Fields can be explicitly defined as well in the DS instead of using LikeRec
CHAIN %KDS(FileKeys) FILENAME1 ;
If we need to use partial key, then need to mention the number of Key fields to be considered.
CHAIN %KDS(FileKeys : 2) FILENAME1 ;
I hope this post has been a bit of help in understanding more about CHAIN and how is it different from READ.
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