Skip to main content

What we need to know about CHAIN (RPGLE) & How is it different from READ?

CHAIN

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

Syntax:

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
ENDIF ;

%FOUND() is used to verify if the Record (with Search Arguments) is found in the file or not. 

IF %FOUND() ;
// Program Logic 
ENDIF ; 

E.g.:

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.


If you have any Suggestions or Feedback, Please leave a comment below or use Contact Form.
Labels:

Comments

Post a Comment

Popular posts from this blog

All about READ in RPGLE & Why we use it with SETLL/SETGT?

READ READ is one of the most used Opcodes in RPGLE. As the name suggests main purpose of this Opcode is to read a record from Database file. What are the different READ Opcodes? To list, Below are the five Opcodes.  READ - Read a Record READC - Read Next Changed Record READE - Read Equal Key Record READP - Read Prior Record READPE - Read Prior Equal Record We will see more about each of these later in this article. Before that, We will see a bit about SETLL/SETGT .  SETLL (Set Lower Limit) SETLL accepts Key Fields or Relative Record Number (RRN) as Search Arguments and positions the file at the Corresponding Record (or Next Record if exact match isn't found).  SETGT (Set Greater Than) SETGT accepts Key Fields or Relative Record Number (RRN) as Search Arguments and positions the file at the Next Record (Greater Than the Key value). Syntax: SETLL SEARCH-ARGUMENTS/KEYFIELDS FILENAME SETGT  SEARCH-ARGUMENTS/KEYFIELDS FILENAME One of the below can be passed as Search Arguments. Key Fiel
4 comments
Read more

Extract a portion of a Date/Time/Timestamp in RPGLE - IBM i

%SUBDT Extracting Year, Month, Day, Hour, Minutes, Seconds or Milli seconds of a given Date/Time/Timestamp is required most of the times.  This can be extracted easily by using %SUBDT. BIF name looks more similar to %SUBST which is used to extract a portion of string by passing from and two positions of the original string. Instead, We would need to pass a value (i.e., Date, Time or Timestamp ) and Unit (i.e., *YEARS, *MONTHS, *DAYS, *HOURS, *MINUTES, *SECONDS or *MSECONDS) to %SUBDT.  Valid unit should be passed for the type of the value passed. Below are the valid values for each type. Date - *DAYS, *MONTHS, *YEARS Time - *HOURS, *MINUTES, *SECONDS Timestamp - *DAYS, *MONTHS, *YEARS, *HOURS, *MINUTES, *SECONDS, *MSECONDS Syntax: %SUBDT(value : unit { : digits { : decpos} }) Value and Unit are the mandatory arguments.  Digits and Decimal positions are optional and can only be used with *SECONDS for Timestamp. We can either pass the full form for the unit or use the short form. Below i
2 comments
Read more