IBM DB400 Question:

CHAIN is used for random retrieval from a File. In other
words, whatever value used by programmer in Factor 1/Key
against CHAIN opcode will be used as search/find criteria in
database file. NO VALUE RETRIEVED IN NO CRITERIA MATCH. It
can be well understand, in case of using composite keys.

But in case of SETLL and SETGT, program will return some
value, if any value present in the database/physical file.

CHAIN = Sets pointer on exact value

SETLL = Sets pointer on Lower Limit of the value

SETGT = Sets pointer on Greater Limit of the value

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It is recommended, use CHAIN only, if record exist in
database file, instead of SETLL and SETGT, which can be used
otherwise.