The 'command' used to change contents of one database using the contents of…

In relational database terminology, JOIN and UPDATE name two distinct kinds of operations. A JOIN is a query/retrieval operation: it combines rows from two or more tables using a common attribute (key) field so a record in one table can be matched against and read alongside the related record in another table, but it does not by itself persist any change to stored data. An UPDATE is a data-modification (DML) command: it writes new values into a table's existing rows, and in several database systems (e.g. the classic xBase/FoxPro UPDATE command) those new values can be sourced from a second table matched on a common key field - which does persist a cross-table change.

This question's phrasing - 'the command used to change contents of one database using the contents of another ... by linking them on a common key field' - literally describes a write operation driven by a cross-table match, which is the closest technical fit to the UPDATE definition above (JOIN alone never writes/changes stored contents). Even so, this is an ISRO 2009 Computer Science PYQ, and two independent, question-specific solved-paper sources that name this exact numbered question with these exact four options (cracku.in, edurev.in) report the intended/circulated answer as 'Join', which is why this record keeps option 'Join' as the key. No authentic official ISRO answer-key document could be located to settle the dispute beyond doubt, and the question's own wording is imprecise - it also conflates 'database' with 'table' (a JOIN or UPDATE ordinarily operates across tables within one database, not across two separate databases).

• Replace overwrites a value already stored in a single table; it names no mechanism for matching records across two tables.

• Change is an informal, generic word, not a defined database command.

• Update modifies values already stored in a table and, in several systems, can source those values from a second table matched on a common key field - the closest literal match to this question's own wording, though it is not the option this PYQ's reported key selects.

Per the reported ISRO 2009 answer for this exact question, 'Join' is kept as the correct option here; readers should note the genuine, source-documented ambiguity around whether 'Update' better fits the stem's literal wording.