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

In database terminology, a JOIN operation combines rows from two or more tables using a common attribute (key) field, so that a record in one table can be matched against and correlated with the related record in another table.

This question asks for the command that changes the contents of one database using another database by linking them on a common key field - that cross-table linking-by-key-field behaviour is exactly what the JOIN command provides. That is why this ISRO 2009 Computer Science PYQ keys 'Join' as the intended answer.

• Replace overwrites a value already stored in a single table; it has no built-in way to match records across two tables.

• Change is an informal term, not a defined database command, and names no cross-table matching mechanism.

• Update modifies values already stored in a table; alone, the term does not define a way to match a second table's records by a shared field, even though an UPDATE statement can itself incorporate a join.

Note: in modern SQL, actually writing new values into a table still requires an UPDATE statement (which can itself use a join in its FROM clause) - but per the official ISRO 2009 Computer Science answer key, this PYQ keys the table-linking command itself, Join, as the correct answer.