Explain how the SELECT FOR UPDATE command works in the context of transaction management and locking in a database.

The SELECT ... FOR UPDATE command is used in relational databases to lock selected rows during the current transaction.

When SELECT ... FOR UPDATE is executed, the database locks these rows for other transactions to prevent their modification or deletion until the current transaction is completed. This ensures data consistency and prevents race conditions during concurrent access.

Example of usage:

BEGIN TRANSACTION;
SELECT * FROM accounts WHERE account_id = 123 FOR UPDATE;
-- here you can safely update data, knowing that other transactions will not modify these rows
UPDATE accounts SET balance = balance - 100 WHERE account_id = 123;
COMMIT;

In the context of Java, when working with JDBC, SELECT ... FOR UPDATE can be used to explicitly lock rows if sequential access to data within a transaction is required.

Thus, SELECT FOR UPDATE helps implement row-level locking mechanisms, which are important for correct operation with concurrent transactions.