What methods are used to maintain data consistency in relational databases?

The following methods are used to maintain data consistency in relational databases:

1. Transactions — allow executing a set of operations as a single unit. If one operation fails, all changes are rolled back, ensuring data integrity.

2. ACID principles:

   • Atomicity — operations within a transaction are executed completely or not at all.
   • Consistency — the database transitions from one correct state to another.
   • Isolation — parallel transactions do not affect each other.
   • Durability — after a transaction is committed, changes are saved even in case of failures.

3. Integrity constraints: primary keys, foreign keys, unique constraints, checks (CHECK), which prevent incorrect data.

4. Locks and isolation levels — manage concurrent access, prevent conflicts, and ensure consistency.

Example in Go using a transaction:

trx, err := db.Begin()
if err != nil {
    // handle error
}
_, err = trx.Exec("UPDATE accounts SET balance = balance - 100 WHERE id = ?", fromID)
if err != nil {
    trx.Rollback()
    // handle error
}
_, err = trx.Exec("UPDATE accounts SET balance = balance + 100 WHERE id = ?", toID)
if err != nil {
    trx.Rollback()
    // handle error
}
err = trx.Commit()
if err != nil {
    // handle error
}

In this example, transferring money between accounts occurs within a single transaction, which guarantees data consistency.