Row locking refers to when a database record is updated and the SQL engine locks the row to ensure referential integrity
Row locking is more common in modern RDBMS than in older ones. Row locking avoids collisions and deadlocking much more readily by locking only rows that are being edited. Locks are most commonly used with database transactions, where a lock is requested and granted before any data is updated. Locks help ensure referential integrity, especially with regards to foreign-keys.
In mysql, for instance, the myisam engine would use table locking. When the innodb engine was released it became popular quickly, in no small part because it offered row locking. InnoDB is now the default engine for MySQL 5.5 and later.
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