While each of the four types of locks have specific purposes, all the types interact and therefore may cause a locking conflict between transactions. To ensure database consistency, only one transaction should change any one row at any one time. Otherwise, two simultaneous transactions might try to change one value to two different new ones. So, it is important that a row write lock be exclusive. In contrast, no difficulty arises if more than one transaction wants to read a row. Since neither is changing it, there is no conflict. So, row read locks may be shared across many connections.

The following table identifies the combination of locks that conflict. Schema locks are not included because they do not apply to rows.