Read Thread #874330 Message #874739 and others

SQL use Sets, not flat tables.

If you not define a Index you do not have control for lock a specific row.

If you want pessimistic Lock, you must define good indexes,
and on WHERE clause lock specific index keys.

On simple case, define a primary key, and on WHERE lock a specific PK value.
With this and lock keys range you can lock PK that not exists
( useful in order to reserve of the values )

Fabio