For access lock to be placed we have to specify it explicitly as LOCKING FOR ACCESS.
Access lock is one wherein the table is not locked means you can do insert/update/delete on the table while access lock will access the table ,in this the dirty reads may happen , which means you will not get the latest changes whatever happened on table to be reflected in your answer set.
READ lock will lock the table wherein you can not do insert/update and structural changes in table.
It is placed by simple SELECT statement for by explicitly specifying as LOCKING FOR READ.
read lock :
Locking table for Access;
access lock :
Locking table for Read Access
The main difference between read lock and access lock is data Integrity.On placing a read lock the user expects data integrity, while as for access lock the user cannot expect data integrity.
Consider following scenarios
1. User A places READ lock and User B WRITE places lock
User B will have to wait for User A to complete its read in order to start insert/updates/deletes on the data.
2. User A places ACCESS lock and User B WRITE places lock.
User A & B access the data simultaneously, hence User A cannot expect to get consistent results.