Distinguish between sequential access and direct access. Give an example of a type of application for which each is particularly appropriate?
Sequential access requires that one pass through the first n-1 records in a data set to get to the nth record. Some examples of applications that lend themselves to sequential access appear in the second paragraph in Section 1.2. Direct access allows for the retrieval of the nth record in a data set without having to look at the previous n-1 records. ‘Behind the scenes’ this is often accomplished by the use of hashing algorithms where, for example, the physical location of a record is determined by taking its identifier (e.g., an account number) and dividing it by some prime number and using the remainder to determine the physical location of the record on the direct access storage device. Direct access is commonly used for ad hoc querying (e.g., perhaps an advisor might want to look at the demographic data the university maintains on a student prior to speaking with him or her about what courses to take during a given semester).