Slideshow:

Where do variable length fields arise ?

Problem caused by a variable length data field

Storage technique for records that contains a variable length field

Example 1 on how to store variable length records

Name: 30 bytes
Gender (G): 4 bytes
Address: variable length string
birth date: 16 bytes

Example 1 on how to store variable length records

Note: you must unpack a variable length record...

Example 2 on how to store variable length records

Name: variable length string
Gender (G): 4 bytes
Address: variable length string
birth date: 16 bytes

Example 2 on how to store variable length records

Same note: you must unpack a variable length record...

Need for variable size records

Situations where record size is variable:

Record contains a repeating field

Example: employee working on multiple projects

Employee: +-------+--------+--------------------------+ | SSN | Dept | proj1, proj2, proj3, ... | +-------+--------+--------------------------+

The problem of records containing "variable size" (data) fields

Main problem with records that contains a variable length field:

Fields following the variable size field have variable starting location
Graphically:
We must know the starting location of every field in the record (to access the data field)
Therefore:

How to store a record with variable size attributes

How to store records with variable length field that uses mimimal number of starting location information:

Store all of the fixed length fields (attributes) first
Then store the variable size attributes
You do not need to store the offset of the first variable size attribute
Because:
You must store the offset of the 2nd, 3rd, ... (etc) variable size attributes