Explain multivalued dependencies
Database Management System
Computer Science Engineering
655
Pari
Multivalued dependency would occur whenever two separate attributes in a given table happen to be independent of each other. And yet, both of these depend on another third attribute.
The multivalued dependency contains at least two of the attributes dependent on the third attribute. This is the reason why it always consists of at least three of the attributes.
Example
Suppose that there is a car manufacturing company that produces two of the colours in the market, i.e., red and yellow of each of their models, every year.
CAR_MODEL | MANUF_MONTH | COLOUR |
---|---|---|
S2011 | JAN | Yellow |
S2001 | FEB | Red |
S3001 | MAR | Yellow |
S3001 | APR | Red |
S4006 | MAY | Yellow |
S4006 | JUN | Red |
In this case, the columns COLOUR and MANUF_MONTH are dependent on CAR_MODEL, and they are independent of each other. Thus, we can call both of these columns multivalued. These are, as a result, dependent on CAR_MODEL. Here is a representation of the dependencies we discussed above:
CAR_MODEL → → MANUF_MONTH
CAR_MODEL → → COLOUR
We can read this as “CAR_MODEL multi-determined MANUF_MONTH” and “CAR_MODEL multi-determined COLOUR”.
We always use multivalued conditions when we encounter these two different ways: