i create a database for video rental
3 tables are

Films (Film_No,Film_Name,Description)
Members(Member_ID,Name,Address)
RentalInfo(Fim_No,DateRented,DateReturn,RentalCost ,Member_ID)

can anyone give me an example query that cannot be expressed in relational algebra?...and please tell me why it can't

This sounds suspiciously like an exam question...

he he...it is....but it is past exam paper...i am just curious to know

According to my Fundamentals of Database Systems text, the following cannot be performed with standard relational algebra:

Aggregate Functions (ie COUNT, MAX, MIN)

Recursive Closure Operations (doesn't really apply to the tables you gave, but it would be answering questions like "show me all the employees under the President" where there are several levels of management.

Arithmatic operations (+/-/*)

Don't know if there are others, but that's what I've got

Relational database has also its limitations.
There is another forms of databases that are deducted databases or logical form of database
and they have their own language.It has been at the masters level.
I wll give you an example "how you find a location or map from any web site like mapquest"
Deducted Databases is infact more powerful than relational databases

Oh....Thank a lotttttttttttttttttttttttttt......