Question

I’m just started using SQL and ran into a problem.

In my database, I presently have two tables, Cinemas and Theatres.
I’m trying to create a column “# of Theatres# in the Cinemas table that counts the number of Theatres in the Theatres table with the same CinemaID (foreign key) as a Cinema in the Cinemas table. I got it to work as an SQL query:

SELECT cinemas.CinemaID,Town,COUNT(*) AS '# of Theatres'
FROM cinemax.cinemas,cinemax.theatres
WHERE cinemas.CinemaID=theatres.CinemaID
GROUP BY cinemas.CinemaID;

But wanted to know if it’s possible to create a column in the Cinemas table, that automatically performs the above query and inserts the value into each row.

It is going to be a very small database so speed isn’t really an issue, I just want to learn how to make such a computed column (if even possible).

Answer 1

it is possible. but you should not do it.

that is called denormalization – and is usually not a good idea at all.

sometimes however, you might absolutely need to denormalize something for some odd reason (your example is nowhere near a good reason). in these cases, you need to add trigger code to automatically manage the values whenever anything changes in the system that would affect the results.