Re: Fun with SQL

Benjamin Walling wrote:

>It's interesting that you assumed the simplest database design for the
>problem.  I assumed something more complex, given that he struggled to
>create the query.
>  
>
Don't assume my struggles came from a complex problem ... ;-)   but 
thanks for the generosity.

>The query gets much more difficult when the Students table does not keep
>summary data about tardies.  
>
Actually, just requiring the tardy dates in the select statement would 
suffice to make it a bit more complex, though your point is valid, many 
wouldn't store the calculated summary data in the db (unless for 
performance reasons, AFAIK).

>What we don't know is whether his database
>is keeping a record of days that were attended or days that were tardy.
>A record of days that were attended makes the query much more fun -
>finding the holes among known school days instead of having a list of
>them.
>
True enough. For this example, we can keep it to a child table from 
STUDENTS that list only tardy dates.

>  Also adding to the difficulty (although not much) is whether you
>have a field on the Students table that identifies the date of the last
>tardy message, or whether you have a table that keeps track of the tardy
>messages and have to check it for the last date sent per student.
>  
>
Right again -- we can keep the notifications in a field in the STUDENTS 
table.

-- 
Chris
http://clabs.org/blogki



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