How to design link tables?

I have been having a similar discussion on the MS Access newsgroup for the
last w and I wanted to discuss the issues in the context of MS SQL in
addition to MS Access. I hope this form of cross posting is not offensive to
anyone.
Lets assume I have to relations: student and course and I want to create a
junction table to record which students are taking which courses.
(1) If I create a two column table containing fkStudent and fkCourse where
the primary consists of these two columns. Now I want to perform a join to
find out all the courses a particular student is taking. Since the primary
key index structure requires both foreign keys and I'm only providing the
value for fkStudent and not fkCourse, will I be preforming a linear search
on the junction table when I preform a join to determine what courses I am
taking?
My guess is yes. If your answer is yes, would you anticipate a linear search
to be a problem? I would. If you agree that we should avoid linear searches,
then what would you recommend for a primary key on this link table? Do we
need a pimary key at all? Most folks say yes -- but I'm not clear on why.
(2) Lets say I have many thousands of job titles -- too many for a combo
box. I want to have a M:M relationship between Job Titles and Job Postings.
Let's say I find a job posting on the web and I need to find the foriegn key
for a job title (if it exists) or create a job title (if it does not exist).
I can assume it does not exist and try an SQL INSERT and, if that fails, use
a SELECT. Or, I can assume it does exist and use SELECT and if I don't find
any, use INSERT. Either way, the worst case scenerio requires two redundant
lookups. Is there a better approach?
Thanks,
SiegfriedHi
1) Look at [Order Details] table in Northwind database.
It is a "junction table" between Orders and Product tables.
2)
IF NOT EXISTS (SELECT * FROM Table WHERE....)
INSERT INTO ......
ELSE
SELECT <columns> FROM ......
"Siegfried Heintze" <siegfried@.heintze.com> wrote in message
news:%23ofWEqlsFHA.460@.TK2MSFTNGP15.phx.gbl...
>I have been having a similar discussion on the MS Access newsgroup for the
> last w and I wanted to discuss the issues in the context of MS SQL in
> addition to MS Access. I hope this form of cross posting is not offensive
> to
> anyone.
> Lets assume I have to relations: student and course and I want to create a
> junction table to record which students are taking which courses.
> (1) If I create a two column table containing fkStudent and fkCourse where
> the primary consists of these two columns. Now I want to perform a join to
> find out all the courses a particular student is taking. Since the primary
> key index structure requires both foreign keys and I'm only providing the
> value for fkStudent and not fkCourse, will I be preforming a linear search
> on the junction table when I preform a join to determine what courses I am
> taking?
> My guess is yes. If your answer is yes, would you anticipate a linear
> search
> to be a problem? I would. If you agree that we should avoid linear
> searches,
> then what would you recommend for a primary key on this link table? Do we
> need a pimary key at all? Most folks say yes -- but I'm not clear on why.
> (2) Lets say I have many thousands of job titles -- too many for a combo
> box. I want to have a M:M relationship between Job Titles and Job
> Postings.
> Let's say I find a job posting on the web and I need to find the foriegn
> key
> for a job title (if it exists) or create a job title (if it does not
> exist).
> I can assume it does not exist and try an SQL INSERT and, if that fails,
> use
> a SELECT. Or, I can assume it does exist and use SELECT and if I don't
> find
> any, use INSERT. Either way, the worst case scenerio requires two
> redundant
> lookups. Is there a better approach?
> Thanks,
> Siegfried
>