MS Access 07 - Q re lookup column vs many-to-many; Q re checkboxes in many-to-many forms_问答_开发者

I'm creating a database with Access. This is just a test database, similar to my requirements, so I can get my skills up before creating one for work. I've created a database for a fictional school as this is a good playground and rich data (many students have many subjects have many teachers, etc).

Question 1 What is the di开发者_StackOverflow社区fference, if any, between using a Lookup column and a many-to-many associate table? Example: I have Tables 'Teacher' and 'Subject'. Many teachers have many subjects. I can, and have, created a table 'Teacher_Subject' and run queries with this. I have then created a lookup column in teachers table with data from subjects. The lookup column seems to take the place of the teacher_subject table. (though the data on relationships is obviously duplicated between lookup table and teacher_subject and may vary). Which one is the 'better' option? Is there a snag with using lookup tables? (I realize that this is a very 'general' question. Links to other resources and answers saying 'that depends...' are appreciated)

Question 2 What attracts me to lookup tables is the following: When creating a form for entering subjects for teachers, with lookup I can simply create checkboxes and click a subject for a teacher 'on' or 'off'. Each click on/off creates/removes a record in the lookup column (which replaces teacher_subject). If I use a form from a query from teacher subject with teacher as main form and subject as subform I run into this problem: In the subform I can either select each subject that teacher has in a bombo box, i.e. click, scroll down, select, go to next row, click, scroll down, etc. (takes too long) OR I can create a list box listing all available subjects in each row but allowing me to select only one. (takes up too much space). Is it possible to have a click on/off list box for teacher_subject, creating/removing a record there with each click?

Note - I know zero SQL or VB. If the correct answer is "you need to know SQL for this" then that's cool. I just need to know.

Thanks!

Lookup columns in tables will cause you more stress than joy. Unless you need them for Sharepoint, they should be avoided. You may wish to read http://r937.com/relational.html and http://www.mvps.org/access/tencommandments.htm

I wouldn't use them. Your example is fine, but there are limitations. What do you do when you need to reference another field from the Subject table other than the name? How would you differentiate subjects that are only offered on a semester basis?

You have no way of getting a count of how many subjects each teacher is assigned without some ugly coding.

Another limitation, is when you start identifying who taught what courses during a given school year.

I'm kind of unclear on your second question, but it sounds to me like you need a subform with a dropdown list.

If you want to do the checkbox thing, it quickly becomes a lot more complicated. To me, you're starting from user interface and working backwards to structure, instead of going the other direction.

I hesitate to mention it, but in terms of full disclosure you should know that in A2007 and A2010, you have multi-value fields available, and they are presented with exactly the UI you describe. But they have many of the same problems as lookup fields, and are quite complex to work with in code. Behind the scenes, they are implemented with a standard many-to-many join table, but it's all hidden from you.

I wish MS would make the listbox with checkbox control that is used with MV fields available for all listboxes, but binding that to a many-to-many join table would be complex if the listbox control were not designed for that (with link child/link master properties, for instance).

I tried to come up with a way to offer you the UI feature you prefer from multi-value fields without actually using multi-value fields. That seems challenging to me.

The closest I could come up with is to load a disconnected recordset with your "List" choices and a check box field. Then create a form, or subform, based on that recordset which you present in datasheet view. It could look similar to a combo bound to a multi-value field. In the after update event of the checkbox field, you would need code to add or remove a record from the junction table as required.

However, I don't know if this is something you would care to tackle. Earlier you indicated a willingness to learn SQL if needed; the approach I'm suggesting would also require VBA. Maybe take a look at Danny Lesandrini's article, Create In-Memory ADO Recordsets, to see whether it is something you could use.

OTOH, maybe the most appropriate answer for you is to keep the multi-value fields and get on with the rest of your life. I'm stuck. But now that we know you are actually using multi-value fields, perhaps someone else will be able to offer you a more appropriate suggestion.