I have recently been introduced to the new Access 2007 feature which is multivalued fields. My initial impression is that it is a bad idea to make use of multiple values in a single field. Traditionally if you wanted to allow for a record to have several values for a field you would create another two tables and link them with foreign keys. This allows for easy querying and ensures that duplicate values reference the same item. Keeping lists in a cell seems like a violation of the purpose of databases.
Are there good uses for these fields which don't make me feel dirty?
The idea of multivalued fields was to support easy creation of report / interface objects, in addition, one can create a form that displays say categories for an issue. Instead of doing some intense work, god forbid joins, it was supposedly simplier to store:
Mechanical, Electrical
as a value in a field rather than
Mechanical Electrical
Personnally I do not like it and assume this type of field was created for non technical personnel like accountants :) (just kidding). No seriously, do not use this unless you are creating a silly tool that rarely anyone will use and rarely anyone will ever have to tap into.
The proper way to handle this is joins, no duplicates, and no multi values inside of columns (this is all 3nf anyhow).
Another reason this was created was to support the multi values inside of a sharepoint list.
Jon
See:
Multivalued datatypes considered harmful: How dangerous can a data type be?
I had a long talk with Suraj Poozhiyil, the Access Program Manager... both Suraj and I agree wholeheartedly that developers do not need to use multi-valued fields. People who understand databases already have a good way of implementing many to many relationships and will gain no benefit from multi-valued fields.
So, my clear and certain advice to developers is not to use multi-valued fields. They have nothing to offer us except potential pain.
Not really answering the question here, but readers might like to note that there is an entire niche industry around the idea of MultValued Databases:
These databases differ from a relational database in that they have features that support and encourage the use of attributes having a list of values, rather than all attributes having a single value
Since in this case the database engine has extensions to it's query language to accommodate the multi-dimensional nature of it's tables (which I assume Access probably does not) then it's not really comparable to multivalued fields in Access. But an interesting parallel in any case (for anyone who's not previously even heard of MultValued Databases).
A big segment of the Access market is non-developer, but kind of technical, users. They might not understand the value of normalization, but they can get something to work. They just need something easy and it's better than a free-text field where people type in, where you hope they all type the same thing.
As they learn more, they might start using other tables and foreign keys. But, sometimes, a multi-valued field is good enough.
multivalued fields can easily save you from having to create a new table and relationship.
Soda --> Types
Why do I need a whole new table just to say that Pepsi comes in regular, diet, and more.
I wish they allow us to give multivalued fields columns, then they would be just like a table, but with far less work
JUST SAY NO!
if you are learning SQL, learn the right way and normalize your tables. if you know database design do it properly. Not every feature has to be used.
Necro-post... I think the question should have been revised when the thread first started, but I won't go through the edit process now.
The question is "Multivalued Fields a Good Idea?"
The real question that should have been asked is "Multivalued Fields in RDBMS a Good Idea?"
As others have noted there is an entire MVDBMS model supporting multi-valued fields. I'm an expert in this area and have been working with the model for over 30 years. Of course it's a good idea in my opinion and to others who use the platform every day. And yes, Caché not only has a great multidimensional model itself but it also supports the MVDBMS model. So in this respect, the answer to the question is YES.
But for a RDBMS and specifically MS ACCESS the answer is almost certainly NO because neither the RDBMS model nor that platform inherently support the concept.
The accepted answer is correct, IMO, as it doesn't just answer the question asked, it answers the question that was intended to be asked. But to be meticulous, for the exact question asked, the accepted answer is incorrect.
I believe the real answer is "It's only a good idea if the DBMS platform supports it, YES for MVDBMS and perhaps other NoSQL platforms, NO for RDBMS."
I really don't like the multi-valued fields. Maybe they did it to make it easier to interface with other multi-valued systems like the old PICK/Unidata system. I bet it's fun upsizing an Access database with heavy use of this new feature to SQL Server.
来源：https://stackoverflow.com/questions/1461582/multivalued-fields-a-good-idea