At 19:26 -0700 7/7/04, John Mistler wrote:
I am not certain from the documentation whether it is advisable to create a
unique multi-column index on two columns that are already individually
indexed. The individual indexes I assume I need for when I do a SELECT on
those particular columns. The multi-column one I need for the reasons
discussed below. Any one know?
If you have indexes on column A and column B, then if you create
a multiple-column unique index on (A,B), you could remove the index
on A. The reason for this is that MySQL can use a leftmost prefix
of a multiple-column index as if you had an index on just the leftmost
columns.
In other words, an index on (A, B) can be use when you search for combinations
of A and B, or when you search for just A.
You cannot remove the index that you have on just B, because B is not
a leftmost index of (A, B).
http://dev.mysql.com/doc/mysql/en/Multiple-column_indexes.html
Thanks,
John
on 7/7/04 2:21 PM, Joshua J. Kugler at [EMAIL PROTECTED] wrote:
Certainly, it's called making a unique index on the field(s) you
want to keep
unique.
Hope that helps.
j----- k-----
On Wednesday 07 July 2004 12:48 pm, John Mistler said something like:
Is there a way to do an INSERT on a table only if no row already exists
with the same info for one or more of the columns as the row to be
inserted? That is, without using a method outside SQL?
Thanks,
John
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