[sql] What's the best way to select the minimum value from several columns?

The best way to do that is probably not to do it - it's strange that people insist on storing their data in a way that requires SQL "gymnastics" to extract meaningful information, when there are far easier ways to achieve the desired result if you just structure your schema a little better :-)

The right way to do this, in my opinion, is to have the following table:

ID    Col    Val
--    ---    ---
 1      1      3
 1      2     34
 1      3     76

 2      1     32
 2      2    976
 2      3     24

 3      1      7
 3      2    235
 3      3      3

 4      1    245
 4      2      1
 4      3    792

with ID/Col as the primary key (and possibly Col as an extra key, depending on your needs). Then your query becomes a simple select min(val) from tbl and you can still treat the individual 'old columns' separately by using where col = 2 in your other queries. This also allows for easy expansion should the number of 'old columns' grow.

This makes your queries so much easier. The general guideline I tend to use is, if you ever have something that looks like an array in a database row, you're probably doing something wrong and should think about restructuring the data.


However, if for some reason you can't change those columns, I'd suggest using insert and update triggers and add another column which these triggers set to the minimum on Col1/2/3. This will move the 'cost' of the operation away from the select to the update/insert where it belongs - most database tables in my experience are read far more often than written so incurring the cost on write tends to be more efficient over time.

In other words, the minimum for a row only changes when one of the other columns change, so that's when you should be calculating it, not every time you select (which is wasted if the data isn't changing). You would then end up with a table like:

ID   Col1   Col2   Col3   MinVal
--   ----   ----   ----   ------
 1      3     34     76        3
 2     32    976     24       24
 3      7    235      3        3
 4    245      1    792        1

Any other option that has to make decisions at select time is usually a bad idea performance-wise, since the data only changes on insert/update - the addition of another column takes up more space in the DB and will be slightly slower for the inserts and updates but can be much faster for selects - the preferred approach should depend on your priorities there but, as stated, most tables are read far more often than they're written.

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