问题
I have a table defined like this:
Column: Version Message
Type: varchar(20) varchar(100)
----------------------------------
Row 1: 2.2.6 Message 1
Row 2: 2.2.7 Message 2
Row 3: 2.2.12 Message 3
Row 4: 2.3.9 Message 4
Row 5: 2.3.15 Message 5
I want to write a T-Sql query that will get message for the MAX version number, where the "Version" column represents a software version number. I.e., 2.2.12 is greater than 2.2.7, and 2.3.15 is greater than 2.3.9, etc. Unfortunately, I can't think of an easy way to do that without using CHARINDEX or some complicated other split-like logic. Running this query:
SELECT MAX(Version) FROM my_table
will yield the erroneous result:
2.3.9
When it should really be 2.3.15. Any bright ideas that don't get too complex?
回答1:
One solution would be to use a table-valued split function to split the versions into rows and then combine them back into columns so that you can do something like:
Select TOP 1 Major, Minor, Build
From ( ...derived crosstab query )
Order By Major Desc, Minor Desc, Build Desc
Actually, another way is to use the PARSENAME function which was meant to split object names:
Select TOP 1 Version
From Table
Order By Cast(Parsename( Z.Version , 3 ) As Int) Desc
, Cast(Parsename( Z.Version , 2 ) As Int) Desc
, Cast(Parsename( Z.Version , 1 ) As Int) Desc
回答2:
Does it have to be efficient on a large table? I suggest you create an indexed persisted computed column that transform the version into a format that ranks correctly, and use the computed column in your queries. Otherwise you'll always scan end to end.
If the table is small, it doesn't matter. Then you can use a just-in-time ranking, using a split function, or (ab)using the parsename as Thomas suggested.
来源：https://stackoverflow.com/questions/3057532/how-to-get-max-value-of-a-version-number-varchar-column-in-t-sql