Question

I have a T-SQL table that contains the following columns: Date, StationCode, HDepth, and MaxDepth. Each row in the MaxDepth column is set to 0 by default. What I am trying to do is find the maximum HDepth by Date and StationCode, and update the MaxDepth to a column on these rows. I have written a SELECT statement to find where the maximums occur and it is:

SELECT StationCode, [Date], MAX(HDepth) AS Maximum FROM dbo.[DepthTable] GROUP BY [Date], StationCode

How could I put this query into an Update statement to set the MaxDepth to 1 on the rows that are returned by this query?

Answer 1

You might try something like this:

UPDATE a
SET MaxDepth = 1
FROM dbo.[DepthTable] AS a
    JOIN (
        -- Your original query
        SELECT StationCode, [Date], MAX(HDepth) AS Maximum
        FROM dbo.[DepthTable]
        GROUP BY [Date], StationCode
    ) AS b ON a.StationCode = b.StationCode
        AND a.[DATE] = b.[DATE]
        AND a.HDepth = b.Maximum -- Here we get only the max rows

However, if a column is simply based upon other columns, then you might think about putting this logic into a view (to avoid update anomalies). The select for such a view might look like:

SELECT a.[Date], a.StationCode, a.HDepth,
    CASE WHEN b.Maximum IS NULL THEN 0 ELSE 1 END AS MaxDepth
FROM dbo.[DepthTable] AS a
    LEFT JOIN (
        -- Your original query
        SELECT StationCode, [Date], MAX(HDepth) AS Maximum
        FROM dbo.[DepthTable]
        GROUP BY [Date], StationCode
    ) AS b ON a.StationCode = b.StationCode
        AND a.[DATE] = b.[DATE]
        AND a.HDepth = b.Maximum -- Here we get only the max rows