finding the sum of a column [closed]

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I have three columns in a table first one for start time, second one for end time and the third one for the duration.

Now I want to find out the sum of the duration column, any ideas how can do it?

however I am using .net as programming language

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select sum(duration)
from tablename

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I don't know why three columns should be different but if I got your questions you can use SUM oprator:

SELECT SUM(Column3) FROM TABLE1

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Me.txtTotalDuration.Text = table.Compute("sum(duration)", String.Empty)