Q:

ssrs expression sum doesn't sum all column

I have a column like:
LEFT_PIN_HEIGHT_MIN

 0
 0
 0
 1
 1
 0

I wrote it to Tablix as below,
=Sum(Fields!LEFT_PIN_HEIGHT_MIN.Value)
I want to sum the fields and result must be "2" but it doesn't sum the column 
and writes all the rows to Tablix. 

A:

I agree with the previous answer that your first choice ought to be to calculate this in the SQL, but sometimes that is not as practical.   
Are you trying to display the column's sum in each row?   If so, add the dataset name as a second parameter in the sum function, as in
=Sum(Fields!LEFT_PIN_HEIGHT_MIN.Value,"Dataset1")

Replace "Dataset1" with the name of your dataset.   The Sum function you're currently using is defining the sum within the context of each row in your tablix.   Adding the second parameter changes that context to return the sum for the entire dataset in each row.  
If your tablix is large, this may result in a performance hit, since the expression will evaluate each time it is displayed, hence the preference toward doing it in your dataset query.