How to detect and remove a column that contains only null values?

Question

In my table table1 there are 6 columns Locations,a,b,c,d,e.

Locations [a]   [b]   [c]  [d]   [e]

[1]       10.00 Null  Null 20.00 Null

[2]         


        

Answer 1

How to detect whether a given column has only the NULL value:

SELECT 1  -- no GROUP BY therefore use a literal
  FROM Locations
HAVING COUNT(a) = 0 
       AND COUNT(*) > 0;

The resultset will either consist of zero rows (column a has a non-NULL value) or one row (column a has only the NULL value). FWIW this code is Standard SQL-92.