using alias in where clause

I know I cannot use an alias in a where clause, but I am trying to
figure out how to achieve what I need.

If I could have an alias in a where clause my sql would look like this:

SELECT IFNULL(f_tag_bottom,
                IFNULL(f_tag_bottom_major_axis,
                 IFNULL(f_tag_bottom_minor_axis,
                  IFNULL(f_tag_ch_x_bottom,
                   IFNULL(f_tag_ch_y_bottom, NULL))))) as ftag,
               STDDEV(ch_x_top)
FROM data_cst
WHERE ftag = 'E-CD7'
GROUP BY wafer_id, lot_id

But I can't use ftag in the where clause. I can't put it in a having
clause, as that would exclude the already aggregated rows and I want
to filter then before the aggregation. Anyone have any idea how I can
do this?

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