Find all those columns which have only null values, in a MySQL table

Question

The situation is as follows:

I have a substantial number of tables, with each a substantial number of columns. I need to deal with this old and to-be-deprecated data

Answer 1

I think you can do this with GROUP_CONCAT and GROUP BY:

select length(replace(GROUP_CONCAT(my_col), ',', ''))
from my_table
group by my_col

(untested)

EDIT: the docs don't seem to state that GROUP_CONCAT needs a corresponding GROUP BY, so try this:

select 
    length(replace(GROUP_CONCAT(col_a), ',', '')) as len_a
    , length(replace(GROUP_CONCAT(col_b), ',', '')) as len_b
    , length(replace(GROUP_CONCAT(col_c), ',', '')) as Len_c
from my_table