Mysql Unknown column in where clause union all

Kanien Notiros :

Still related to my Previous Question, have a table(tb_data) which like this

+---------+---------------------+---------------------+---------------------+---------------------+-------+ 
| Disease | Additional_Disease1 | Additional_Disease2 | Additional_Disease3 | Additional_Disease4 | Room  |
+---------+---------------------+---------------------+---------------------+---------------------+-------+
| A01     | A03                 | A02                 |                     |                     | Man   |
| A03     | A02                 |                     |                     |                     | Woman |
| A03     | A05                 |                     |                     |                     | Child |
| A03     | A05                 |                     |                     |                     | Man   |
| A02     | A05                 | A01                 | A03                 |                     | UGD   |
+---------+---------------------+---------------------+---------------------+---------------------+-------+

My question is how to make it like this

+---------+-------+ 
| Disease | Total |
+---------+-------+
| A03     | 2     |
| A02     | 1     |
| A01     | 1     |
| A05     | 1     |
+---------+-------+

Here's my code attempt

    select Disease, count(*) total
    from (
    select Disease from tb_data
    union all select Additional_Disease1 from tb_data
    union all select Additional_Disease2 from tb_data
    union all select Additional_Disease3 from tb_data
    union all select Additional_Disease4 from tb_data
) t
where Disease is not Null
and Room = 'Man'
group by Disease
order by total desc, Disease

Which result the error

Error Code: 1054. Unknown column 'Room' in 'where clause'

Nick :

Your issue is that your derived table doesn't include the Room column. You can either filter by Room in the derived table:

select Disease, count(*) total
from (
    select Disease from tb_data where Room = 'Man'
    union all select Additional_Disease1 from tb_data where Room = 'Man'
    union all select Additional_Disease2 from tb_data where Room = 'Man'
    union all select Additional_Disease3 from tb_data where Room = 'Man'
    union all select Additional_Disease4 from tb_data where Room = 'Man'
) t
where Disease is not Null
group by Disease
order by total desc, Disease

Or include the Room column in the derived table:

select Disease, count(*) total
from (
    select Room, Disease from tb_data
    union all select Room, Additional_Disease1 from tb_data
    union all select Room, Additional_Disease2 from tb_data
    union all select Room, Additional_Disease3 from tb_data
    union all select Room, Additional_Disease4 from tb_data
) t
where Disease is not Null
and Room = 'Man'
group by Disease
order by total desc, Disease

Mysql Unknown column in where clause union all

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