use select case in Mysql - mysql

Question

The question I try to solve:
Return the value for "prefix" following these conditions:
If model_hyouji is a number and ranging from 0 to 9, set prefix = 0
If model_hyouji is a letter and ranging from a to z or A to Z, set prefix = 1
In all other cases, set prefix = 2
I used this SQL:
SELECT model_code,
model_hyouji,
model_name_prefix,
model_kana_prefix,
model_count,
case prefix
when 'a' then 1
when 'b' then 2
else 3
end
FROM webikept.mst_model_v2
where model_maker_code = 1
and model_displacement between 51 and 125
and (SELECT substring(model_hyouji,1,1) as prefix
FROM webikept.mst_model_v2 )
and I get this error:
Unknown column 'prefix' in 'field list'

Answer 1

assuming you column model_hyouji is varchar(1) (otherwise use substr(model_hyouji,1) )
SELECT model_code,
model_hyouji,
model_name_prefix,
model_kana_prefix,
model_count,
case
when model_hyouji between '0' and '9' then 0
when model_hyouji between 'a' and 'z' then 1
when model_hyouji between 'A' and 'Z' then 1
else 2
end prefix
FROM webikept.mst_model_v2
where model_maker_code = 1
and model_displacement between 51 and 125
and (SELECT substring(model_hyouji,1,1) as prefix
FROM webikept.mst_model_v2 )
or you can use regexp
SELECT model_code,
model_hyouji,
model_name_prefix,
model_kana_prefix,
model_count,
case
when model_hyouji REGEXP '^[0-9]{1}$' then 0
when model_hyouji REGEXP '^[A-Za-z]$' then 1
else 2
end prefix
FROM webikept.mst_model_v2
where model_maker_code = 1
and model_displacement between 51 and 125
and (SELECT substring(model_hyouji,1,1) as prefix
FROM webikept.mst_model_v2 )

Answer 2

SELECT
model_code,model_hyouji,model_name_prefix,model_kana_prefix,model_count,
case
when prefix = 'a' then 1 when 'b' then 2 else 3 end
You need CASE WHEN prefix =
But you also dont specify which prefix column you want to use hence the error on unknown column
E.G
SELECT
model_code,model_hyouji,model_name_prefix,model_kana_prefix,model_count,
case
WHEN model_kana_prefix = 'a' then 1
WHEN model_kana_prefix = 'b' then 2
else 3 end