to write case when to another case when in one select Google Big Query - google-bigquery

Question

I'm a new user in Google Big Query and I want to write two "Case when...." in one select, and they should be connected to each other
For example:
Case when number = 1 then 'one' when number=2 then 'two' else 'other' end as mark
Case when mark = 'one' then 'red' when mark='two' then 'orange' else 'other' end as color
But I have mistake like 'Unrecognized name: mark at [8:14]'
I used all the options with apostrophe but still have the same result.
I also try to find answer in BigQuery Documentation but it was unsuccessfully.
Is it even possible to do this?

Answer 1

I assume you're using Google BigQuery's current SQL syntax support, as opposed to their older SQL implementation.
SQL does not allow column expressions to refer to other column aliases in the same level, so you will need to use an outer-query. Yes, it's dumb, blame the ISO SQL committee for creating such a terribly unergonomic language.
You'll want something like this:
I'm not a BigQuery user so I don't know if this syntax is fully supported, but I believe the query below is ISO SQL-compliant).
Note the use of CASE <expr> WHEN <value> instead of CASE WHEN <expr> = <value> btw.
SELECT
t.*
t.number,
CASE t.mark
WHEN 'one' THEN 'red'
WHEN 'two' THEN 'orange' ELSE 'other'
END AS color
FROM
(
SELECT
y.*,
CASE y.number
WHEN 1 THEN 'one'
WHEN 2 THEN 'two' ELSE 'other'
END AS mark
FROM
someTable AS y
) AS t
Another alternative is to use a CTE, which is essentially the same as the above but the inner-query is expressed above the outer-query instead of inside it (and it can be reused more easily in the same query):
WITH t AS (
SELECT
y.*,
CASE y.number
WHEN 1 THEN 'one'
WHEN 2 THEN 'two' ELSE 'other'
END AS mark
FROM
someTable AS y
)
SELECT
t.*
t.number,
CASE t.mark
WHEN 'one' THEN 'red'
WHEN 'two' THEN 'orange' ELSE 'other'
END AS color
FROM
t;

Answer 2

Consider also below option (BigQuery)
select *,
Case mark when 'one' then 'red' when 'two' then 'orange' else 'other' end as color
from data, unnest([
struct(Case number when 1 then 'one' when 2 then 'two' else 'other' end as mark)
])
Obviously this is a matter of preferences, but in my practice, I am using above approach much more frequently than subquery and/or cte
You can test it using dummy data as in below example
with data as (
select 1 number union all
select 2 union all
select 3
)
select *,
Case mark when 'one' then 'red' when 'two' then 'orange' else 'other' end as color
from data, unnest([
struct(Case number when 1 then 'one' when 2 then 'two' else 'other' end as mark)
])
with output