using where clause in SQL - sql

Question

I'm using postgre sql and I have a problem with where clause.
What I wanted to do was:
compare each month's data to that of previous month
select location where august to july ratio was lower than 0.7
Below is my code. I used cast as decimal because without that the result was just integers (either 0 or 1).
If I run the query, it does not return any results. It doesn't say error, but that there is no data where august < 0.7 (But there is! I checked in the original table without the where clause)
SELECT location
, round(cast(june as decimal)/may, 3) as june
, round(cast(july as decimal)/june, 3) as july
, round(cast(august as decimal)/july, 3) as august
FROM per_country
WHERE august < 0.7
Can you tell me what is wrong with my code? Thank you in advance

Answer 1

You cannot reuse an alias defined in the select clause in the where clause. You need to repeat the expression, or use a derived table (subquery, cte):
SELECT location
, round(june::numeric/may, 3) as june
, round(july::numeric/june, 3) as july
, round(august::numeric/july, 3) as august
FROM per_country
WHERE august::numeric/july < 0.7
Note that your existing query indicates of a bad design. You should not be storing each month in a separate table, for many reasons (scalability, efficiency, maintainability, ...). Instead, you should have each month on a separate row.