SQL，如果条件为真，则仅计数一次

Question

I have a table with three columns: Name, Country, Price and I need a SQL query that creates a fourth Boolean column named Qualify. This column needs to be true if Price<100 and there is no other row with price<100 and the same country.

Example:

Name   - Country- Price-  Qualify
Daniel - ES   -   98    - TRUE 
John  -  PT   -   45   -  TRUE 
Maria  - UK   -   102   - FALSE 
Anna   - PT   -   31   -  FALSE (because there is already a row with PT and Price<100)
Joseph - UK   -   25   -  TRUE 
Miriam  -DK   -   105  -  FALSE   

All this is because I do not want to count volumes more than one time if the price is under 100 and the country is the same. Is this even possible? Thanks

Answer 1

Think exists . In MySQL, you don't even need a case expression:

select t.*,
       (t.price < 100 and
        not exists (select 1
                    from t t2
                    where t2.country = t.country and t2.name <> t.name and t2.price < 100
                   )
       ) as flag
from t;

This assumes that name is unique, at least with respect to country.

Answer 2

Just providing another option using CASE statement:

Select 
@row_number:=CASE
        WHEN @country = Country AND Price < 100 AND @price < 100  
        THEN 1
        ELSE 0 END AS Qualify,
@country:= Country As Country,
@price:= Price As Price
FROM
Test
ORDER BY Country, Price

Here is the demo