Как создать проверку, если запись не существует в mysql?

Question

Привет, я хочу показать графство в следующем запросе, но если графство не существует, верните город вместо этого.

 SELECT P.id AS id, SUM(P.price) AS price, 
        P.tax, 
        county 
       FROM Purchases AS P 
       JOIN Status AS S  ON S.id = T.status_id 
       JOIN Shipping AS Ship ON S.shipping_id= Ship.id 
       JOIN Shipping_Addresses AS SA  ON Ship.shipping_address_id = SA.id 
       JOIN Zip_Codes   AS ZIP  ON ZIP.zip_code = SA.zip_code 
       JOIN Counties   AS C  ON ZIP.city = C.city 
       JOIN States    AS STATE ON STATE.id = C.county_state_id 

       GROUP BY ZIP.zip_code 
       ORDER BY county 

Благодаря

Answer 1

Попробуйте использовать COALESCE():

COALESCE(county, city) AS location 

Если county это NULL будет возвращать значение city вместо.

Итак:

SELECT P.id AS id, SUM(P.price) AS price, 
    P.tax, 
    COALESCE(county, city) AS location