如何在mysql中不存在记录时创建检查？

Question

Hi I want to show the county in the following query but if county does not exist, return the city instead.

        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

Thanks

Answer 1

Try using COALESCE() :

COALESCE(county, city) AS location

If county is NULL it will return the value of city instead.

So:

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