Supply the Ar alias in the WHERE clause
SELECT
COALESCE(CONCAT(Ar.usaf, '-', Ar.wban),"NONE") AS TABLE_NAME
FROM
`metadata`.`ISH-HISTORY_HASPOS` A
INNER JOIN `metadata`.`Artificial` Ar ON (Ar.id = A.mergeId)
WHERE
A.usaf = Ar.usaf
AND A.wban = Ar.wban;
Looking at this query, you could improve the JOIN even more in three(3) ways
First, add the WHERE clause elements into the ON section of the JOIN
Second, make it a LEFT JOIN so as to accommodate NULL
situations
SELECT
COALESCE(CONCAT(Ar.usaf, '-', Ar.wban),"NONE") AS TABLE_NAME
FROM
`metadata`.`ISH-HISTORY_HASPOS` A
LEFT JOIN `metadata`.`Artificial` Ar
ON (Ar.id = A.mergeId) AND A.usaf = Ar.usaf AND A.wban = Ar.wban;
Third, add these indexes
ALTER TABLE `metadata`.`ISH-HISTORY_HASPOS` ADD INDEX (mergeId,usaf,wban);
ALTER TABLE `metadata`.`Artificial` ADD INDEX (id,usaf,wban);
Give it a Try !!!