从查询中获取结果的空值

Question

我有这样的疑问：

SELECT U.LOCATION,SUM(S.EXISTENCIA) AS Total
FROM LOCATIONS U
LEFT JOIN CANT_WAREHOUSE S ON U.LOCATION= S.LOCATION
WHERE U.ENABLED= 1 
GROUP BY U.LOCATION

并像这样返回数据：

LOCATION  | Total
2A-1      | 566.0000000
2A-2      | 14.0000000
2A-3      | 1.0000000
2A-4      | (null)
2A-5      | 1.0000000
2A-6      | (null)
2A-7      | 6.0000000
2B-1      | 560.0000000
2B-2      | (null)

有没有办法使用子查询或类似的方法来获取'LOCATION‘，其中'TOTAL’= null？

Answer 1

使用HAVING子句

SELECT U.LOCATION,SUM(S.EXISTENCIA) AS Total 
FROM LOCATIONS U
LEFT JOIN CANT_WAREHOUSE S ON U.LOCATION= S.LOCATION
  and U.ENABLED= 1 
GROUP BY U.LOCATION
HAVING SUM(S.EXISTENCIA) IS NULL

Answer 2

使用NOT EXISTS

SELECT U.LOCATION,
       NULL AS Total
FROM   LOCATIONS U
WHERE  NOT EXISTS (SELECT 1
                   FROM   CANT_WAREHOUSE S
                   WHERE  U.LOCATION = S.LOCATION)
       AND U.ENABLED = 1
GROUP  BY U.LOCATION