该查询为什么不返回结果计数?如何显示计数
SELECT COUNT (*) AS MWith
FROM member m
JOIN Channel mc ON mc.MemberID = m.id
JOIN Client c ON c.id = m.clientid
JOIN packages p ON p.id = m.packageid
WHERE Enroll > '2018'
AND EXISTS (
SELECT * FROM
activity a
WHERE a.memberid = m.id
AND a.code IN ('785', 'a599')
)
GROUP BY m.id;
输出
MWith
1
1
1
答案 0 :(得分:1)
空集是因为子句group by
。
解决方法是:
GROUP BY
,因为无论如何m.id都不是输出的一部分GROUP BY ALL
原始示例:
SELECT COUNT(*) AS MWith
FROM member m
JOIN Channel mc
ON mc.MemberID = m.id
JOIN Client c
ON c.id = m.clientid
JOIN packages p
ON p.id = m.packageid
WHERE Enroll > '2018'
AND EXISTS
(
SELECT *
FROM activity a
WHERE a.memberid = m.id
AND a.code IN ( '785', 'a599' )
)
-- GROUP BY m.id;
其他更简单的示例来显示差异:
-- returns an empty resultset
SELECT COUNT(*) FROM sys.databases
WHERE 1=0
GROUP BY name
-- returns: a single row with 0
SELECT COUNT(*) FROM sys.databases
WHERE 1=0
-- Another example with GROUP BY ALL
-- it returns one row per grouped value, with expected count = 0
SELECT COUNT(*) FROM sys.databases
WHERE 1=0
GROUP BY ALL name