首先，看如下两个查询语句：
第一种
SELECT t.id, t.meetStyle, t.title,
count(p.id) AS personCount,
count(f.id) AS fileCount
FROM meeting_info t
LEFT JOIN meeting_person p
ON t.id= p.mId
LEFT JOIN meeting_file f
ON t.id=f.mId where t.ID='info0000000234'
第二种
SELECT t.id, t.meetStyle, t.title,
count(DISTINCT p.id) AS personCount,
count(DISTINCT f.id) AS fileCount
FROM meeting_info t
LEFT JOIN meeting_person p
ON t.id= p.mId
LEFT JOIN meeting_file f
ON t.id=f.mId where t.ID='info0000000234'
这两条语句虽然只有一词之差，实际却相差千里，因为第一条语句的查询结果和如下语句查询结果相同：
SELECt f1.id,f1.MeetStyle,f1.title,f1.personCount,f2.fileCount from
(SELECT t.id, t.meetStyle, t.title,
count(p.id) AS personCount
FROM meeting_info t
LEFT JOIN meeting_person p
ON t.id= p.mId
where t.ID='info0000000234' ) f1
,
(SELECT t.id, t.meetStyle, t.title,
count(f.id) AS fileCount
FROM meeting_info t
LEFT JOIN meeting_file f
ON t.id=f.mId where t.ID='info0000000234') f2
)
由上面两种查询可以看出，count(DISTINCT p.id)具有过滤当数值为空的情况，使其不再计入总数
