如果要计算多个列之间的重复项，请使用
group by：
select ColumnA, ColumnB, ColumnC, count(*) as NumDuplicatesfrom tablegroup by ColumnA, ColumnB, ColumnC
如果只希望重复的值，则计数大于1。可使用以下
having子句获得此值：
select ColumnA, ColumnB, ColumnC, count(*) as NumDuplicatesfrom tablegroup by ColumnA, ColumnB, ColumnChaving NumDuplicates > 1
如果您实际上希望所有重复的行都返回，则将最后一个查询返回到原始数据：
select t.*from table t join (select ColumnA, ColumnB, ColumnC, count(*) as NumDuplicates from table group by ColumnA, ColumnB, ColumnC having NumDuplicates > 1 ) tsum on t.ColumnA = tsum.ColumnA and t.ColumnB = tsum.ColumnB and t.ColumnC = tsum.ColumnC
假设所有列值都不为NULL，这将起作用。如果是这样，请尝试：
on (t.ColumnA = tsum.ColumnA or t.ColumnA is null and tsum.ColumnA is null) and (t.ColumnB = tsum.ColumnB or t.ColumnB is null and tsum.ColumnB is null) and (t.ColumnC = tsum.ColumnC or t.ColumnC is null and tsum.ColumnC is null)
编辑：
如果有
NULL值，也可以使用
NULL-safe运算符：
on t.ColumnA <=> tsum.ColumnA and t.ColumnB <=> tsum.ColumnB and t.ColumnC <=> tsum.ColumnC
