sum(case when ct.tradeTotal >= 0 then 1 else 0 end)的意思

String hql = "select new com.ks.admin.report.dto.ReportMonthWithDrawalDto(" +
				"count(*)," +
				"sum(ct.tradeTotal)," +
				"sum(case when ct.tradeTotal >= 0 then 1 else 0 end)," +
				"sum(case when ct.tradeTotal >= 0 then ct.tradeTotal else 0 end)" +
				") " + "from CustTrade ct " + "where ct.tradeType = 'DR' " + "and ct.tradeTime between ? and ? " + "and ct.tradeDesc like '%提现%'";

count（*）返回的是Long类型，Sum(Double类型)返回Double类型， sum(case when ct.tradeTotal >= ? then ct.1 else 0 end) 返回Long类型。

sum(case when ct.tradeTotal >= 0 then 1 else 0 end)的意思是当ct.tradeTotal>=0的时候会返回1，否则返回0，而sum会累加，因为0累加之后还是0，所以只会返回1的累加之后才会改变值，因此这表示统计ct.tradeTotal>=0的数据总数。为什么不用count呢？因为试验过，count里面无法用条件。

sum(case when ct.tradeTotal >= 0 then ct.tradeTotal else 0 end)的意思是当ct.tradeTotal>=0的时候返回ct.tradeTotal的值，否则返回0，同理，如果条件成立，会累加ct.tradeTotal>=0的值。

如果以上描述有错误，请提出，一起共同学习。