我有这个查询：
SELECT *
FROM gross_profit
WHERE client_name = 'john'
AND timestamp = 'lm153-1531439443738'
这是结果：
+--------+-------------+---------------------+-------------+------------------+------------------+
| id | client_name | timestamp | profit_loss | profit_loss_unit | profit_loss_type |
+--------+-------------+---------------------+-------------+------------------+------------------+
| 500392 | john | lm153-1531439443738 | 1.90000000 | ADA | profit |
| 500393 | john | lm153-1531439443738 | 0.00210900 | USDT | fraction |
| 500394 | john | lm153-1531439443738 | 0.00000000 | BTC | normal |
| 509189 | john | lm153-1531439443738 | -1.43713843 | ADA | fee |
+--------+-------------+---------------------+-------------+------------------+------------------+
4 rows in set (0.01 sec)
现在，我要消除任何具有timestamp的{{1}}。
所以我将查询修改为此：
profit_loss_type = fee当然还有结果：SELECT *
FROM gross_profit
WHERE client_name = 'john'
AND timestamp = 'lm153-1531439443738'
GROUP BY timestamp
HAVING SUM(CASE WHEN profit_loss_type = 'fee' THEN 1 ELSE 0 END) = 0
但是为什么，当我将该查询用作子查询时：
Empty set (0.01 sec)它给了我：SELECT * FROM (
SELECT timestamp
FROM gross_profit
WHERE client_name = 'john'
AND profit_loss > 0
GROUP BY timestamp
HAVING SUM(CASE WHEN profit_loss_type = 'fee' THEN 1 ELSE 0 END) = 0
) AS t1
WHERE t1.timestamp = 'lm153-1531439443738'
和1 row in set (0.60 sec)显示为结果。
这真的使我感到困惑。为什么当我用作单个查询和子查询时结果不同？
答案 0 :(得分：1)
我认为这是由于WHERE条件造成的：
AND profit_loss > 0
在
然后，“费用” profit_loss类型之前已被过滤掉，结果不再为空。
尝试删除此行。