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下面是表:MySQL的,困惑的嵌套计数
+------+--------+------+--------+
| sID | sName | GPA | sizeHS |
+------+--------+------+--------+
| 123 | Amy | 3.9 | 1000 |
| 234 | Bob | 3.6 | 1500 |
| 345 | Craig | 3.5 | 500 |
| 456 | Doris | 3.9 | 1000 |
| 567 | Edward | 2.9 | 2000 |
| 678 | Fay | 3.8 | 200 |
| 789 | Gary | 3.4 | 800 |
| 987 | Helen | 3.7 | 800 |
| 876 | Irene | 3.9 | 400 |
| 765 | Jay | 2.9 | 1500 |
| 654 | Amy | 3.9 | 1000 |
| 543 | Craig | 3.4 | 2000 |
+------+--------+------+--------+
我想不通的逻辑是这样的查询背后究竟
select *
from Student S1
where (select count(*) from Student S2
where S2.sID <> S1.sID and S2.GPA = S1.GPA) =
(select count(*) from Student S2
where S2.sID <> S1.sID and S2.sizeHS = S1.sizeHS);
这是返回什么:
+------+--------+------+--------+
| sID | sName | GPA | sizeHS |
+------+--------+------+--------+
| 345 | Craig | 3.5 | 500 |
| 567 | Edward | 2.9 | 2000 |
| 678 | Fay | 3.8 | 200 |
| 789 | Gary | 3.4 | 800 |
| 765 | Jay | 2.9 | 1500 |
| 543 | Craig | 3.4 | 2000 |
+------+--------+------+--------+
count是一个聚合命令,它是否能够等于另一个聚合并在它是where条件时返回一个表?
很好的解释 – Andrew