费用表....状态1显示费用在所有班级中都很常见...... 和状态2是不常见的费用................
+--------+------------------+-------------+--------+
| fee_id | fee_name | duration_id | status |
+--------+------------------+-------------+--------+
| 1 | Admission Fee | 4 | 1 |
| 2 | Re-admission Fee | 4 | 1 |
| 3 | Tuition Fee | 1 | 1 |
| 4 | Computer Lab Fee | 1 | 2 |
| 5 | Science Lab Fee | 1 | 2 |
| 6 | Exam Fee | 2 | 1 |
| 7 | Electricity Fee | 3 | 1 |
| 8 | Transport Fee | 1 | 1 |
| 9 | Late Fee | 1 | 1 |
+--------+------------------+-------------+--------+`
班级表....................
+----------+------------+
| class_id | class_name |
+----------+------------+
| 1 | Nursery |
| 2 | L.K.G |
| 3 | U.K.G |
| 4 | 1 |
| 5 | 2 |
| 6 | 3 |
| 7 | 4 |
| 8 | 5 |
| 9 | 6 |
| 10 | 7 |
| 11 | 8 |
| 12 | 9 |
| 13 | 10 |
+----------+------------+
`在这里，我将不常见的费用映射到课堂上。
+----------+--------+
| class_id | fee_id |
+----------+--------+
| 10 | 4 |
| 10 | 5 |
| 11 | 4 |
| 11 | 5 |
| 12 | 4 |
| 12 | 5 |
| 13 | 4 |
| 13 | 4 |
+----------+--------+
我必须为类ID 10获取fee_names，包括所有常见费用和不常见费用属于该class_id。
答案 0 :(得分：1)
试试这个（如果我理解你的要求）：
select f.fee_name from fee f join uncommon_class c where f.fee_id = c.fee_id
and c.class_id=10 union select fee_name from fee where status =1
答案 1 :(得分：0)
- 这对我来说似乎有点复杂，但应该是一种方法。 我们从费用中获取所有记录，然后离开加入课程，但仅限于class_ID 10。 然后我们只包含记录，其中class_ID为空，费用状态为1。
class_Fee表中只有class_ID的记录是10。
SELECT Fee_name
FROM Fee
LEFT JOIN Class_fee CF
on Fee.Fee_ID = CF.Fee_ID
and class_ID = 10
WHERE (CF.Class_ID is not null OR Fee.Status = 1)
- 工会方法似乎更直接
SELECT Fee_name
FROM Fee
WHERE status = 1
UNION
SELECT Fee_Name
FROM Fee
INNER JOIN Class_Fee CF
on CF.Fee_ID= Fee.Fee_ID
WHERE Class_ID = 10
如果你可以保证只有状态2记录得到映射，那么union可以是一个联合，通过避免distinct来获得一点性能。或者如果可以接受重复的名称，你可以使用union all。