name表
| id | name |
| 1 | tom |
| 2 | boy |
school表：
| name | school |
| tom | 哈佛 |
| boy | 清华 |
city表：
| school | city |
| 哈佛 | 纽约 |
| 清华 | 北京 |
通过id查city
SELECT
t3.id,
t4.city
FROM
(
SELECT
t1.id,
t2.school
FROM
idtable t1
LEFT JOIN nametable t2 ON t1.NAME = t2.NAME
WHERE
t1.id = 1
) t3
LEFT JOIN city t4 ON t3.school = t4.school
玩似的
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