mysql - 如果表中的字段中有 1，则需要检查其他表

Question

我有三个表

students: first_name, last_name, high_school_id, other_high_school

high_schols: id, title

other_high_schools: id, title, student_id

我想显示每个学生的列表，其中包含以下信息:名字、姓氏、高中

high_schools 表包含一个预先填写的高中列表，学生可以从中选择，如果他们没有找到他们的高中，他们会在“其他高中”字段中填写他们的高中。当学生提交表格时，他们的信息被存储，将 students.other_high_school 字段设置为 1 并将他们的 ID (student.id = other_high_school.student_id) 和他们高中的标题存储到 other_high_schools 表 (other_high_school.title) 中。

select 
 first_name, last_name, hs.title as high_school
from
 students s
left join
 high_schools hs
on
 s.high_school_id = hs.id

返回 first_name、last_name、high_school，但是否可以修改该查询以检测 students.other_high_school = 1，然后加入 other_high_school 而不是 high_schools 表？

这行不通，但应该有助于解释我想要完成的事情:

select 
 first_name, last_name, hs.title as high_school
from
 students s

CASE s.other_high_school
 WHEN 0 THEN
  left join
   high_schools hs
  on
   s.high_school_id = hs.id
 WHEN 1 THEN
  left join
   other_high_school hs
  ON
   s.id = hs.student_id
 ELSE
  left join
   other_high_school hs
  ON
   s.id = hs.student_id
END CASE

已解决

select 
 first_name, last_name, 
 IF(s.other_high_school = 1, ohs.title, hs.title) high_school
from
 students s
left join
 high_schools hs
on
 s.high_school_id = hs.id
left join
 other_high_schools ohs
on
 s.id = ohs.student_id

Answer 1

首先，我会重新考虑您的架构。您真的需要不同表中的 high_schools 和 other_high_schools 吗？或者您是否可以在一张表中添加一个额外的标志，说明它是默认高中还是用户添加的高中？

其次，我感觉您的解决方案不会很好地扩展，因为您在所有三个表上都采用 LEFT JOINing。随着表的增长，我怀疑性能会很快下降。我会建议一个替代方案:

SELECT first_name, last_name,title
FROM (
SELECT first_name, last_name,title
FROM students s
    INNER JOIN high_schools hs ON s.high_school_id = hs.id
WHERE s.other_high_school=0
UNION
SELECT first_name, last_name,title
FROM students s
    INNER JOIN other_high_schools ohs ON s.high_school_id = ohs.id
WHERE s.other_high_school=1
) AS combined_schools
ORDER BY last_name,first_name

使用正确的索引，这应该与您的解决方案一样快，并且可能会更好地扩展(但这完全取决于您的真实数据集的形状)。我添加了一个 ORDER BY 来展示您可以如何操作组合结果。