场景
在查询学生信息中，允许学生宿舍信息为空，如果有获取，没有为空
select t.*, cs_sc_studenthostel.name as hostelname
from (
select cs_sc_student.id as student_id,
cs_sc_student.name as studentname,
cs_sc_class.name as classname,
cs_sc_student.sc_studenthostel_id
from cs_sc_student, cs_sc_class
where cs_sc_student.sc_class_id = cs_sc_class.id) t
left join cs_sc_studenthostel on t.sc_studenthostel_id = cs_sc_studenthostel.id
第一步 获取到学生的基础信息
(
select cs_sc_student.id as student_id,
cs_sc_student.name as studentname,
cs_sc_class.name as classname,
cs_sc_student.sc_studenthostel_id
from cs_sc_student, cs_sc_class
where cs_sc_student.sc_class_id = cs_sc_class.id) t
第二步 进行左连接
left join cs_sc_studenthostel on t.sc_studenthostel_id = cs_sc_studenthostel.id
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