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