MySQL第七课 left join左连接

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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转载自blog.51cto.com/fengyuzaitu/2429210
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