LEFTJOIN ON关键字会从左表那里返回所有的行,即使在右表中没有匹配的行。意思就是向左关联某个表记录,以左边的那个表的记录为基准,通过关联条件,将关联表的相关符合要求的记录一起找出来,找出来的记录条数就是左边表的记录数
具体用法如下:
SELECT column_name(s) FROM table_name1 LEFTJOINtable_name2ONtable_name1.name = table_name2.name
SELECT column_name(s) FROM table_name1 LEFTJOINtable_name2ONtable_name1.name = table_name2.name
也可以三级联动查询 :如下数据表
查询语句:
select u.user_id, u.user_name,u.user_sex,c.college_name,s.subject_name, s.achievementfrom user uLEFT JOIN subject s ON s.user_number=u. user_idLEFT JOIN college c ON c.subject_number=s.subject_id;
参考博客:/djr2ss666666/article/details/79808235