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内连接：select * from A inner join B on A.Key = B.Key;

左连接：select * from A left join B on A.Key = B.Key;

右连接：select * from A right join B on A.Key = B.Key;

等值连接：select A_name,B_name,B_mobile from A, B where A.A_id = B.B_id

自连接：select p1.user_id, p1.user_name from usertable as p1, usertable as p2 where p1.user_id = p2.user_id and p2.user_id = '10086';

交叉连接：卡迪尔积，没有建立联结关系的表，行数为两表行数相乘

等值连接和内连接查询效果等效，用on建立连接关系，用where进行筛选

mysql 不支持 full join