在Oracle中， 除了inner join，left join，right join，还有
full join
SELECT column_name(s) FROM table_name1 FULL JOIN table_name2 ON table_name1.column_name=table_name2.column_name
只要其中某个表存在匹配，FULL JOIN 关键字就会返回行
在Mysql中，是没有full join的，怎么去实现呢
左联 + 右联 + union
SELECT column_name(s) from table_name1 left join table_name2 on table_name1.column_name = table_name2.column_name union SELECT column_name(s) from table_name1 right join table_name2 on table_name1.column_name = table_name2.column_name