mysql数据库(11)--进阶一之join

一、查看存储引擎

show variables like '%storage_engine%';

二、MyISAM与InnoDB的比较

 三、SQL书写顺序  vs 执行顺序

 

 四、Join连接（7种）

• 内连接：select <select list>  from tableA inner join tableB on A.key = B.key
• 左连接：select <select list>  from tableA left join tableB on A.key = B.key  （不满足的补NULL）
• 右连接：select <select list>  from tableA right join tableB on A.key = B.key  （不满足的补NULL）
• 左连接-内连接：select <select list>  from tableA left join tableB on A.key = B.key where B.key is NULL
• 右连接-内连接：select <select list>  from tableA right join tableB on A.key = B.key where A.key is NULL
• 全外连接= A独有+B独有+A、B共有：
  • 　　select <select list>  from tableA left join tableB on A.key = B.key union select <select list>  from tableA right join tableB on A.key = B.key  （union具有去重的功能）
• 全外连接-内连接：
  • 　　select <select list>  from tableA left join tableB on A.key = B.key where B.key is NULL union select <select list>  from tableA right join tableB on A.key = B.key where A.key is NULL （union具有去重的功能）