两者是一样的 left join左连接 left outer join 左外连接 只是写法不同,相同的概念 ,缩写而已。
确实,3L说对了,记错了,inner join没有所谓的左连右连 测试一下就知道了select a.value, b.value from (select 1 as id, 'a' as value) a inner join (select 0 as id, 'b' as value) b on a.id = b.idselect a.value, b.value from (select 1 as id, 'a' as value) a left join (select 0 as id, 'b' as value) b on a.id = b.id
左外连接也是以左表为主,右表有关联的选出,没有关联的用null代替选出,也就是左表的数据件数大于等于右表,也就是总件数以左表为准
比如 左表有 1,2,3,右表有1,3
左连接选出结果是1,3,1,3,共2件
左外连接选出结果是1,2,3,1,null,3,共3件
left join左连接
left outer join 左外连接
只是写法不同,相同的概念 ,缩写而已。
测试一下就知道了select a.value, b.value from
(select 1 as id, 'a' as value) a inner join (select 0 as id, 'b' as value) b
on a.id = b.idselect a.value, b.value from
(select 1 as id, 'a' as value) a left join (select 0 as id, 'b' as value) b
on a.id = b.id