SQL中的左连接、右连接,外连接,内连接关系
how : {‘left’, ‘right’, ‘outer’, ‘inner’}, default ‘inner’
Type of merge to be performed.
left: use only keys from left frame, similar to a SQL left outer join; preserve key order.
right: use only keys from right frame, similar to a SQL right outer join; preserve key order.
outer: use union of keys from both frames, similar to a SQL full outer join; sort keys lexicographically.
inner: use intersection of keys from both frames, similar to a SQL inner join; preserve the order of the left keys.