When making OUTER JOINs (ANSI-89 or ANSI-92), filtration location matters because criteria specified in the
ON clause is applied before the JOIN is made. Criteria against an OUTER JOINed table provided in the WHERE clause is applied after the JOIN is made.
This can produce very different result sets. In comparison, it
doesn't matter for INNER JOINs if the criteria is provided in the ON or WHERE clauses -- the result will be the same.所谓外连接指left join /right join /full outer join. 内连接就是其他的