ITPub博客

半连接的定义A semi-join is a join between two sets of data (tables) where rows from the first set are returned, basedon the presence or absence of at least one matching row in the other set.[@more@]半连接的定义A semi-join is a join between two sets of data (tables) where rows from the first set are returned, basedon the presence or absence of at least one matching row in the other set.The main difference between a normal inner joinand a semi-join is that with a semi-join, each record in the firs

一，先看看官方文档的解释：Often the database can improve the performance of queries that contain subqueries by unnesting the subqueries and converting them into joins. Most subqueries are unnested by the query transformer. For those subqueries that are not unnested, separate subplans are generated. 大概意思就是说oracle可以对包含子查询的查询语句可以通过非嵌套子查询和把子查询转换成普通表连接方式来提高查询的效率[@more@]并且大多数子查询可以通过查询转换操作以非嵌套的方式来处理。对于嵌套的子查询优化器