select * from user where exists (select 1); 对user表的记录逐条取出,由于子条件中的select 1永远能返回记录行,那么user表的所有记录都将被加入结果集,所以与 select * from user;是一样的 又如下 select * from user where exists (select * from user where userId = 0); 可以知道对user表进行loop时...
select /*+ monitor */ * from T1 where exists(select 1 from T2 where T1.sql_id = T2.sql_id) ; select /*+ monitor */ * from T1 where T1.sql_id in (select T2.sql_id from T2) ;SQL>select sql_id, sql_text from v$sqlwheresql_text like'%T2.sql_id%'SQL_ID SQL_TEXT ---...
select * from user where exists (select 1); 对user表的记录逐条取出,由于子条件中的select 1永远能返回记录行,那么user表的所有记录都将被加入结果集,所以与 select * from user;是一样的 又如下 select * from user where exists (select * from user where userId = 0); 可以知道对user表进行loop时...
select count(*) from test1 t1 where not exists(select 1 from test2 t2 where t1.object_id=t2.object_id); 执行计划相同,此处就省略了。 执行计划相同;效率一样 not in 比 not exists 效率高的例子(依然和转载文章结果不符,结果还是效率相同,后来我用hint改变了not in的执行计划才能显示出not in的优势...
有两个简单例子,以说明 “exists”和“in”的效率问题 1) select * from T1 where exists(select 1 from T2 where T1.a=T2.a) ; T1数据量小而T2数据量非常大时,T1<<T2 时,1) 的查询效率高。 2) select * from T1 where T1.a in (select T2.a from T2) ; T1数据量非常大而T2数据量小时,...
1、关于在 Oracle8i 时代中in和exists的区别这里有条SQL语句:select * from A where id in(select id from B) 以上查询使用了in语句,in()只执行一次,它查出B表中的所有id字段并缓存起来.之后,检查A表的id是否与B表中的id相等,如果相等则将A表的记录加入结果集中,直到遍历完A表的所有记录;它的查询过程类似...
http://download.oracle.com/docs/cd/E11882_01/server.112/e17118/sql_elements005.htm#i59110 A condition that evaluates to UNKNOWN acts almost like FALSE.For example, a SELECT statement with a condition in the WHERE clausethat evaluates toUNKNOWN returns no rows. However, a cond...
Oracle中 IN 和 EXISTS 的性能区别和使用注意事项 (1). select * from T1 where exists(select 1 from T2 where T1.a=T2.a) ; T1数据量小而T2数据量非常大时,T1<<T2 时,(1)的查询效率高。 (2). select * from T1 where T1.a in (select T2.a from T2) ;...
简介:--在in和exists中,oracle可以把父查询推进到子查询SQL> SELECT COUNT(*) 2 FROM T0 LT 3 WHERE EXISTS (SELECT NULL FROM T1 LA WHERE LA. --在in和exists中,oracle可以把父查询推进到子查询 SQL> SELECT COUNT(*) 2 FROM T0 LT 3 WHERE EXISTS (SELECT NULL FROM T1 LA WHERE LA.T1_ID = ...
问Oracle INSERT、SELECT和NOT EXISTSEN远程连接 MySQL mysql -h <ip> -P 3306 -u root -p 然后...