发现和上面的not exist差不多!你是不是又恢复对not in的信心了呢? 但是 deletefromtb_scwhere(studentid,courseid,score)notin(selectstudentid,courseid,max(score)asscorefromtb_scgroupbystudentid,courseid) 确实是慢得让人发指,而将delete换成select之后,却并不慢,不信大家请执行下面sql: selecta.*fromtb...
发现和上面的not exist差不多!你是不是又恢复对not in的信心了呢? 但是 deletefromtb_scwhere(studentid,courseid,score)notin(selectstudentid,courseid,max(score)asscorefromtb_scgroupbystudentid,courseid) 1. 确实是慢得让人发指,而将delete换成select之后,却并不慢,不信大家请执行下面sql: selecta.*fr...
--in关键字尽量要少使用,因为性能比较低,可以使用 exists 来代替性能很高 select*fromdept twheret.deptnoin(selectdistinctdeptnofromemp); --exists()子查询的记录数是 0 则整个表达式是 false 如果大于 0 为 true, --exists 子查询一般是要和外侧查询关联的 select*fromemp twhereexists(select*fromdept dwh...
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SQL> select * from t2 where not exists (select 1 from t1 where t1.c2=t2.c2); C1 C2 --- --- 1 3、结论 in和exists结果相同(都会排除掉null,无论内部表中有无null) not in会过滤掉外部表中的null(即使内部表中无null) not exists不会过滤掉外部...
This entry was posted in,SQLNOT EXISTS,NOT IN,NULLsubqueryUnnestingon15. October 2024. first,secondandthird). When I was preparing a talk about optimizer statistics for data engineers recently, I took a closer look at it again. Oracle has now continuously improved the feature over several ver...
1、稳定给定SQL语句的执行计划,防止执行环境或对象统计信息等因子的改变对SQL语句的执行计划产生影响 2、减少数据库中出现SQL语句性能退化的概率,理论上不允许一条语句切换到一个比已经执行过的执行计划慢很多的新的执行计划上(可以通过OPTIMIZER_USE_SQL_PLAN_BASELINE实现) ...
Oracle publishing requires the registry entry to exist and to be set to1for 64 bit Distributors. If the entry does not exist, SQL Server will attempt to create it. If the entry exists, but is set to0, the setting will not be changed; the configuration of the Oracle Publisher will fail...
This is not only cumbersome, it also results in very poor performance—a true anti-pattern. However, turning rows into columns is the pivot problem in its purest form. Therefore, these steps should be followed again: (1) use group by to reduce the rows to one row per entity, (2) use...
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