SQL开发实战系列(五):从执行计划看IN、EXISTS 和 INNER JOIN效率,我们要分场景不要死记网上结论 - 一、组合相关的行相对查询单表中的数据来说,平时更常见的需求是要在多个表中返回数据。比如,显示部门10的员工编码、姓名及所在部门名称和工作地址。select a.empno,a.d
该exists关键字可以以这种方式被使用，但实际上它的预期的一种方式，以避免计数：--this statement needs...
SQL>select /*+ monitor */ SQL_ID, SQL_PLAN_HASH_VALUE, SQL_PLAN_LINE_ID, count(*)2 from T1 3 where T1.sql_id in (select T2.sql_id from T2) 4 group by SQL_ID, SQL_PLAN_HASH_VALUE, SQL_PLAN_LINE_ID 5 order by 1; SQL_ID SQL_PLAN_HASH_VALUE SQL_PLAN_LINE_ID COUNT(*)...
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=363 Card=1 Bytes=3) 1 0 SORT (AGGREGATE) 2 1 FILTER 3 2 TABLE ACCESS (FULL) OF 'BIG_TABLE' (Cost=363 Card=1011 2 Bytes=30336) 4 2 INDEX (RANGE SCAN) OF 'IDX_SMALL_TABLE' (NON-UNIQUE) ( Cost=1 Card=1 Bytes=3) Statistics --- 0 r...
1.子查询结果集小,用IN2.外表小,子查询表大,用EXISTS这是完全错误的观点。在8i时代,这经常是正确的,但是现在已经11G了,马上12C就要面世了。其实在ORACLE 9i CBO就已经优化了IN,EXISTS的区别,ORACLE优化器有个查询转换器,很多SQL虽然写法不同,但是ORACLE优化器会根据既定规则进行查询重写,重写为优化器觉得效率最高...
end Any help would be greatly appreciated! On another note, I read that there are plans, or at least a feature request, to support something along the lines of T-SQL's @@ERROR, where in you can check if an error occurred exactly after a statement. Example: ...
Learn how to simulate the EXISTS clause in conditional statements, which is not supported by natively compiled stored procedures in SQL Server.
Transact-SQL syntax conventionsSyntaxsyntaxsql Copy EXISTS ( subquery ) Argumentssubquery Is a restricted SELECT statement. The INTO keyword is not allowed. For more information, see the information about subqueries in SELECT (Transact-SQL).
Pastaba To view Transact-SQL syntax for SQL Server 2014 (12.x) and earlier versions, see Previous versions documentation.Argumentssubquery Is a restricted SELECT statement. The INTO keyword is not allowed. For more information, see the information about subqueries in SELECT (Transact-SQL)....
CREATE EVENT IF NOT EXISTSis always replicated, whether or not the event named in the statement already exists on the source. PREVHOMEUPNEXT