SQL> alter table if exists t1 add (id number); alter table if exists t1 add (id number) * ERROR at line 1: ORA-01430: column being added already exists in table SQL> CREATE OR REPLACE We can't mixCREATE OR REPL
When you write EXISTS in a where clause,you're telling the optimizer that you want the outer query to be run first, using each value tofetch a value from the inner query. In many cases, EXISTS is better because itrequires you to specify a join condition, which can invoke an INDEX scan...
The syntax for the EXISTS condition in Oracle/PLSQL is: WHERE EXISTS ( subquery ); Parameters or Arguments subquery The subquery is a SELECT statement. If the subquery returns at least one record in its result set, the EXISTS clause will evaluate to true and the EXISTS condition will be me...
NOT EXISTS = NOT IN ,意思相同不过语法上有点点区别 SELECT ID,NAME FROM A WHERE ID NOT IN (SELECT AID FROM B) 下面是普通的用法: SQL中IN,NOT IN,EXISTS,NOT EXISTS的用法和差别: IN:确定给定的值是否与子查询或列表中的值相匹配。 IN 关键字使您得以选择与列表中的任意一个值匹配的行。 当要获...
The syntax for using EXISTS in Oracle is as follows: SELECT column1, column2, ... FROM table_name WHERE EXISTS (subquery); The subquery within the EXISTS clause returns either a TRUE or FALSE value. If the subquery returns any rows, the EXISTS condition is evaluated as TRUE, and the ou...
这个例子比较了两个语义类似的查询。第一个查询使用 EXISTS 而第二个查询使用 IN。注意两个查询返回相同的信息。 USEpubsGOSELECTDISTINCTpub_nameFROMpublishersWHEREEXISTS(SELECT*FROMtitlesWHEREpub_id=publishers.pub_idANDtype='business' )GO -- Or, using the IN clause: ...
Oracle EXISTS vs. IN# TheEXISTSoperator stops scanning rows once the subquery returns the first row because it can determine the result whereas theINoperator must scan all rows returned by the subquery to conclude the result. Additionally theINclause can’t compare anything withNULLvalues, but th...
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...
MySQL should support IF [NOT] EXISTS clauses for CREATE, DROP and ALTER USER commands. This will allow for distribution of accounts using replication without triggering replication failures in the event of non-synchronized accounts on master and slave. It also simplifies user scripting of account ...
1.解读in和exists 这两个关键字的区别主要是在于子查询上面,in是独立子查询,exists是相关子查询,例如: 用in查询有员工的部门 :select dept_name from dept where id in (select dept_id from emp); 用exists查询有员工的部门:select dept_name fr......