exists

下面为您介绍sql下用了判断各种资源是否存在的代码，需要的朋友可以参考下，希望对您学习sql的函数及数据库能够有所帮助。 库是否存在 if exists(select * from master..sysdatabases where name=N'库名') print 'exists' else print 'not exists' --------------- -- 判断要创建的表名是否存在 if exists (select * from dbo.sysobjects where id = object_id(N'[dbo].[表名]') and OBJECTPROPERTY(id, N'IsUserTable') = 1) -- 删除表 drop table [dbo].[表名] GO --------------- --判断要创建临时表是否存在 If Object_Id('Tempdb.dbo.#Test') Is Not Null Begin print '存在' End Else Begin print '不存在' End --------------- -- 判断要创建的存储过程名是否存在 if exists (select * from dbo.sysobjects where id = object_id(N'[dbo].[存储过程名]') and

Sql Server中判断表或者数据库是否存在 if exists(select 1 from master..dbo.sysdatabases where name='example') print 'DataBase existed' else print 'Database not existed' 2.表 IF Exists(Select 1 From sysObjects Where Name ='表名' And Type In ('S','U')) Print 'Exists Table' Else Print 'Not Exists Table' Sql Server中判断表或者数据库是否存在 分类:软件开发2007.8.12 14:31 作者：stone | 评论：1 | 阅读：2996 1.数据库 if exists(select 1 from master..sysdatabases where name='example') print 'DataBase existed' else print 'Database not existed' 2.表 IF Exists(Select 1 From sysObjects Where Name ='表名' And Type In ('S','U')) Print 'Exists Table' Else Print

IN与OR与EXISTS 使用IN的子查询 ： SELECT * FROM t1 WHERE x IN (SELECT y FROM t2); -- 等价于 SELECT * FROM t1, (SELECT DISTINCT y FROM t2) t WHERE t1.x= t.y; 可见，先t2表进行了全表的扫描并唯一排序，再根据t2的y列的值去索引匹配x列的值； 使用EXISTS的子查询 ： SELECT * FROM t1 WHERE EXISTS (SELECT 1 FROM t2 WHERE y = t1.x); -- 等价于 FOR tab1 IN (SELECT x FROM t1) LOOP IF(EXISTS SELECT y FROM t2 WHERE y = tab1.x) THEN OUTPUT THE RECORD; END IF; END LOOP; 可见，先t1表全表检索，再通过索引 IN 和OR 的效率，在有索引的情况下，没有区别；没有索引时，IN 的效率要高于OR；OR的效率为O(n)，而IN的效率为O(log n) 子查询，小表驱动大表，内层表小用IN，表大用EXISTS；NOT EXISTS 无论什么情况都要大于NOT IN； 来源： https://www.cnblogs.com/f1ynn/p/11515255.html

在做文件上传时遇到一个需求，上传时判断文件名称是否重复，如果重复则在后面标记数字，重新命名后继续上传。 比如文件名为“测试.jpg”，上传时假如该文件存在，则改名为“测试（1）.jpg”上传。 下面为代码展示 效果图 public function uploadFile(Request $request) { // $data = json_decode(file_get_contents("php://input"), true); $userId = request()->userid; if (!$userId) return $this->responseJson(self::CODE_USER_NOT_EXIST, trans('api.user.notExist')); $file = $request->file('file'); Log::channel('notify')->info($file); //获取文件名 $fileName = $file->getClientOriginalName(); $fileSize = $file->getClientSize(); $fileType = $file->guessClientExtension(); $path = 'upload\\' . $userId . '\\'; //判断文件是否存在

Which one of the two would perform better(I was recently accused of not being careful with my code because I used the later in Oracle): Select * from Tab1 Where (not) exists(Select 1 From Tab2 Where Tab1.id = Tab2.id) Select * from Tab1 Where (not) exists(Select Field1 From Tab2 Where Tab1.id = Tab2.id) Or are they both same? Please answer both from SQL Server perspective as well as Oracle perspective. I have googled (mostly from sql-server side) and found that there is still a lot of debate over this although my present opinion/assumption is the optimiser in both the RDMBS are mature enough

本节重点： 多表连接查询 符合条件连接查询 子查询 准备工作：准备两张表，部门表（department）、员工表（employee） create table department( id int, name varchar(20) ); create table employee( id int primary key auto_increment, name varchar(20), sex enum('male','female') not null default 'male', age int, dep_id int ); #插入数据 insert into department values (200,'技术'), (201,'人力资源'), (202,'销售'), (203,'运营'); insert into employee(name,sex,age,dep_id) values ('tom','male',18,200), ('mike','female',48,201), ('jack','male',38,201), ('lucy','female',28,202), ('lili','male',18,200), ('alice','female',18,204) ; # 查看表结构和数据 mysql> desc department; +-------+

一、多表连接查询 　　按照如下命令创建department，employee两个表格： #创建表 create table department( id int, name varchar(20) ); create table employee( id int primary key auto_increment, name varchar(20), sex enum('male','female') not null default 'male', age int, dep_id int ); #插入数据 insert into department values (200,'技术'), (201,'人力资源'), (202,'销售'), (203,'运营'); insert into employee(name,sex,age,dep_id) values ('egon','male',18,200), ('alex','female',48,201), ('wupeiqi','male',38,201), ('yuanhao','female',28,202), ('liwenzhou','male',18,200), ('jingliyang','female',18,204) ; #重点：外链接语法 SELECT 字段列表 FROM 表1 INNER|LEFT