exists

问题 检索选修所有课程的学生的学号 解决方案 SELECT SN FROM S WHERE NOT EXISTS (SELECT * FROM C WHERE NOT EXISTS (SELECT * FROM SC WHERE SC.CNO=C.CNO AND SC.SNO=S.SNO)); 解释 EXISTS 子查询找到的提交 NOT EXISTS 子查询中 找不到的提交 建立程序循环的概念，这是一个动态的查询过程，如 FOR循环 %伪代码 for S =1:m for C = 1:n if S has C, None else, 记录 S %S现在为没有选择全部课程的学生 return S 的补集 来源： https://www.cnblogs.com/kexve/p/11760096.html

Possible Duplicate: Is there an “exists” function for jQuery Yes, I know this has been asked a lot. But, it confuses me, since the results on google for this search show different methods (listed below) $(document).ready(function() { if ($('#DivID').length){ alert('Found with Length'); } if ($('#DivID').length > 0 ) { alert('Found with Length bigger then Zero'); } if ($('#DivID') != null ) { alert('Found with Not Null'); } }); Which one of the 3 is the correct way to check if the div exists? EDIT: It's a pitty to see that people do not want to learn what is the better approach from the three

Zookeeper客户端java代码操作 上篇博客记录了shell命令操作zookeeper集群的方式，这次尝试采用java代码来操作。通过查阅API，发现并不困难。 1. 首先获得客户端与服务器的连接 //zookeeper客户端 private ZooKeeper zkCli; //连接地址 private static final String CONNECT_STRING = "hadoop102:2181,hadoop103:2181,hadoop104:2181"; //session过期时间 private static final int SESSION_TIMEOUT = 2000; /** * 创建客户端实例对象 * * @throws IOException */ @Before public void before() throws IOException { zkCli = new ZooKeeper(CONNECT_STRING, SESSION_TIMEOUT, (event) -> { System.out.println("默认的回调函数"); }); } 2. 列出根节点下的子节点(非递归) @Test public void ls() throws KeeperException, InterruptedException { List

题目来源：数据库系统教程（施伯乐第三版）... 供理解和学习SQL语句使用，长期不定时更新例题。 已知教学书库中4个关系分别为,（主键加了下划线，外键颜色分辨） 教师关系 T( T# ,TNAME,TITLE) 课程关系 C( C# ,CNAME, T# ) 学生关系 S( S# ,SNAME,AGE,SEX) 选课关系 SC( S# , C# , SCORE) //该表主键为S#和C# /*1.检索学习课程号为C2的学生学号与成绩*/ SELECT S#,SCORE FROM SC WHERE C#='C2'; /*2.检所学习课程号为C2的学生学号与姓名*/ SELECT S.S#,SNAME FROM S,SC WHERE S.S#=SC.S# AND C#='C2' /*3.检索至少选修LIU老师所授课程中一门课程的学生学号与姓名*/ SELECT S.S#,SNAME FROM S,SC,C,T WHERE S.S#=SC.S# AND SC.C#=C.C# AND C.T#=T.T# AND TNAME='LIU'; /*4.检索至少选修课程号为C2或C4课程的学生学号*/ SELECT DISTINCT S# FROM SC WHERE SC.C#='C2' OR SC.C#='C4'; /*5.检索至少选修课程号为C2和C4课程的学生学号*/ SELECT X.S#

前面一篇文章( SpringBoot-定时任务 )中介绍了如何用SpringBoot框架中的注解方式来实现定时任务，这种方式的好处是不使用第三方的依赖，仅凭几个方便的注解，即可编写一个简单的定时任务处理。 实际开发中为了满足复杂的业务场景，比如任务暂停、恢复、删除等操作。上面这种方式就不能满足了，这一节我们来学习SpringBoot集成Quartz框架来实现复杂的任务调度处理。 Quzrtz简介 Quartz是一个由Java语言编写的开源任务调度框架，具有简单高效、容错、支持分布式等优点。 主要API： Scheduler ：与调度器交互的主要API。 Job ：需要被调度器调度的任务必须实现的接口。 JobDetail ：用于定义任务的实例。 Trigger ：用于定义调度器何时调度任务执行的组件。 JobBuilder ：用于定义或创建JobDetail的实例 。 TriggerBuilder ：用于定义或创建触发器实例。 数据库准备 Quertz需要将任务的相关信息保存到数据库中，所以我们需要提前创建好数据库，库名自己定义好后，执行以下SQL语句： DROP TABLE IF EXISTS QRTZ_FIRED_TRIGGERS ; DROP TABLE IF EXISTS QRTZ_PAUSED_TRIGGER_GRPS ; DROP TABLE IF EXISTS

I'm converting this query from Netezza to run with my RedShift dw. I'm continuously getting this error. Amazon Invalid operation: This type of IN/NOT IN query is not supported yet; I have tried converting it using 'EXISTS/NOT EXISTS' condition but still not successful. Can someone give his/her input how should i convert that IN, NOT IN part? CREATE TEMP TABLE TMP_EMPLY_BRND AS ( Select EMPLY_SRRGT_ID, EMPLY_PRMRY_BRND_PRDCT_DPRTMNT_EFF_STRT_DT_KEY, EMPLY_PRMRY_PRDCT_DPRTMNT_GRP_SRRGT_ID, From (( (SELECT E.EMPLY_SRRGT_ID, E.EMPLY_PRMRY_BRND_PRDCT_DPRTMNT_EFF_STRT_DT_KEY, FROM INS_EDW_CP.EMPLY

SELECT * FROM employees e WHERE NOT EXISTS ( SELECT name FROM eotm_dyn d WHERE d.employeeID = e.id ) And SELECT * FROM employees a LEFT JOIN eotm_dyn b on (a.joinfield=b.joinfield) WHERE b.name IS NULL Which is more efficient,some analysis? Assuming the column values involved can not be NULL - MySQL: LEFT JOIN/IS NULL is more efficient than NOT EXISTS - read this article for details . Oracle: They are equivalent . SQL Server: NOT EXISTS is more efficient than LEFT JOIN/IS NULL - read this article for details . Postgres: Like Oracle, they are equivalent . If you have trouble with the detail

I have the following table: +-----------+--------+ | FirstName | Active | +-----------+--------+ | Rob | TRUE | | Jason | TRUE | | Mike | FALSE | +-----------+--------+ I would like to insert 'John' (with Active=True) only if an entry for John doesn't exist already where Active=True. I try the following: insert into testTable (FirstName, Active) values ('John',True) where not exists (select 1 from testTable where FirstName='John' and Active=True) but i get 'Query input must contain at least one table or query'. Can anybody help with what I am trying to achieve? You can't combine Values with a

How can I check if a specific primary key (a string variable) already exists on the table and if not insert a new record otherwise just update the existing one with new values using c#? I tried this MySqlCommand cmd2 = new MySqlCommand("INSERT INTO mapdisplay (ID,Distance) VALUES (@r,@c,) ON DUPLICATE KEY UPDATE mapdisplay (Distance) VALUES (@c,)", conn); but I think the syntax is wrong. INSERT INTO mapdisplay (HexID,FlightNo,Lat,Lon,Alt,Course,Groundspeed,Verticalrate,Distance) VALUES (@r,@c,@f,@t,@w,@q,@u,@e,@y) ON DUPLICATE KEY UPDATE FlightNo = @c ,Lat = @f ,Lon = @t ,Alt = @w ,Course = @q