exists

EXIST和IN 比如在Northwind数据库中有一个查询为 SELECT c.CustomerId,CompanyName FROM Customers c WHERE EXISTS( SELECT OrderID FROM Orders o WHERE o.CustomerID=c.CustomerID) 这里面的EXISTS是如何运作呢？子查询返回的是OrderId字段，可是外面的查询要找的是CustomerID和CompanyName字段，这两个字段肯定不在OrderID里面啊，这是如何匹配的呢？ EXISTS用于检查子查询是否至少会返回一行数据，该子查询实际上并不返回任何数据，而是返回值True或False EXISTS 指定一个子查询，检测 行 的存在。 语法： EXISTS subquery 参数： subquery 是一个受限的 SELECT 语句 (不允许有 COMPUTE 子句和 INTO 关键字)。 结果类型： Boolean 如果子查询包含行，则返回 TRUE ，否则返回 FLASE 。 例表A：TableIn 例表B：TableEx （一）. 在子查询中使用 NULL 仍然返回结果集 select * from TableIn where exists(select null) 等同于： select * from TableIn （二）. 比较使用

主机IP: 192.168.137.210 1.关闭防火墙和SElinux 2 . 安装epel源 [root@sky ~] # cd /etc/yum.repos.d/ [root@sky yum.repos.d] # yum -y install epel-release 安装cobbler [root@sky yum.repos.d] # yum install -y cobbler cobbler-web tftp xinetd dhcp cman pykickstart debmirror syslinux net-tools 安装cobbler依赖包 [root@sky yum.repos.d] # yum install -y ed patch perl perl-Compress-Zlib perl-Digest-SHA1 perl-LockFile-Simple perl-libwww-perl fence-agents 3. 设置服务开机启动 [root@sky yum.repos.d] # systemctl enable httpd.service [root@sky yum.repos.d] # systemctl enable dhcpd.service [root@sky yum.repos.d] # systemctl enable xinetd

I have found my self in quite a pickle. I have tables of only one column (supression or inclusion lists) that are more or less varchar(25) but the thing is I won't have time to index them before using them in the main query and, depending how inportant it is, I won't know how many rows are in each table. The base table at the heart of all this is some 1.4 million rows and some 50 columns. My assumptions are as follows: IN shouln't be used in cases with a lot of values (rows) returned because it looks though the values serially, right? (IN on a subquery not passed the values directly) Joins

懒得多写了，懂的看建表脚本就懂了。。。 -- ---------------------------- -- Table structure for ucb_user -- ---------------------------- DROP TABLE IF EXISTS `ucb_user`; CREATE TABLE `ucb_user` ( `id` char(32) NOT NULL COMMENT '主键(UUID)', `user_type` tinyint(3) unsigned NOT NULL DEFAULT '0' COMMENT '用户类型：0、未定义；1、内部用户；2、合作方用户；3、外部用户', `source` tinyint(3) DEFAULT '0' COMMENT '来源', `code` varchar(8) DEFAULT NULL COMMENT '用户编码', `name` varchar(64) NOT NULL COMMENT '名称', `account` varchar(64) NOT NULL COMMENT '登录账号', `mobile` varchar(32) DEFAULT NULL COMMENT '手机号', `email` varchar(64) DEFAULT NULL COMMENT '电子邮箱',

I am familiar with Sybase which allows queries with format: IF EXISTS () THEN ... ELSE ... END IF (or very close). This a powerful statement that allows: "if exists, then update, else insert". I am writing queries for DB2 on IBM iSeries box. I have seen the CASE keyword, but I cannot make it work. I always receive the error: "Keyword CASE not expected." Sample: IF EXISTS ( SELECT * FROM MYTABLE WHERE KEY = xxx ) THEN UPDATE MYTABLE SET VALUE = zzz WHERE KEY = xxx ELSE INSERT INTO MYTABLE (KEY, VALUE) VALUES (xxx, zzz) END IF Is there a way to do this against DB2 on IBM iSeries? Currently, I

内存存储RAMJobStore Quartz默认使用RAMJobStore，它的优点是速度。因为所有的 Scheduler 信息都保存在计算机内存中，访问这些数据随着电脑而变快。而无须访问数据库或IO等操作，但它的缺点是将 Job 和 Trigger 信息存储在内存中的。因而我们每次重启程序，Scheduler 的状态，包括 Job 和 Trigger 信息都丢失了。 Quartz 的内存 Job 存储的能力是由一个叫做 org.quartz.simple.RAMJobStore 类提供。在我们的quartz-2.x.x.jar包下的org.quartz包下即存储了我们的默认配置quartz.properties。打开这个配置文件，我们会看到如下信息 # Default Properties file for use by StdSchedulerFactory # to create a Quartz Scheduler Instance, if a different # properties file is not explicitly specified. # org.quartz.scheduler.instanceName: DefaultQuartzScheduler org.quartz.scheduler.rmi.export: false org.quartz

I'm looking for a way to find if element referenced in javascript has been inserted in the document. Lets illustrate a case with following code: var elem = document.createElement('div'); // Element has not been inserted in the document, i.e. not present document.getElementByTagName('body')[0].appendChild(elem); // Element can now be found in the DOM tree Jquery has :visible selector, but it won't give accurate result when I need to find that invisible element has been placed somewhere in the document. Here's an easier method that uses the standard Node.contains DOM API to check in an element