exists | 易学教程

基础查询-SQL和Linq相互化

阅读更多关于基础查询-SQL和Linq相互化

目录 SELECT SQL SELECT DISTINCT 语句 WHERE 和操作符 BETWEEN 和操作符 LIKE 和通配符 ORDER BY 排序 TOP In Alias(as) EXISTS 和 NOT EXISTS SELECT SELECT 语句用于从表中选取数据，是 SQL 最基本的操作之一。通过 SELECT 查询的结果被存储在一个结果表中（称为结果集）。 SQL SELECT 语法只查询某个列的数据： SELECT [列名称] FROM [表名称] 查询所有列的数据 SELECT * FROM [表名称] SQL 查询所有数据 SELECT * FROM categories 查询一列或多列 SELECT category_id,category_name FROM categories 查询表达式 var list = from categories in context.Categories select new { categories.CategoryId, categories.CategoryName }; var lists = from categories in context.Categories select categories; 查询语句 var list = context.Categories.Select

exists 原理

阅读更多关于 exists 原理

---查询有员工的部门信息用exists作用 select * from dept d1 where exists ( select * from emp e where e.deptno=d1.deptno) exist的原理查询语句，不是一次性的查询出来。是查询一条，显示一条。比如先查询部门1，然后去关联表中，遍历，看看有没有对应部门1的员工数据。如果有，返回部门1，如果没有对应员工，部门1不返回。查询一条，显示1条来源： https://www.cnblogs.com/bingyizhihun/p/11355374.html

Test if a variable exists

阅读更多关于 Test if a variable exists

问题 I want to test if a variable exists and if it doesn't, create it. 回答1: The open() & varnum() functions can be used. Non-zero output from varnum() indicates the variable exists. data try; input var1 var2 var3; datalines; 7 2 2 5 5 3 7 2 7 ; data try2; set try; if _n_ = 1 then do; dsid=open('try'); if varnum(dsid,'var4') = 0 then var4 = .; rc=close(dsid); end; drop rc dsid; run; 回答2: data try2; set try; var4 = coalesce(var4,.); run; (assuming var4 is numeric) 回答3: Assign var4 to itself. The

Mongo: find items that don't have a certain field

阅读更多关于 Mongo: find items that don't have a certain field

问题 How to search for documents in a collection that are missing a certain field in MongoDB? 回答1: Yeah, it's possible using $exists: db.things.find( { a : { $exists : false } } ); // return if a is missing When is true, $exists matches the documents that contain the field, including documents where the field value is null. If is false, the query returns only the documents that do not contain the field. 回答2: If you don't care if the field is missing or null (or if it's never null ) then you can

redis 入门之string（2）

阅读更多关于 redis 入门之string（2）

set 用法 #set key value 设置value为字符串的键值对redis> SET key "value" #对不存在的key设置value OK redis> GET key "value" redis> SET key "new-value" # 对已经存在的key设置value OK redis> GET key "new-value" redis> SET key-with-expire-time "hello" EX 10086 #使用EX选项，设置过期时间，单位为秒OK redis> GET key-with-expire-time "hello" redis> TTL key-with-expire-time (integer) 10069 redis> SET key-with-pexpire-time "moto" PX 123321 #使用PX选项，设置时间为毫秒 OK redis> GET key-with-pexpire-time "moto" redis> PTTL key-with-pexpire-time (integer) 111939 redis> SET not-exists-key "value" NX #NX选项，只对不存在的key设置value OK # 键不存在，设置成功 redis> GET not-exists-key

Check if method exists in the same class

阅读更多关于 Check if method exists in the same class

问题 So, method_exists() requires an object to see if a method exists. But I want to know if a method exists from within the same class. I have a method that process some info and can receive an action, that runs a method to further process that info. I want to check if the method exists before calling it. How can I achieve it? Example: class Foo{ public function bar($info, $action = null){ //Process Info $this->$action(); } } 回答1: You can do something like this: class A{ public function foo(){

【MySQL】查询语句优化

阅读更多关于【MySQL】查询语句优化

原文: http://106.13.73.98/__/189/ MySQL的性能优化包罗甚广：索引优化、查询优化、查询缓存、服务器设置优化、操作系统及硬件优化、应用层优化（web服务器、缓存）等等。本文提到的优化技巧更适用于开发人员，都是从网络上收集和自己整理的，主要是查询语句上面的优化，其它层面的优化技巧在此不做记录。整理如下合理创建索引 count 的优化避免使用不兼容的数据类型索引字段上进行运算会使索引失效尽量避免使用 != 、 is null 、 is not null 、 in 、 not in 这样的操作符尽量使用数字型字段合理使用 exists 、 not exists 子句能够用 between 的就不要用 in 能够用 distinct 的就不要用 group by 尽量不要使用 select info 语句，它会导致表锁，阻止其它用户访问该表必要时强制查询优化器使用某个索引消除对大型表行数据的顺序存取程序中如果需要一次性对同一个表插入多条数据，应写成一条语句查询的开销指标执行时间检查的行数返回的行数建立索引的几个准则合理的建立索引能够加快数据读取效率，不合理的建立索引反而会拖慢数据的响应速度。索引越多，更新数据的速度越慢。尽量在采用 MyISAM 作为存储引擎的时候使用索引（因为MySQL以Btree存储索引），而不是

Check if record exists from controller in Rails

阅读更多关于 Check if record exists from controller in Rails

In my app a User can create a Business. When they trigger the index action in my BusinessesController I want to check if a Business is related to the current_user.id : If yes: display the business. If no: redirect to the new action. I was trying to use this: if Business.where(:user_id => current_user.id) == nil # no business found end But it always returns true even when the business doesn't exist... How can I test if a record exists in my database? Why your code does not work? The where method returns an ActiveRecord::Relation object (acts like an array which contains the results of the where

LINQ - Where not exists

阅读更多关于 LINQ - Where not exists

问题 What is the equivalent of following statement in LINQ: Select t1.appname, t1.julianDte, t1.cat From table1 t1 Where NOT EXISTS ( Select * from table t2 where t1.cat = t2.cat AND t2.julianDte < t1.julianDte ) 回答1: Try this Not Any pattern. var query = db.table1 .Where(t1 => !db.table2 .Any(t2 => t2.cat == t1.cat && t2.julianDte < t1.julianDte) ); 回答2: Query syntax version of @Amy B's answer (with !Any inverted to All): from t1 in db.Table1 where db.Table2.All(t2 => t1.cat != t2.cat || t2

mysql 求时间段平均值

阅读更多关于 mysql 求时间段平均值

考虑下面的需求，在一段时间内，间隔一段时间，取一个平均值，把所有的平均值取出来，怎么办？思路：在存储过程中，拼接sql语句。根据起始时间和结束时间，while循环每次加一段时间。 DROP PROCEDURE IF EXISTS `get_avg`; DELIMITER ;; CREATE DEFINER=`root`@`%` PROCEDURE `get_avg`(in iStartTime datetime, in iEndTime datetime) BEGIN declare vSql varchar(10240) default ''; declare vNextTime datetime; while(iStartTime < iEndTime) do -- 每次加一个小时 set vNextTime = date_add(iStartTime,interval 3600 second); -- 单引号是特殊字符，要表示单引号，使用 '' 进行转义 set vSql = concat(vSql,'union select 100, avg(`value`) from t1 where time between ''',iStartTime,''' and ''', vNextTime,''' '); set iStartTime = vNextTime; end