exists

MySql question: What is the difference between [NOT] IN and [NOT] EXIST when doing subqueries in MySql. EXISTS EXISTS literally is for checking for the existence of specified criteria. In current standard SQL, it will allow you to specify more than one criteria for comparison - IE if you want to know when col_a and col_b both match - which makes it a little stronger than the IN clause. MySQL IN supports tuples, but the syntax is not portable, so EXISTS is a better choice both for readability and portability. The other thing to be aware of with EXISTS is how it operates - EXISTS returns a

Is there a single way of detecting if a directory/file/symlink/etc. entity (more generalized) exists? I need a single function because I need to check an array of paths that could be directories, files or symlinks. I know File.exists?"file_path" works for directories and files but not for symlinks (which is File.symlink?"symlink_path" ). The standard File module has the usual file tests available: RUBY_VERSION # => "1.9.2" bashrc = ENV['HOME'] + '/.bashrc' File.exist?(bashrc) # => true File.file?(bashrc) # => true File.directory?(bashrc) # => false You should be able to find what you want

代码如下：UPDATE T1 SET T1.FMONEY = (select T2.FMONEY from T2 where T2.FNAME = T1.FNAME) WHERE EXISTS(SELECT 1 FROM T2 WHERE T2.FNAME = T1.FNAME); 来源： https://www.cnblogs.com/Cuimc/p/11363265.html

原文: http://blog.gqylpy.com/gqy/252 " 目录 多表链接查询 笛卡尔积 内链接 inner join 外链接之左链接 left join 外链接之右链接 right join 全外链接 符合条件链接查询 子查询 先准备两张表：部门表（department)、员工表（employee) # 部门表create table department( id int primary key auto_increment, name varchar(20) not null ); # 员工表create table employee( id int primary key auto_increment, name varchar(20) not null, sex enum('male', 'female') not null default 'male', age int not null, dep_id int not null ); # 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),(

转自 https://www.cnblogs.com/xuanhai/p/5810918.html 来源： https://www.cnblogs.com/yongan/p/11362595.html

--存在 select * from A where exists(select 1 from B where A.a = B.b) --不存在 select * from A where not exists(select 1 from B where A.a = B.b) 来源： https://www.cnblogs.com/lydg/p/11362343.html

#DDL /* 数据定义语言 库和表的管理 一、库的管理 创建、修改、删除 二、表的管理 创建、修改、删除 创建： create 修改： alter 删除： drop */ #一、库的管理 #1、库的创建 /* 语法： create database [if not exists]库名; */ #案例：创建库Books CREATE DATABASE IF NOT EXISTS books ; #2、库的修改 RENAME DATABASE books TO 新库名; #更改库的字符集 ALTER DATABASE books CHARACTER SET gbk; #3、库的删除 DROP DATABASE IF EXISTS books; #二、表的管理 #1.表的创建 ★ /* 语法： create table 表名( 列名 列的类型【(长度) 约束】, 列名 列的类型【(长度) 约束】, 列名 列的类型【(长度) 约束】, ... 列名 列的类型【(长度) 约束】 ) */ #案例：创建表Book CREATE TABLE book( id INT,#编号 bName VARCHAR(20),#图书名 price DOUBLE,#价格 authorId INT,#作者编号 publishDate DATETIME#出版日期 ); DESC book; #案例