Summary: in this tutorial, we will show you how to use the MySQL subquery to write complex queries and explain the correlated subquery concept. A MySQL subquery is a query that is nested inside another query such as SELECT, INSERT, UPDATEor DELETE. I…

我们可以在一个 SQL 语句中放入另一个 SQL 语句.当我们在 WHERE 子句或 HAVING 子句中插入另一个 SQL 语句时,我们就有一个 subquery 的架构. Subquery 的作用是什么呢?第一,它可以被用来连接表格.另外,有的时候 subquery 是唯一能够连接两个表格的方式. Subquery 的语法如下: SELECT "栏位1"  FROM "表格"  WHERE "栏位2" [比较运算素]  (SELECT &qu…

子查询:嵌套在其他查询中的查询称之. 子查询又称内部,而包含子查询的语句称之外部查询(又称主查询). 所有的子查询可以分为两类,即相关子查询和非相关子查询 1>非相关子查询是独立于外部查询的子查询,子查询总共执行一次,执行完毕后将值传递给外部查询. 2>相关子查询的执行依赖于外部查询的数据,外部查询执行一行,子查询就执行一次. 故非相关子查询比相关子查询效率高   查询工资大于所有员工平均工资的员工姓名: --非相关子查询 SELECT department_id, FIRST_NAME FR…

原文地址:https://dev.mysql.com/doc/refman/5.7/en/explain-output.html 9.8.2 EXPLAIN Output Format The EXPLAIN statement provides information about the execution plan for a SELECT statement. EXPLAIN returns a row of information for each table used in the S…

接触MySQL已经有一段时间了,了解如何优化它也迫在眉睫了,话说工欲善其事,必先利其器.最近我就打算了解下几个优化MySQL中经常用到的工具.今天就简单介绍下EXPLAIN. 环境准备 Explain 介绍 id select_type table type possible_keys key key_len ref rows Extra Explain extended 选项介绍 环境准备 1. MySQL版本 mysql> select version(); +---------------…

commands : show sys connect sys as sysdba or connect system as sysdba logout or disc clear screen or cle screen (utlsample.sql  —> unlock scott tables.sql  —> all tables data.sql  —> add data into tables) connect scott/tiger start C:\utlsample.sq…

最近慢慢接触MySQL,了解如何优化它也迫在眉睫了,话说工欲善其事,必先利其器.最近我就打算了解下几个优化MySQL中经常用到的工具.今天就简单介绍下EXPLAIN. 内容导航 id select_type table type possible_keys key key_len ref rows Extra 环境准备 MySQL版本: 创建测试表 CREATE TABLE people( id bigint auto_increment primary key, zipcode char(32…

最近慢慢接触MySQL,了解如何优化它也迫在眉睫了,话说工欲善其事,必先利其器.最近我就打算了解下几个优化MySQL中经常用到的工具.今天就简单介绍下EXPLAIN. 内容导航 id select_type table type possible_keys key key_len ref rows Extra 环境准备 MySQL版本: 创建测试表 CREATE TABLE people( id bigint auto_increment primary key, zipcode ) not n…

Subqueries in the FROM Clause Subqueries in the WHERE Clause Subqueries in the FROM Clause SELECT ... FROM (subquery) name ... SELECT ... FROM (subquery) AS name ...   (Note: Only valid starting with Hive 0.13.0) Hive supports subqueries only in the…

Suppose that you are given the following simple database table called Employee that has 2 columns named Employee ID and Salary: Employee Employee ID Salary 3 200 4 800 7 450 Write a SQL query to get the second highest salary from the table above. Als…