Mysql SELECT、FROM或WHERE中嵌套的SELECT子句和差异

1.我想知道我们什么时候在select子句中使用子查询的结构

2.如果在1中写入任何查询。结构具有其他嵌套形式之一的等效项：

  SELECT ... from ... where .. IN (SELECT ...)
    SELECT ... from (SELECT ...)

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谢谢

为了回答这个问题，我将第一个选择称为外部选择，第二个选择称为内部选择

Get the population of cities which are in Texas
SELECT city, population FROM cities WHERE city IN (SELECT city FROM states WHERE state = 'Texas')

Get employees who have earned more than $500 this week (derived table x)
SELECT name, ROUND(wages) FROM (SELECT name, per_hour * hours_worked AS wages FROM timesheet) AS x WHERE wages > 500

您将使用选择。。。从…起哪里在选择。。。如果您想查找与您所知道的记录相似的记录。可以将内部选择的项目与外部选择的项目进行比较

Get the population of cities which are in Texas
SELECT city, population FROM cities WHERE city IN (SELECT city FROM states WHERE state = 'Texas')

Get employees who have earned more than $500 this week (derived table x)
SELECT name, ROUND(wages) FROM (SELECT name, per_hour * hours_worked AS wages FROM timesheet) AS x WHERE wages > 500

您将使用选择。。。从…起选择如果要为语句创建派生表。然后，可以在外部选择上使用在内部选择上计算的项目

Get the population of cities which are in Texas
SELECT city, population FROM cities WHERE city IN (SELECT city FROM states WHERE state = 'Texas')

Get employees who have earned more than $500 this week (derived table x)
SELECT name, ROUND(wages) FROM (SELECT name, per_hour * hours_worked AS wages FROM timesheet) AS x WHERE wages > 500

还有第三种方法是相关子查询，它将显示为SELECT。。。从…起哪里…=或>或<或！=等选择。。。如果要基于计算选择行，则使用和

Get cities with bigger populations than 2 times the average
SELECT city, population FROM cities WHERE population > 2 * (SELECT AVG(population) FROM cities)

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为了回答这个问题，我将第一个选择称为外部选择，第二个选择称为内部选择

Get the population of cities which are in Texas
SELECT city, population FROM cities WHERE city IN (SELECT city FROM states WHERE state = 'Texas')

Get employees who have earned more than $500 this week (derived table x)
SELECT name, ROUND(wages) FROM (SELECT name, per_hour * hours_worked AS wages FROM timesheet) AS x WHERE wages > 500

您将使用选择。。。从…起哪里在选择。。。如果您想查找与您所知道的记录相似的记录。可以将内部选择的项目与外部选择的项目进行比较

Get the population of cities which are in Texas
SELECT city, population FROM cities WHERE city IN (SELECT city FROM states WHERE state = 'Texas')

Get employees who have earned more than $500 this week (derived table x)
SELECT name, ROUND(wages) FROM (SELECT name, per_hour * hours_worked AS wages FROM timesheet) AS x WHERE wages > 500

您将使用选择。。。从…起选择如果要为语句创建派生表。然后，可以在外部选择上使用在内部选择上计算的项目

Get the population of cities which are in Texas
SELECT city, population FROM cities WHERE city IN (SELECT city FROM states WHERE state = 'Texas')

Get employees who have earned more than $500 this week (derived table x)
SELECT name, ROUND(wages) FROM (SELECT name, per_hour * hours_worked AS wages FROM timesheet) AS x WHERE wages > 500

还有第三种方法是相关子查询，它将显示为SELECT。。。从…起哪里…=或>或<或！=等选择。。。如果要基于计算选择行，则使用和

Get cities with bigger populations than 2 times the average
SELECT city, population FROM cities WHERE population > 2 * (SELECT AVG(population) FROM cities)