MySQL8.0新特性CTE(Common Table Expression)

本文主要是介绍MySQL8.0新特性CTE(Common Table Expression)，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

CTE(Common Table Expression)可以认为是派生表(derived table)的替代，在一定程度上，CTE简化了复杂的join查询和子查询，提高了SQL的可读性和执行性能。CTE是ANSI SQL 99标准的一部分，在MySQL 8.0.1版本被引入。

原文地址：
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1. CTE优势

查询语句的可读性更好
在一个查询中，可以被引用多次
能够链接多个CTE
能够创建递归查询
能够提高SQL执行性能
能够有效地替代视图

2. 如何创建和使用CTE

CTE类似于使用子查询时的派生表，但是CTE的定义不在SQL主体中，而是提到SQL最前端，声明CTE的需要使用语法WITH。

2.1 CTE的使用

先看一个派生表实现的例子：

mysql> SELECT Name, Population FROM (SELECT * FROM country WHERE continent='Europe') AS derived_t ORDER BY Population DESC LIMIT 5;

使用CTE改写后，SQL变成这样：

mysql> WITH cte AS (SELECT * FROM country WHERE continent='Europe') SELECT Name, Population FROM cte ORDER BY Population DESC LIMIT 5;

CTE的语法也比较简单，在SQL主体查询之前，使用WITH语法，定义一个或者多个CTE，然后就可以在查询SQL的主体中引用一次或多次CTE，可以把CTE看成是一类提前物化的临时表，以便于查询主体引用。

2.2 为CTE指定具体的字段名称

使用圆括号为CTE指定字段名称，如下eur_name和eur_population为CTE的字段：

mysql> WITH cte(eur_name, eur_population) AS (SELECT Name, Population FROM country WHERE continent='Europe')SELECT eur_name, eur_population FROM cteORDER BY eur_opulation DESC LIMIT 5;

2.3 CTE也可以被用作数据源来更新其他表

CTE可以作为数据源，来更新或者删除其他表，如下：

mysql> WITH cte(eur_code, eur_population) AS (SELECT Code, Population FROM country WHERE continent='Europe')  UPDATE country_2020, cte SET Population_2020 = ROUND(eur_population*1.1) WHERE Code=cte.eur_code;mysql> WITH cte AS (SELECT Code FROM country WHERE continent <> 'Europe') DELETE country_2020 FROM country_2020, cte WHERE country_2020.Code=cte.Code;

2.4 CTE也可以用于insert … select 语句

mysql> INSERT INTO largest_countriesWITH cte AS (SELECT Code, Name, SurfaceArea FROM country ORDER BY SurfaceArea DESC LIMIT 10)SELECT * FROM cte;

2.5 CTE作为提前物化的临时表

定义多个CTE，作为提前物化的临时表，在主查询里面可以多次引用这些临时表。如下：

WITH cte1 AS (SELECT ... FROM ... WHERE ...),cte2 AS (SELECT ... FROM ... WHERE ...)
SELECT ...
FROM table1, table1, cte1, cte2 ....
WHERE .....

2.6 CTE的可见性

下面两个例子，第一个例子中cte对于顶层SELECT可见，第二个例子中，cte对顶层SELECT不可见。为了避免这种不可见的问题，通常将CTE定义在最前面，以便能够在查询主体的任何地方都能引用到CTE。

WITH cte AS (SELECT Code FROM country WHERE Population<1000000)
SELECT * FROM city WHERE city.CountryCode IN 
(SELECT Code FROM cte);   # cte对于顶层SELECT是可见的SELECT * FROM city WHERE city.CountryCode IN 
(WITH cte AS (SELECT Code FROM country WHERE Population<1000000)
SELECT Code from cte);   # cte对于顶层SELECT不可见

2.7 CTE引用链

如果在一个查询中创建多个CTE，可能会出现一个CTE引用前一个CTE，导致CTE引用链的产生。下面这个例子展示了CTE引用链：

WITH density_by_country(country,density) AS (SELECT Name, Population/SurfaceArea FROM country WHERE Population>0 and surfacearea>0), max_density(country,maxdensity,label) AS (SELECT country, density, 'max density' FROM density_by_country WHERE density=(SELECT MAX(density) FROM density_by_country)), min_density(country,mindensity,label) AS (SELECT country, density, 'min density' FROM density_by_country WHERE density=(SELECT MIN(density) FROM density_by_country)) SELECT * FROM max_density UNION ALL SELECT * FROM min_density;

上述SQL如果使用派生表的方式改写，也将是非常庞大和复杂的。

2.8 使用CTE代替视图

如果你的用户没有权限创建视图，而同时又有需要使用视图，不妨试试CTE来代替视图。

mysql> CREATE VIEW city_pop_by_country AS (SELECT countrycode, SUM(population) sum_population FROM city GROUP BY countrycode);mysql> SELECT name, city_pop_by_country.sum_population/country.population ratio FROM country, city_pop_by_country WHERE country.code=city_pop_by_country.countrycode AND country.population > (SELECT 10*AVG(sum_population) FROM city_pop_by_country);

视图改写为CTE，如下：

WITH city_pop_by_country AS (SELECT countrycode, SUM(population) sum_population FROM city GROUP BY countrycode)SELECT name, city_pop_by_country.sum_population/country.population ratio FROM country, city_pop_by_country WHERE country.code=city_pop_by_country.countrycode AND country.population > (SELECT 10*AVG(sum_population) FROM city_pop_by_country);

使用CTE代替视图能够有效提高执行效率，在本案例中，视图的执行时间大概是0.0097秒，而CTE大概是0.0054秒，CTE更快，因为只需要一次物化临时表，可以被多次引用。