MySQL8.0新特性CTE(Common Table Expression)

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。

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 cte
   ORDER 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_countries
    WITH 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。

3. 总结

在MySQL 8.0 中引入CTE新特性,在大多数场景下,能够简化SQL,提高可读性,同时也能使用CTE代替视图,提高整体性能。另外CTE也能实现递归查询,下一篇文章将详细介绍。

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