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 Population0),
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。
本文译自:
https://www.percona.com/blog/2020/02/10/introduction-to-mysql-8-0-common-table-expressions-part-1/