Question

嘿嘿，我需要在不使用子查询的情况下重写以下代码：

SELECT country, region, name
FROM wcities
WHERE( country, region ) NOT IN ( SELECT country, code FROM regions )
AND country = 'IT'
AND region is NOT NULL;

我尝试过：

select c.country, c.region, c.name
from wcities c
join regions r on c.country <> r.country AND c.region <> r.code
where c.country = 'IT' 
AND c.region is NOT NULL;

我明白为什么这不起作用，但我看不到我需要做什么。

提前致谢:)

Answer 1

作为 left join 这将是：

SELECT c.country, c.region, c.name
FROM wcities c LEFT JOIN
     regions r
     ON r.country = c.country and r.region = c.region
WHERE r.country IS NULL AND
      c.country = 'IT' AND
      c.region is NOT NULL;

也就是说，我会为此目的推荐NOT EXISTS：

SELECT c.country, c.region, c.name
FROM wcities c         
WHERE NOT EXISTS (SELECT 1
                  FROM regions r
                  WHERE r.country = c.country and r.region = c.region
                 ) AND
      c.country = 'IT' AND
      c.region is NOT NULL;

如何使用连接重写子查询

1 个答案: