Question

我想将2个查询合并为一个。我想从last_name中提取一封信，并寻找以该字母开头的计数器。我的查询给了我一封以上的信，我不知道为什么。

    SELECT CONCAT(LOWER(SUBSTR(country, 1, LENGTH(country) - 1)), 
           UPPER(substr(country, -1))) AS Kraj,
           COUNT(C.country_id) AS ilosc
FROM city AS C
JOIN country AS CO ON C.country_id = CO.country_id
WHERE country LIKE
    (SELECT CONCAT(LEFT(last_name, 1), '%')
     FROM actor
     ORDER BY RAND()
     LIMIT 1)
GROUP BY C.country_id
HAVING COUNT(C.country_id) > 1;

Answer 1

首先将其更改为INNER_JOIN会对您来说很容易理解：JOIN country AS CO ON C.country_id = CO.country_id第二个：您正在从表中提取数据，但看不到数据。如果您只想为该示例检索一个结果，则可以考虑使用DISTINCT

SELECT DISTINCT CONCAT(LOWER(SUBSTR(country, 1, LENGTH(country) - 1)), 
       UPPER(substr(country, -1))) AS Kraj,
       COUNT(C.country_id) AS ilosc
FROM city AS C
INNER JOIN country AS CO ON C.country_id = CO.country_id
WHERE country LIKE
    (SELECT CONCAT(LEFT(last_name, 1), '%')
     FROM actor
     ORDER BY RAND()
     LIMIT 1)
GROUP BY C.country_id
HAVING COUNT(C.country_id) > 1;

Answer 2

对于每个国家，DBMS都会评估WHERE子句中的条件，从而一次又一次地执行子查询，并产生各种字母，而不仅仅是一个字母。

使用CROSS JOIN代替随机提供一个字母。

SELECT
  CONCAT(LOWER(SUBSTR(country, 1, LENGTH(country) - 1)), UPPER(substr(country, -1))) AS Kraj,
  COUNT(C.country_id) AS ilosc
FROM city AS C
JOIN country AS CO ON C.country_id = CO.country_id
CROSS JOIN 
(
  SELECT CONCAT(LEFT(last_name, 1), '%') AS letter
  FROM actor
  ORDER BY RAND()
  LIMIT 1
) the_one
WHERE country LIKE the_one.letter
GROUP BY C.country_id
HAVING COUNT(C.country_id) > 1;

将两个查询合二为一

2 个答案: