MySQL的SELECT COUNT

Question

我想數着至少有一個產品從下列查詢

SELECT count(*) 
FROM company c 
JOIN product p on c.id = product.company_id 
WHERE p.is_deleted = 0 
AND c.is_customer = 1 
AND c.company_type_id = 5 
GROUP by c.id 

所以公司的數量，這說明我的所有公司的名單，以及產品對每個計公司。

我試圖實現的是從上述結果中計算出的公司數量。

這是可以實現如下：

SELECT count(*) 
FROM ( 
    SELECT count(*) 
    FROM company c 
    JOIN product p on c.id = product.company_id 
    WHERE p.is_deleted = 0 
    AND c.is_customer = 1 
    and c.company_type_id = 5 
    GROUP by c.id) AS t1 

所以，這給了我正確的結果，但我只是想知道是否有做事情的更有效的方式。

Answer 1

我相信你可以把它簡化爲這樣：

SELECT count(distinct c.id) 
FROM company c 
JOIN product p on c.id = product.company_id 
WHERE p.is_deleted = 0 
AND c.is_customer = 1 
AND c.company_type_id = 5