在查詢中帶有多個計數的SQL煩人

Question

我已經提供了模式和示例數據的sql小提琴。

http://sqlfiddle.com/#!2/e9d22/7/0

如果我想知道在泰國有多少省多少城市。

Country Name | No. Provinces | No. Cities 
    Thailand | 77   | 1234 

我想它需要使用多個COUNT（*），但我不知道如何使用它。

有人知道請提出解決方案嗎？

Answer 1

您需要使用GROUP BY和COUNT：

SELECT c.name, count(distinct p.id) provincecoutn, count(distinct city.id) citycount 
FROM country c 
    LEFT JOIN province p on c.id = p.country_id 
    LEFT JOIN City on p.id = city.province_id 
GROUP BY c.name 

好運。

Answer 2

試試這個：

SELECT 
    C.Name, COUNT(DISTINCT P.Id) NoProvance, COUNT(CC.Id) NoCities 
FROM country C 
    JOIN province P 
     ON C.Id = P.COUNTRY_ID 
    JOIN city CC 
     ON P.Id = CC.province_id 
WHERE C.Name = 'Thailand' 
GROUP BY C.Name 

SQL FIDDLE DEMO

Answer 3

它可能更快前計數每個省城市加入到省：（！）

SELECT c.name   AS "Country Name" 
     ,count(p.id)  AS "No. Provinces" 
     ,sum(ci.city_ct) AS "No. Cities" 
FROM country  c 
LEFT JOIN province p ON p.country_id = c.id 
LEFT JOIN (
    SELECT province_id, count(*) AS city_ct FROM city GROUP BY 1 
    ) ci ON ci.province_id = p.id 
GROUP BY 1 

-> sqlfiddle PostgreSQL的