Mysql計數返回0如果沒有找到記錄

Question

我有兩個表。

cities - id_city, city_name 
properties - id_property, id_city, property_name 

我想顯示cities.city_name和旁邊[properties.count(id_city)]

如何進行查詢，如果不找到任何記錄，而不是NULL仍返回零，讓我得到的結果是這樣的：

London [123] 
New York [0] 
Berlin [11] 

其中「紐約」是[0]，而不是NULL而不是1？

Answer 1

使用外連接：

select cities.city_name, count(properties.id_city) 
    from cities left join properties on cities.id_city = properties.id_city 
    group by 1 

Answer 2

查詢：

SELECT cities.*, COUNT(properties.id_city) as num 
FROM cities 
LEFT JOIN properties on cities.id_city=properties.id_city 
GROUP BY cities.id_city 

應該回到你想要它一個0計數，雖然我不是100％肯定它工作在MySQL那樣。

Answer 3

我想下面會爲你做，雖然我還沒有測試過它。訣竅是獲取一個表中的屬性計數，然後將該表連接到城市表，使用IFNULL函數將NULL轉換爲0。

SELECT city_name, IFNULL(property_count, 0) 
FROM cities 
LEFT JOIN 
    (SELECT id_city, count(*) as property_count 
    FROM properties 
    GROUP BY id_city) city_properties 
    USING (id_city); 

Answer 4

簡單。使用「（Count（*）+ 0）」。任何null加0都會返回一個數字。