如何使用單個查詢從兩個表中檢索數據？

Question

我需要一份活動列表，列出每項活動註冊的每個性別的子女數量。

結構：

CREATE TABLE Child(
child_id INT AUTO_INCREMENT PRIMARY KEY NOT NULL, 
pc_id INTEGER NOT NULL, 
child_fname VARCHAR(20) NOT NULL, 
child_lname VARCHAR(20) NOT NULL, 
child_dob DATE NOT NULL, 
child_gender ENUM ('F','M') DEFAULT 'F' NOT NULL, 
CONSTRAINT FOREIGN KEY(pc_id) REFERENCES Parent_Carer(pc_id)) 
ENGINE=InnoDB; 

CREATE TABLE Child_Activity(
child_id INTEGER NOT NULL, 
activity_name_id ENUM('Art','Football','IT') DEFAULT 'IT' NOT NULL, 
activity_day ENUM('Tuesday','Wednesday','Thursday') NOT NULL, 
CONSTRAINT PRIMARY KEY(child_id,activity_name_id), 
CONSTRAINT FOREIGN KEY(child_id)references Child(child_id), 
CONSTRAINT FOREIGN KEY(activity_name_id)references Activity(activity_name_id)) 
ENGINE=InnoDB; 

我有這個疑問

SELECT activity_name_id, 
     COUNT(*)  AS 'Number of Children', 
     child_gender AS 'Childs Gender' 
FROM child c, 
     child_activity ca 
WHERE c.child_id = ca.child_id 
GROUP BY child_gender, 
      activity_name_id; 

謝謝。

Answer 1

Select Child_Activity.activity_name_id, count(child_m.id) as boys, count(child_f.id) as girls, 
FROM Child_Activity 
INNER JOIN Child as child_m 
ON (child_m.child_id = Child_Activity.child_id AND child_m.gender = 'M') 
INNER JOIN Child child_f 
ON (child_f.child_id = Child_Activity.child_id AND child_f.gender = 'F') 

我希望這個作品

Answer 2

使用joins

例如

SELECT * FROM Child AS c 
JOIN Child _Activity as ca 
ON c.child_id = ca.child_id 

Answer 3

谷歌 '的MySQL表連接' 更多的例子。您需要將下面的字段替換爲您想要返回的字段的名稱。

SELECT fields FROM Child AS C 
INNER JOIN Child_Activity AS CA 
ON CA.child_id = C.child_id