Question

表1： tbl_users ：

+----+--------+
| id | name   |
+----+--------+
| 1  | waheed |
+----+--------+
| 2  | fareed |
+----+--------+

表2： tbl_watched ：

+------------+----+--------+
| id_watched | id | name   |
+------------+----+--------+
| 1          | 2  | fareed |
+------------+----+--------+

我想获取总记录，如果tbl_watched不存在，它应该返回zero或null

输出：

+----+--------+--------+
| id | name   |watched |
+----+--------+--------+
| 1  | waheed |  90    |
+----+--------+--------+
| 2  | fareed |  null  |
+----+--------+--------+

我怎样才能得到这样的结果。？

Answer 1

使用LEFT JOIN

Select t1.id , t1.name, count(*) as total from tbl_users as t1 
left join tbl_watched as t2 on t1.id=t2.id_watched
group by t1.id , t1.name

Answer 2

您可以使用LEFT JOIN和users之间的watched来实现这一目标。这将保留左表中的所有行，如果右侧没有匹配的记录，则将分配NULL。在此结果集上，您可以GROUP BY用户的姓名，并计算每个人看到的内容。

select  t1.name, count(t2.id)
from    tbl_users t1
left join
        tbl_watched t2
on      t1.id = t2.watched_id
group by t1.name

Answer 3

您可以加入表格。我不确定那里的id_watched列，假设它是来自另一个表的外键。在SqlFiddle为您创建了一个示例模式和查询。请注意，第三人没有任何观看内容，因此在这种情况下，计数将为0。请参阅here。

示例架构：

create table tbl_users (
  id INT(8) UNSIGNED AUTO_INCREMENT PRIMARY KEY,
  name VARCHAR(266) NOT NULL
);

create table tbl_movies (
  id INT(8) UNSIGNED AUTO_INCREMENT PRIMARY KEY,
  name VARCHAR(266) NOT NULL
);

create table tbl_watched (
  id INT(8) UNSIGNED AUTO_INCREMENT PRIMARY KEY,
  user_id int(8) UNSIGNED,
  movie_id int(8) UNSIGNED,
  FOREIGN KEY (user_id) REFERENCES tbl_users(id),
  FOREIGN KEY (movie_id) REFERENCES tbl_movies(id)
);

INSERT INTO tbl_users (name) VALUES 
("John Doe"),("Jane Doe"),("Jamie Fox");

INSERT INTO tbl_movies (name) VALUES 
("The Pianist"),("Django Unchained"),("Pulp Fiction"),("Wanted");

INSERT INTO tbl_watched (user_id,movie_id) VALUES 
(1,1),(1,2),(1,3),(2,3),(2,4);

示例查询：

SELECT u.*
     , COUNT(w.id) watched_count 
  FROM tbl_watched w
 RIGHT 
  JOIN tbl_users u
    ON u.id = w.user_id
 GROUP  
    BY u.id;

如何从总记录中将现有记录保留为空。

3 个答案: