CREATE TABLE `t_video` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
`describe` varchar(255) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;
CREATE TABLE `t_comment` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`video_id` int(11) DEFAULT NULL,
`comment` varchar(255) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `video_id` (`video_id`),
CONSTRAINT `t_comment_ibfk_1` FOREIGN KEY (`video_id`) REFERENCES `t_video` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;
CREATE TABLE `t_praise` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`video_id` int(11) DEFAULT NULL,
`create_time` datetime DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `video_id` (`video_id`),
CONSTRAINT `t_praise_ibfk_1` FOREIGN KEY (`video_id`) REFERENCES `t_video` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;
INSERT INTO `test`.`t_video` (`id`, `name`, `describe`) VALUES ('1', '自拍', '自拍');
INSERT INTO `test`.`t_video` (`id`, `name`, `describe`) VALUES ('2', '小猫咪', '小猫咪');
INSERT INTO `test`.`t_video` (`id`, `name`, `describe`) VALUES ('3', '小狗狗', '小狗狗');
INSERT INTO `test`.`t_video` (`id`, `name`, `describe`) VALUES ('4', '驴蛋蛋', '驴蛋蛋');
INSERT INTO `test`.`t_video` (`id`, `name`, `describe`) VALUES ('5', '小脑腐', '小脑腐');
INSERT INTO `test`.`t_comment` (`id`, `video_id`, `comment`) VALUES ('1', '1', '啊好好看');
INSERT INTO `test`.`t_comment` (`id`, `video_id`, `comment`) VALUES ('2', '1', '啊好丑啊');
INSERT INTO `test`.`t_comment` (`id`, `video_id`, `comment`) VALUES ('3', '1', '你是魔鬼吗');
INSERT INTO `test`.`t_comment` (`id`, `video_id`, `comment`) VALUES ('4', '2', '喵喵喵');
INSERT INTO `test`.`t_comment` (`id`, `video_id`, `comment`) VALUES ('5', '2', '喵喵喵');
INSERT INTO `test`.`t_comment` (`id`, `video_id`, `comment`) VALUES ('6', '3', '汪汪汪');
INSERT INTO `test`.`t_praise` (`id`, `video_id`, `create_time`) VALUES ('1', '1', '2018-12-12 16:59:08');
INSERT INTO `test`.`t_praise` (`id`, `video_id`, `create_time`) VALUES ('2', '1', '2018-12-12 16:59:17');
INSERT INTO `test`.`t_praise` (`id`, `video_id`, `create_time`) VALUES ('3', '2', '2018-12-12 16:59:24');
INSERT INTO `test`.`t_praise` (`id`, `video_id`, `create_time`) VALUES ('4', '2', '2018-12-12 16:59:30');
INSERT INTO `test`.`t_praise` (`id`, `video_id`, `create_time`) VALUES ('5', '2', '2018-12-12 16:59:38');
INSERT INTO `test`.`t_praise` (`id`, `video_id`, `create_time`) VALUES ('6', '2', '2018-12-12 16:59:47');
mysql> select * from t_video;
+----+-----------+-----------+
| id | name | describe |
+----+-----------+-----------+
| 1 | 自拍 | 自拍 |
| 2 | 小猫咪 | 小猫咪 |
| 3 | 小狗狗 | 小狗狗 |
| 4 | 驴蛋蛋 | 驴蛋蛋 |
| 5 | 小脑腐 | 小脑腐 |
+----+-----------+-----------+
5 rows in set (0.00 sec)
mysql> select * from t_comment;
+----+----------+-----------------+
| id | video_id | comment |
+----+----------+-----------------+
| 1 | 1 | 啊好好看 |
| 2 | 1 | 啊好丑啊 |
| 3 | 1 | 你是魔鬼吗 |
| 4 | 2 | 喵喵喵 |
| 5 | 2 | 喵喵喵 |
| 6 | 3 | 汪汪汪 |
+----+----------+-----------------+
6 rows in set (0.00 sec)
mysql> select * from t_praise;
+----+----------+---------------------+
| id | video_id | create_time |
+----+----------+---------------------+
| 1 | 1 | 2018-12-12 16:59:08 |
| 2 | 1 | 2018-12-12 16:59:17 |
| 3 | 2 | 2018-12-12 16:59:24 |
| 4 | 2 | 2018-12-12 16:59:30 |
| 5 | 2 | 2018-12-12 16:59:38 |
| 6 | 2 | 2018-12-12 16:59:47 |
+----+----------+---------------------+
6 rows in set (0.00 sec)
-- group by c.video_id 会导致左表没有评论的视频（在右表中不存在记录的）将不会被展示，他们会统一归到c.video_id为null的这一条记录中
-- 统计c.video_id c表中video_id为null的肯定为空，所以就是0
SELECT v.id,v.`name`,c.video_id,COUNT(c.video_id) from t_video v LEFT JOIN t_comment c ON c.video_id=v.id GROUP BY c.video_id;
mysql> SELECT v.id,v.`name`,c.video_id,COUNT(c.video_id) from t_video v LEFT JOIN t_comment c ON c.video_id=v.id GROUP BY c.video_id;
+----+-----------+----------+-------------------+
| id | name | video_id | COUNT(c.video_id) |
+----+-----------+----------+-------------------+
| 4 | 驴蛋蛋 | NULL | 0 |
| 1 | 自拍 | 1 | 3 |
| 2 | 小猫咪 | 2 | 2 |
| 3 | 小狗狗 | 3 | 1 |
+----+-----------+----------+-------------------+
4 rows in set (0.00 sec)
-- group by c.video_id 会导致左表没有评论的视频（在右表中不存在记录的）将不会被展示，他们会统一归到c.video_id为null的这一条记录中
-- 统计v.id v表中有两条没有评论的记录被归到了 c.video_id为null的这条记录中，所以count为2
SELECT v.id,v.`name`,c.video_id,COUNT(v.id) from t_video v LEFT JOIN t_comment c ON c.video_id=v.id GROUP BY c.video_id;
mysql> SELECT v.id,v.`name`,c.video_id,COUNT(v.id) from t_video v LEFT JOIN t_comment c ON c.video_id=v.id GROUP BY c.video_id;
+----+-----------+----------+-------------+
| id | name | video_id | COUNT(v.id) |
+----+-----------+----------+-------------+
| 4 | 驴蛋蛋 | NULL | 2 |
| 1 | 自拍 | 1 | 3 |
| 2 | 小猫咪 | 2 | 2 |
| 3 | 小狗狗 | 3 | 1 |
+----+-----------+----------+-------------+
4 rows in set (0.00 sec)
-- group by v.id 按v.id 分组 统计 v.id 那么count至少就是1
SELECT v.id,v.`name`,c.video_id,COUNT(v.id) from t_video v LEFT JOIN t_comment c ON c.video_id=v.id GROUP BY v.id;
mysql> SELECT v.id,v.`name`,c.video_id,COUNT(v.id) from t_video v LEFT JOIN t_comment c ON c.video_id=v.id GROUP BY v.id;
+----+-----------+----------+-------------+
| id | name | video_id | COUNT(v.id) |
+----+-----------+----------+-------------+
| 1 | 自拍 | 1 | 3 |
| 2 | 小猫咪 | 2 | 2 |
| 3 | 小狗狗 | 3 | 1 |
| 4 | 驴蛋蛋 | NULL | 1 |
| 5 | 小脑腐 | NULL | 1 |
+----+-----------+----------+-------------+
5 rows in set (0.00 sec)
-- 这个才是正确的
SELECT v.id,v.`name`,c.video_id,COUNT(c.video_id) from t_video v LEFT JOIN t_comment c ON c.video_id=v.id GROUP BY v.id;
mysql> SELECT v.id,v.`name`,c.video_id,COUNT(c.video_id) from t_video v LEFT JOIN t_comment c ON c.video_id=v.id GROUP BY v.id;
+----+-----------+----------+-------------------+
| id | name | video_id | COUNT(c.video_id) |
+----+-----------+----------+-------------------+
| 1 | 自拍 | 1 | 3 |
| 2 | 小猫咪 | 2 | 2 |
| 3 | 小狗狗 | 3 | 1 |
| 4 | 驴蛋蛋 | NULL | 0 |
| 5 | 小脑腐 | NULL | 0 |
+----+-----------+----------+-------------------+
5 rows in set (0.00 sec)
-- 再连点赞表
SELECT v.id,v.`name`,p.video_id,COUNT(p.video_id) from t_video v LEFT JOIN t_praise p ON p.video_id=v.id GROUP BY v.id;
mysql> SELECT v.id,v.`name`,p.video_id,COUNT(p.video_id) from t_video v LEFT JOIN t_praise p ON p.video_id=v.id GROUP BY v.id;
+----+-----------+----------+-------------------+
| id | name | video_id | COUNT(p.video_id) |
+----+-----------+----------+-------------------+
| 1 | 自拍 | 1 | 2 |
| 2 | 小猫咪 | 2 | 4 |
| 3 | 小狗狗 | NULL | 0 |
| 4 | 驴蛋蛋 | NULL | 0 |
| 5 | 小脑腐 | NULL | 0 |
+----+-----------+----------+-------------------+
5 rows in set (0.00 sec)
下边这个sql语句怎么看都是没错的是吧，但是！！！我们拆开来看！
SELECT v.id,v.`name`,c.video_id AS cvid , p.video_id AS pvid,COUNT(c.video_id) AS comments,COUNT(p.video_id)AS praises from t_video v LEFT JOIN t_comment c ON c.video_id=v.id LEFT JOIN t_praise p ON p.video_id=v.id GROUP BY v.id;
mysql> SELECT v.id,v.`name`,c.video_id AS cvid , p.video_id AS pvid,COUNT(c.video_id) AS comments,COUNT(p.video_id)AS praises from t_video v LEFT JOIN t_comment c ON c.video_id=v.id LEFT JOIN t_praise p ON p.video_id=v.id GROUP BY v.id;
+----+-----------+------+------+----------+---------+
| id | name | cvid | pvid | comments | praises |
+----+-----------+------+------+----------+---------+
| 1 | 自拍 | 1 | 1 | 6 | 6 |
| 2 | 小猫咪 | 2 | 2 | 8 | 8 |
| 3 | 小狗狗 | 3 | NULL | 1 | 0 |
| 4 | 驴蛋蛋 | NULL | NULL | 0 | 0 |
| 5 | 小脑腐 | NULL | NULL | 0 | 0 |
+----+-----------+------+------+----------+---------+
5 rows in set (0.00 sec)
mysql> SELECT v.id,v.`name`,c.`comment` from t_video v LEFT JOIN t_comment c ON c.video_id=v.id;
+----+-----------+-----------------+
| id | name | comment |
+----+-----------+-----------------+
| 1 | 自拍 | 啊好好看 |
| 1 | 自拍 | 啊好丑啊 |
| 1 | 自拍 | 你是魔鬼吗 |
| 2 | 小猫咪 | 喵喵喵 |
| 2 | 小猫咪 | 喵喵喵 |
| 3 | 小狗狗 | 汪汪汪 |
| 4 | 驴蛋蛋 | NULL |
| 5 | 小脑腐 | NULL |
+----+-----------+-----------------+
8 rows in set (0.00 sec)
mysql> SELECT v.id,v.`name`,p.id FROM t_video v LEFT JOIN t_praise p ON p.video_id=v.id;
+----+-----------+------+
| id | name | id |
+----+-----------+------+
| 1 | 自拍 | 1 |
| 1 | 自拍 | 2 |
| 2 | 小猫咪 | 3 |
| 2 | 小猫咪 | 4 |
| 2 | 小猫咪 | 5 |
| 2 | 小猫咪 | 6 |
| 3 | 小狗狗 | NULL |
| 4 | 驴蛋蛋 | NULL |
| 5 | 小脑腐 | NULL |
+----+-----------+------+
9 rows in set (0.00 sec)
mysql> SELECT v.id,v.`name`,c.video_id AS cvid , p.video_id AS pvid,c.`comment` ,p.id from t_video v
-> LEFT JOIN t_comment c ON c.video_id=v.id LEFT JOIN t_praise p ON p.video_id=v.id ;
+----+-----------+------+------+-----------------+------+
| id | name | cvid | pvid | comment | id |
+----+-----------+------+------+-----------------+------+
| 1 | 自拍 | 1 | 1 | 啊好好看 | 1 |
| 1 | 自拍 | 1 | 1 | 啊好好看 | 2 |
| 1 | 自拍 | 1 | 1 | 啊好丑啊 | 1 |
| 1 | 自拍 | 1 | 1 | 啊好丑啊 | 2 |
| 1 | 自拍 | 1 | 1 | 你是魔鬼吗 | 1 |
| 1 | 自拍 | 1 | 1 | 你是魔鬼吗 | 2 |
| 2 | 小猫咪 | 2 | 2 | 喵喵喵 | 3 |
| 2 | 小猫咪 | 2 | 2 | 喵喵喵 | 4 |
| 2 | 小猫咪 | 2 | 2 | 喵喵喵 | 5 |
| 2 | 小猫咪 | 2 | 2 | 喵喵喵 | 6 |
| 2 | 小猫咪 | 2 | 2 | 喵喵喵 | 3 |
| 2 | 小猫咪 | 2 | 2 | 喵喵喵 | 4 |
| 2 | 小猫咪 | 2 | 2 | 喵喵喵 | 5 |
| 2 | 小猫咪 | 2 | 2 | 喵喵喵 | 6 |
| 3 | 小狗狗 | 3 | NULL | 汪汪汪 | NULL |
| 4 | 驴蛋蛋 | NULL | NULL | NULL | NULL |
| 5 | 小脑腐 | NULL | NULL | NULL | NULL |
+----+-----------+------+------+-----------------+------+
17 rows in set (0.00 sec)
所以会出现count全都变成了两个count的乘积的情况
正确的sql如下
SELECT t.*,COUNT(p.id) FROM (SELECT v.id,v.`name`,c.video_id,COUNT(c.video_id) AS comments from t_video v LEFT JOIN t_comment c ON c.video_id=v.id GROUP BY v.id)t LEFT JOIN t_praise p ON p.video_id=t.id GROUP BY t.id;
mysql> SELECT t.*,COUNT(p.id) FROM (SELECT v.id,v.`name`,c.video_id,COUNT(c.video_id) AS comments from t_video v LEFT JOIN t_comment c ON c.video_id=v.id GROUP BY v.id)t LEFT JOIN t_praise p ON p.video_id=t.id GROUP BY t.id;
+----+-----------+----------+----------+-------------+
| id | name | video_id | comments | COUNT(p.id) |
+----+-----------+----------+----------+-------------+
| 1 | 自拍 | 1 | 3 | 2 |
| 2 | 小猫咪 | 2 | 2 | 4 |
| 3 | 小狗狗 | 3 | 1 | 0 |
| 4 | 驴蛋蛋 | NULL | 0 | 0 |
| 5 | 小脑腐 | NULL | 0 | 0 |
+----+-----------+----------+----------+-------------+
5 rows in set (0.00 sec)
一步一步来是这样的
mysql> SELECT t.*,p.id FROM (SELECT v.id,v.`name`,c.video_id,COUNT(c.video_id) AS comments from t_video v LEFT JOIN t_comment c ON c.video_id=v.id GROUP BY v.id)t LEFT JOIN t_praise p ON p.video_id=t.id ;
+----+-----------+----------+----------+------+
| id | name | video_id | comments | id |
+----+-----------+----------+----------+------+
| 1 | 自拍 | 1 | 3 | 1 |
| 1 | 自拍 | 1 | 3 | 2 |
| 2 | 小猫咪 | 2 | 2 | 3 |
| 2 | 小猫咪 | 2 | 2 | 4 |
| 2 | 小猫咪 | 2 | 2 | 5 |
| 2 | 小猫咪 | 2 | 2 | 6 |
| 3 | 小狗狗 | 3 | 1 | NULL |
| 4 | 驴蛋蛋 | NULL | 0 | NULL |
| 5 | 小脑腐 | NULL | 0 | NULL |
+----+-----------+----------+----------+------+
9 rows in set (0.00 sec)
hiahiahia~