我有 3 张 table :
user
(用户表)post
(包含字段作者的帖子表,该字段作者是用户 ID 的FOREIGN KEY
)friend
(一个表,表示用户与用户之间的关系,其中包含FOREIGN KEY
字段:user_id
和friend
我有一个查询,我可以在其中设置值来过滤结果,如下所示(来自 JavaPreparedStatement):
SELECT post.* FROM post, friend WHERE friend.user_id = ? AND post.author = friend.friend AND post.body LIKE ? AND post.date BETWEEN ? AND ? LIMIT ?,?
在查询中插入值,如下所示:
SELECT post.* FROM post, friend WHERE friend.user_id = 2 AND post.author = friend.friend AND post.body LIKE '%word%' AND post.date BETWEEN '2016-02-10 00:00:00' AND '2016-12-01 00:00:00' LIMIT 1,5;
我得到了 5 个帖子的列表。但现在,我想得到结果的数量。我尝试过:
SELECT COUNT(*) FROM post, friend WHERE friend.user_id = 2 AND post.author = friend.friend AND post.body LIKE '%word%' AND post.date BETWEEN '2016-02-10 00:00:00' AND '2016-12-01 00:00:00' LIMIT 1,5;
但我得到空集
。为什么我得到这个而不是 5?
编辑:
如果有帮助,我会发布数据库代码和一些数据插入。
DROP DATABASE IF EXISTS dbname;
CREATE DATABASE dbname;
USE dbname;
CREATE TABLE user (
id INT NOT NULL AUTO_INCREMENT,
username VARCHAR(20) NOT NULL UNIQUE,
name VARCHAR(70) NOT NULL,
PRIMARY KEY(id)
) ENGINE=InnoDB;
CREATE TABLE post (
id INT NOT NULL AUTO_INCREMENT,
date DATETIME NOT NULL,
title VARCHAR(100) NOT NULL,
body VARCHAR(1000) NOT NULL,
author INT NOT NULL,
PRIMARY KEY(id)
) ENGINE=InnoDB;
CREATE TABLE friend (
user_id INT NOT NULL,
friend INT NOT NULL,
PRIMARY KEY(user_id, friend)
) ENGINE=InnoDB;
ALTER TABLE post
ADD FOREIGN KEY fk_autor_post(author) REFERENCES user(id)
ON DELETE CASCADE
ON UPDATE CASCADE;
ALTER TABLE friend
ADD FOREIGN KEY fk_user_id(user_id) REFERENCES user(id)
ON DELETE CASCADE
ON UPDATE CASCADE;
ALTER TABLE friend
ADD FOREIGN KEY fk_friend_id(friend) REFERENCES user(id)
ON DELETE CASCADE
ON UPDATE CASCADE;
INSERT INTO user VALUES(null, "uname1", "User 1");
INSERT INTO user VALUES(null, "uname2", "User 2");
INSERT INTO user VALUES(null, "uname3", "User 3");
INSERT INTO user VALUES(null, "uname4", "User 4");
INSERT INTO user VALUES(null, "uname5", "User 5");
INSERT INTO friend VALUES(1,2);
INSERT INTO friend VALUES(1,3);
INSERT INTO friend VALUES(1,4);
INSERT INTO friend VALUES(1,5);
INSERT INTO friend VALUES(2,1);
INSERT INTO friend VALUES(2,5);
INSERT INTO friend VALUES(4,1);
INSERT INTO friend VALUES(2,4);
INSERT INTO friend VALUES(2,3);
INSERT INTO post values (null, '2016-01-01 10:00:00', "Title post 1", "Body 1", 1);
INSERT INTO post values (null, '2016-01-01 10:00:00', "Title post 2", "Body 2", 1);
INSERT INTO post values (null, '2016-01-01 10:20:40', "Title post 3", "Body 3", 1);
INSERT INTO post values (null, '2016-01-01 10:20:00', "Title post 4", "Body 4", 4);
INSERT INTO post values (null, '2016-02-05 10:00:00', "Title post 5", "Body 5", 5);
INSERT INTO post values (null, '2016-01-05 10:00:00', "Title post 6", "Body 6", 2);
INSERT INTO post values (null, '2016-02-01 22:00:00', "Title post 7", "Body 7", 2);
INSERT INTO post values (null, '2016-02-01 10:00:00', "Title post 8", "Body 8", 3);
INSERT INTO post values (null, '2016-01-02 10:00:00', "Title post 9", "Body 9", 5);
INSERT INTO post values (null, '2016-01-01 10:40:00', "Title post 10", "Body 10", 1);
查询:
SELECT post.* FROM post, friend WHERE friend.user_id = 2 AND post.author = friend.friend AND post.body LIKE '%%' AND post.date BETWEEN '2016-01-01 00:00:00' AND '2016-12-01 00:00:00' LIMIT 1,5;
返回:
+----+---------------------+--------------+--------+--------+
| id | date | title | body | author |
+----+---------------------+--------------+--------+--------+
| 2 | 2016-01-01 10:00:00 | Title post 2 | Body 2 | 1 |
| 3 | 2016-01-01 10:20:40 | Title post 3 | Body 3 | 1 |
| 4 | 2016-01-01 10:20:00 | Title post 4 | Body 4 | 4 |
| 5 | 2016-02-05 10:00:00 | Title post 5 | Body 5 | 5 |
| 8 | 2016-02-01 10:00:00 | Title post 8 | Body 8 | 3 |
+----+---------------------+--------------+--------+--------+
5 rows in set (0.00 sec)
查询:
SELECT COUNT(*) FROM post, friend WHERE friend.user_id = 2 AND post.author = friend.friend AND post.body LIKE '%%' AND post.date BETWEEN '2016-01-01 00:00:00' AND '2016-12-01 00:00:00' LIMIT 1,5;
返回:
Empty set (0.00 sec)
同样使用 COUNT(post.id)
, COUNT(post.body)
, SUM(1)
, SUM (post.id)
。
使用 COUNT(post.*)
时出错:
最佳答案
我在自己的数据库上尝试了类似的查询,得到了与你相同的结果。我将限制从 1,5 更改为 0,5 并得到了所需的结果。我不知道这是否适合您。
关于mysql - 为什么 MySQL COUNT(*) 查询返回空集?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/36701642/