Question

我有3张桌子：

'#uses "C:\Scripts\pgGlobal.bas.txt" Sub Main MsgBox "Primary Width: " & GetSystemMetrics(SM_CXSCREEN) & _ " x Primary Height: " & GetSystemMetrics(SM_CYSCREEN) & vbCrLf & _ "Number of monitors: " & GetSystemMetrics(SM_CMONITORS) & vbCrLf & _ "Total Width: " & GetSystemMetrics(SM_CXVIRTUALSCREEN) & _ " x Total Height: " & GetSystemMetrics(SM_CYVIRTUALSCREEN) & vbCrLf & _ "Left Pixel: " & GetSystemMetrics(SM_XVIRTUALSCREEN) & _ " x Top Pixel: " & GetSystemMetrics(SM_XVIRTUALSCREEN) End Sub（用户表）
user（带有字段作者的帖子表，用户ID为post）
FOREIGN KEY（表示具有friend字段的用户与用户关系的表：FOREIGN KEY和user_id

我有一个查询，我可以设置值来过滤这样的结果（来自Java PreparedStatement）：

friend

在查询中插入值，如下所示：

SELECT post.* FROM post, friend WHERE friend.user_id = ? AND post.author = friend.friend AND post.body LIKE ? AND post.date BETWEEN ? AND ? LIMIT ?,?

我得到了5个帖子的列表。但现在，我想得到结果的数量。我试过了：

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;

但我得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;。为什么我会这样做5？

修改

如果有帮助，我发布数据库代码和一些数据插入。

Empty set

查询：

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);

返回：

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;

查询：

+----+---------------------+--------------+--------+--------+
| 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)

返回：

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;

使用Empty set (0.00 sec)，COUNT(post.id)，COUNT(post.body)，SUM(1)进行相同操作。使用SUM(post.id)时出错：

Answer 1

我在自己的数据库上尝试过类似的查询，并得到与您相同的结果。我将限制从1.5更改为0.5并获得了预期的结果。我不知道这是不是你的选择。

Answer 2

试试这个

 SELECT COUNT(*) AS TotalCount FROM (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) AS a ;

为什么MySQL COUNT（*）查询返回空集？

2 个答案: