Question

我有4张这样的表

产品表：

+--------------+-------------+------+-----+---------+----------------+
| Field        | Type        | Null | Key | Default | Extra          |
+--------------+-------------+------+-----+---------+----------------+
| productID    | int(11)     | NO   | PRI | NULL    | auto_increment |
| name         | varchar(45) | YES  |     | NULL    |                |
| price        | int(11)     | YES  |     | NULL    |                |
| manufacturer | varchar(45) | YES  |     | NULL    |                |
| rating       | int(2)      | YES  |     | NULL    |                |
| categoryID   | int(11)     | YES  | MUL | NULL    |                |
+--------------+-------------+------+-----+---------+----------------+

类别表：

+--------------+-------------+------+-----+---------+----------------+
| Field        | Type        | Null | Key | Default | Extra          |
+--------------+-------------+------+-----+---------+----------------+
| categoryID   | int(11)     | NO   | PRI | NULL    | auto_increment |
| categoryName | varchar(64) | YES  |     | NULL    |                |
+--------------+-------------+------+-----+---------+----------------+

用户：

+-----------+-------------+------+-----+---------+----------------+
| Field     | Type        | Null | Key | Default | Extra          |
+-----------+-------------+------+-----+---------+----------------+
| userID    | int(11)     | NO   | PRI | NULL    | auto_increment |
| firstname | varchar(45) | YES  |     | NULL    |                |
| age       | int(11)     | YES  |     | NULL    |                |
| location  | varchar(45) | YES  |     | NULL    |                |
| username  | varchar(45) | YES  |     | NULL    |                |
| password  | varchar(45) | YES  |     | NULL    |                |
+-----------+-------------+------+-----+---------+----------------+

修改：

+------------+------------+------+-----+---------+----------------+
| Field      | Type       | Null | Key | Default | Extra          |
+------------+------------+------+-----+---------+----------------+
| reviewID   | int(11)    | NO   | PRI | NULL    | auto_increment |
| reviewTime | date       | YES  |     | NULL    |                |
| reviewData | mediumtext | YES  |     | NULL    |                |
| productID  | int(11)    | YES  | MUL | NULL    |                |
| userID     | int(11)    | YES  | MUL | NULL    |                |
+------------+------------+------+-----+---------+----------------+

我基本上需要一个显示以下

的select语句

reviewID - reviewTime - reviewData - product.name - category.name - user.firstname

到目前为止，我已经提出了以下内容

SELECT r.reviewID, r.reviewTime, r.reviewData, p.name, u.firstname 
  FROM review r 
  INNER JOIN product p on p.productID = r.productID 
  INNER JOIN user u on u.userID = r.userID;

这给了我除了类别名称之外的所有东西，就像我被困住的那样。

数据库是这样链接的;

enter image description here

我只是不明白如何将categoryname从类别表传递到评论表

Answer 1

select r.reviewID, r.reviewTime, r.reviewData, p.name, u.firstname, c.categoryName
from review r
left join product p on p.productID = r.productID
left join user u on u.userID = r.userID
left join category c on c.categoryID = p.categoryID

注意：根据您的架构，我使用了LEFT JOIN，这样无论是否有相关数据，都会返回所有评论。

Answer 2

您当前的疑问：

SELECT r.reviewID, r.reviewTime, r.reviewData, p.name, u.firstname 
FROM review r 
    INNER JOIN product p on p.productID = r.productID 
    INNER JOIN user u on u.userID = r.userID;

由于您已加入产品表并且产品表中引用了categoryID，因此只需添加其他联接：

SELECT 
    r.reviewID, r.reviewTime, r.reviewData, p.name, u.firstname, c.categoryName  
FROM review r 
    INNER JOIN product p on p.productID = r.productID 
    INNER JOIN user u on u.userID = r.userID;
    INNER JOIN category c ON c.categoryID = p.categoryID

Answer 3

添加INNER JOIN category c on c.categoryID = p.categoryID

Answer 4

您没有加入类别表添加

Join category c on c.categoryID = p.categoryID

并添加选择

 c.name

只有在表之间的任何链接可能不存在时，才将连接更改为左连接。

内连接但我不理解另一个表的外键位

4 个答案: