代码之家 › 专栏 › 技术社区 › Thomas Matthews

MySQL:删除左联接上的重复列,3个表

mysql-connector left-join database mysql sql

11

Thomas Matthews · 技术社区 · 15 年前

我有一个表在其他表中使用了3个外键。当我执行左联接时,会得到重复的列。mysql说 USING 语法将减少重复列,但没有多个键的示例。

鉴于:

mysql> describe recipes;
+------------------+------------------+------+-----+---------+-------+
| Field            | Type             | Null | Key | Default | Extra |
+------------------+------------------+------+-----+---------+-------+
| ID_Recipe        | int(11)          | NO   | PRI | NULL    |       |
| Recipe_Title     | char(64)         | NO   |     | NULL    |       |
| Difficulty       | int(10) unsigned | NO   |     | NULL    |       |
| Elegance         | int(10) unsigned | NO   |     | NULL    |       |
| Quality          | int(10) unsigned | NO   |     | NULL    |       |
| Kitchen_Hours    | int(10) unsigned | NO   |     | NULL    |       |
| Kitchen_Minutes  | int(10) unsigned | NO   |     | NULL    |       |
| Total_Hours      | int(10) unsigned | NO   |     | NULL    |       |
| Total_Minutes    | int(10) unsigned | NO   |     | NULL    |       |
| Serving_Quantity | int(10) unsigned | NO   |     | NULL    |       |
| Description      | varchar(128)     | NO   |     | NULL    |       |
| ID_Prep_Text     | int(11)          | YES  |     | NULL    |       |
| ID_Picture       | int(11)          | YES  |     | NULL    |       |
| Category         | int(10) unsigned | NO   |     | NULL    |       |
| ID_Reference     | int(11)          | YES  |     | NULL    |       |
+------------------+------------------+------+-----+---------+-------+
15 rows in set (0.06 sec)

mysql> describe recipe_prep_texts;
+------------------+---------------+------+-----+---------+-------+
| Field            | Type          | Null | Key | Default | Extra |
+------------------+---------------+------+-----+---------+-------+
| ID_Prep_Text     | int(11)       | NO   | PRI | NULL    |       |
| Preparation_Text | varchar(2048) | NO   |     | NULL    |       |
+------------------+---------------+------+-----+---------+-------+
2 rows in set (0.02 sec)

mysql> describe recipe_prep_texts;
+------------------+---------------+------+-----+---------+-------+
| Field            | Type          | Null | Key | Default | Extra |
+------------------+---------------+------+-----+---------+-------+
| ID_Prep_Text     | int(11)       | NO   | PRI | NULL    |       |
| Preparation_Text | varchar(2048) | NO   |     | NULL    |       |
+------------------+---------------+------+-----+---------+-------+
2 rows in set (0.02 sec)

mysql> describe mp_references;
+--------------+---------+------+-----+---------+-------+
| Field        | Type    | Null | Key | Default | Extra |
+--------------+---------+------+-----+---------+-------+
| ID_Reference | int(11) | NO   | PRI | NULL    |       |
| ID_Title     | int(11) | YES  |     | NULL    |       |
| ID_Category  | int(11) | YES  |     | NULL    |       |
+--------------+---------+------+-----+---------+-------+
3 rows in set (0.00 sec)

我的查询语句:

SELECT  *
 FROM  Recipes
LEFT JOIN (Recipe_Prep_Texts, Recipe_Pictures, mp_References)
ON (
 Recipe_Prep_Texts.ID_Prep_Text = Recipes.ID_Prep_Text AND
 Recipe_Pictures.ID_Picture = Recipes.ID_Picture AND
 mp_References.ID_Reference = Recipes.ID_Reference
);

我的目标是从联接中获取所有列中的一行,而不复制列。我使用MySQL C++连接器发送SQL语句并检索结果集。我相信C++连接器有重复列名的问题。

那么我应该使用什么样的SQL语句语法呢?

Reference to MySQL JOIN syntax

4 回复 | 直到 9 年前

1

13

Danny Shisler 15 年前

我认为以下方法应该有效:

SELECT  *
 FROM  Recipes
LEFT JOIN Recipe_Prep_Texts USING (ID_Prep_Text)
LEFT JOIN Recipe_Pictures USING (ID_Picture)
LEFT JOIN mp_References USING (ID_Reference)

2

4

Rob Van Dam 15 年前

因为看起来您要连接的大多数表都有一些列,除了第一列之外,如何处理:

   SELECT Recipes.*,
          Recipe_Prep_Texts.Preparation_Text,
          Recipe_Pictures.Foo, -- describe is missing in OP
          mp_References.ID_Title,
          mp_References.ID_Category

     FROM Recipes
LEFT JOIN (Recipe_Prep_Texts, Recipe_Pictures, mp_References)
       ON (
           Recipe_Prep_Texts.ID_Prep_Text = Recipes.ID_Prep_Text AND
           Recipe_Pictures.ID_Picture = Recipes.ID_Picture AND
           mp_References.ID_Reference = Recipes.ID_Reference
          );

我不能告诉你我希望我有多少次

SELECT (* - foo) FROM table

尤其是在foo是一个像blob这样的大字段的情况下,我只想在不破坏格式的情况下看到其他所有内容。

3

2

tgandrews 15 年前

您正在从组合结果表中选择*。将*限制在您要保留的列中。

4

0

Mike F 9 年前

尝试以下查询:

SELECT  name,ac,relation_name
FROM  table1
LEFT JOIN table2 USING (ID_Prep_Text)
LEFT JOIN table3 USING (ID_Picture);