如何在MySQL中连接这两个表

我有两个表，结构如下：

|=================|
| posts           |
|=================|
| ID | Title      |
|-----------------|
| 1  | Title #1   |
|-----------------|
| 2  | Title #1   |
|-----------------|
| 3  | Title #1   |
|-----------------|
| 4  | Title #1   |
|-----------------|
| 5  | Title #1   |
|-----------------|

及

我需要得到以下单一结果：

|----------------------------------------------------------------|
| ID | Post Title | Meta Key One | Meta Key Two | Meta Key Three |
|----------------------------------------------------------------|
| 1  | Title #1   | value for #1 | value for #1 | value for #1   |
|----------------------------------------------------------------|
| 2  | Title #2   | value for #2 | null         | value for #2   |
|----------------------------------------------------------------|
| 3  | Title #3   | value for #3 | null         | value for #3   |
|----------------------------------------------------------------|

但我不知道怎么做

到目前为止，我构建的SQL查询如下：

SELECT
   `p`.`ID` AS `ID`,
   `p`.`Title` AS `Post Title`,
   `mt1`.`meta_value` AS `Meta Key One`,
   `mt2`.`meta_value` AS `Meta Key One`,
FROM
    posts AS `p`
    LEFT JOIN `meta` AS `mt1` ON ( `p`.`ID` = `mt1`.`post_id` )
    LEFT JOIN `meta` AS `mt2` ON ( `p`.`ID` = `mt2`.`post_id` )
WHERE
    1 = 1
    AND `mt1`.`meta_key` = 'key_one'
    AND `mt2`.`meta_key` = 'key_three';

问题是，如果我在

meta

表中添加第三个

LEFT JOIN

，稍后在

WHERE

子句中使用它，并说

mt1.meta\u key='key\u two'

我只得到一条记录，而不是三条

有人知道我如何通过一个查询实现这一点吗

我不知道这是否有帮助，但我在这里创建了一个SQL FIDLE：

---

请注意，fiddle中的列名与我的示例中的列名不符，但问题仍然相同。

您按列联接表，而不是按列联接列。。因为只有两个表，所以只需要一个联接。从这个查询开始，将

*

更改为您需要的任何列

select 
    * --or whatever
from
posts p
left join meta m on p.id = m.post_id

您可以为此使用条件聚合：

SELECT p.ID, p.Title AS 'Post Title',
       MAX(CASE WHEN meta_key = 'key_one' THEN meta_value END) AS 'Meta Key One',
       MAX(CASE WHEN meta_key = 'key_two' THEN meta_value END) AS 'Meta Key Two',
       MAX(CASE WHEN meta_key = 'key_three' THEN meta_value END) AS 'Meta Key Three'
FROM posts AS p
LEFT JOIN meta AS m ON p.ID = m.post_id
GROUP BY p.ID, p.Title

此方法的好处是只需使用一次

左连接

，因此它很容易扩展以容纳其他键值

试试这个：

SELECT
   `p`.`ID` AS `ID`,
   `p`.`Title` AS `Post Title`,
   `mt1`.`meta_value` AS `Meta Key One`,
   `mt2`.`meta_value` AS `Meta Key Two`,
   `mt3`.`meta_value` AS `Meta Key Three`
FROM
    posts AS `p`
    LEFT JOIN `meta` AS `mt1` 
        ON ( `p`.`ID` = `mt1`.`post_id` AND `mt1`.`meta_key` = 'key_one')
    LEFT JOIN `meta` AS `mt2` 
        ON ( `p`.`ID` = `mt2`.`post_id` AND `mt2`.`meta_key` = 'key_two')
    LEFT JOIN `meta` AS `mt3` 
        ON ( `p`.`ID` = `mt3`.`post_id` AND `mt3`.`meta_key` = 'key_three')

---

为此，您应该使用外部联接，因为否则您将无法获得第二和第三个所需的输出行。您可以检查：

左联接

和

左外部联接

之间没有区别。一般规则是：外部联接表上的条件属于

on

子句，而不是

WHERE

子句。这就是查询失败的地方。

SELECT
   `p`.`ID` AS `ID`,
   `p`.`Title` AS `Post Title`,
   `mt1`.`meta_value` AS `Meta Key One`,
   `mt2`.`meta_value` AS `Meta Key Two`,
   `mt3`.`meta_value` AS `Meta Key Three`
FROM
    posts AS `p`
    LEFT JOIN `meta` AS `mt1` 
        ON ( `p`.`ID` = `mt1`.`post_id` AND `mt1`.`meta_key` = 'key_one')
    LEFT JOIN `meta` AS `mt2` 
        ON ( `p`.`ID` = `mt2`.`post_id` AND `mt2`.`meta_key` = 'key_two')
    LEFT JOIN `meta` AS `mt3` 
        ON ( `p`.`ID` = `mt3`.`post_id` AND `mt3`.`meta_key` = 'key_three')

SELECT 
  `b`.`id` AS `ID`,
  `b`.`title` AS `Title`,
  mt1.meta_value `KeyOne`,
  mt2.meta_value `KeyTwo`,
  mt3.meta_value `KeyThree`
FROM 
  `base` as `b`
  LEFT JOIN `meta` mt1 ON b.id = mt1.base_id AND mt1.meta_key = 'key_one' 
  LEFT JOIN `meta` mt2 ON b.id = mt2.base_id AND mt2.meta_key = 'key_two' 
  LEFT JOIN `meta` mt3 ON b.id = mt3.base_id AND mt3.meta_key = 'key_three'