左联接中仅一个表的where子句

Question

我有两个表，第一个是“ actions”，具有“ id”和“ actionname”列，第二个表是“ completedactions”，具有“ userid”和“ actionid”。 它们看起来如下：

Table: actions

 id | actionname
 ==============
  1 | An action
  2 | Another
  3 | ect
  4 | ect

actions is a table of actions a user can make

-

Table: completedactions

 userid | actionid
 ==============
   1    |  1
   1    |  2
   1    |  3
   2    |  3
   3    |  2
   3    |  4

completedactions holds the userid and action id for each action made

我想加入表以给出所有动作的列表，以及那些动作是由给定用户在查询中指定的位置进行的。 因此，从此示例中，我希望看到类似以下内容：

 id | actionname | Complete by userid=1?
 =======================================
  1 | An action  | 1
  2 | Another    | 1
  3 | ect        | 1
  4 | ect        | Null

我尝试了左联接，并设法获得了所有动作的列表，但是当多个用户执行了一个动作时，会有重复的条目，每个用户要执行一个动作。 然后，我在最后compledtedactions.userid =“ 1”处添加了where子句，但是随后我丢失了该用户尚未执行的所有操作。

我似乎在左联接中仅对一个表没有where子句，那么我应该怎么做呢？

Answer 1

尝试在ON()子句中使用WHERE条件，即ON (c.actionid = a.id AND c.user_id = 1) 。

WHERE过滤器将应用于整个结果集，而联接中的其他条件将联接匹配的结果，并为不匹配的结果提供null 。

SELECT a.*,c.user_id FROM 
actions a 
LEFT JOIN completedactions c 
ON (c.actionid = a.id AND c.user_id = 1)

Answer 2

select 
  actions.id, 
  actions.actionname, 
  count(completedactions.actionid) 
from actions
left join completedactions 
  on completedactions.actionid = actions.id and completedactions.userid = 1
group by 1,2

Answer 3

从动作中选择*

在（completedactions.actionid = actions.id和completedactions.userid = 1）上左外部联接completedactions

Answer 4

如果有人需要使用它，它将为您提供全面的帮助。

select `m`.`id` AS `id`,
`m`.`CategoryParentId` AS `CategoryParentId`,
`m`.`CategoryChildId` AS `CategoryChildId`,
`p`.`CategoryName` AS `ParentCategory`,
`c`.`CategoryName` AS `ChildCategory`,
count(c_p.category_id) AS `prodcuts`,
`img`.`image` AS `CategoryImage`,
`img2`.`image` AS `thumbImage`
from (((((`tblcategoryparentchildassociations` `m` 
    left join `tblmastercategories` `c` on((`c`.`id` =  `m`.`CategoryChildId`))) 
    left join `tblmastercategories` `p` on((`p`.`id` = `m`.`CategoryParentId`)))
    left join `vwimagetypeassociations` `img` on((`c`.`id` = `img`.`domainid` AND `img`.`imagetypeid` = 2 AND `img`.`status` = 1 )))
    left join `vwimagetypeassociations` `img2` on((`c`.`id`= `img`.`domainid`AND `img2`.`imagetypeid` = 3 AND `img`.`status` = 1)))
    left join `tblproductcategoriesassociations` `c_p` on((`m`.`CategoryChildId` = `c_p`.`category_id`)))
group by m.id