MySQL多个联接在一个查询? [英] MySQL Multiple Joins in one query?

问题描述

我有以下查询:

SELECT
  dashboard_data.headline,
  dashboard_data.message,
  dashboard_messages.image_id 
FROM dashboard_data
INNER JOIN dashboard_messages
  ON dashboard_message_id = dashboard_messages.id

所以我正在使用INNER JOIN并抓住image_id.所以现在，我要获取该image_id并将其从图像表中转换为images.filename.

So I am using an INNER JOIN and grabbing the image_id. So now, I want to take that image_id and turn it into images.filename from the images table.

如何将其添加到查询中?

How can I add that in to my query?

推荐答案

您可以像这样简单地添加另一个联接:

You can simply add another join like this:

SELECT dashboard_data.headline, dashboard_data.message, dashboard_messages.image_id, images.filename
FROM dashboard_data 
    INNER JOIN dashboard_messages 
        ON dashboard_message_id = dashboard_messages.id
    INNER JOIN images
        ON dashboard_messages.image_id = images.image_id 

但是请注意，由于它是INNER JOIN，因此如果您收到的消息中没有图像，则会跳过整行.如果可能的话，您可能想做一个LEFT OUTER JOIN，它将仅在存在的情况下返回所有仪表板消息和一个image_filename(否则将为空)

However be aware that, because it is an INNER JOIN, if you have a message without an image, the entire row will be skipped. If this is a possibility, you may want to do a LEFT OUTER JOIN which will return all your dashboard messages and an image_filename only if one exists (otherwise you'll get a null)

SELECT dashboard_data.headline, dashboard_data.message, dashboard_messages.image_id, images.filename
FROM dashboard_data 
    INNER JOIN dashboard_messages 
        ON dashboard_message_id = dashboard_messages.id
    LEFT OUTER JOIN images
        ON dashboard_messages.image_id = images.image_id 

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