加入2个MySQL查询，结果相同

Question

I am a newbie to this coding world. Is it possible to join two Queries into one? I have googled but I didn't get what exactly I am looking for so I reached here: My first query is :

SELECT filename
     , course.cname
     , DATE_FORMAT(pd_course_file.creationDate,"%D %M %Y") as date
     , type 
  FROM pd_course_file
  JOIN course 
    ON pd_course_file.course_id = course.id

Now whatever the result of type column for example the result of type I got pdf then my next query will be

Select icons from fileType  where type = 'pdf'

How can I join both queries? Any help will be highly appreciated.

Answer 1

if the type is the file type you could use a join

SELECT filename
  , course.cname
  ,DATE_FORMAT(pd_course_file.creationDate,"%D %M %Y") as date
  , type 
  , ft.icon 
FROM pd_course_file p
INNER JOIN course ON pd_course_file.course_id = course.id 
INNER JOIN fileType ft ON ft.type = pd_course_file.type  and ft.type='pdf'

oterwise if ever is a pdf but you have not a relatio you could use a cross join

SELECT filename
  , course.cname
  ,DATE_FORMAT(pd_course_file.creationDate,"%D %M %Y") as date
  , type 
  , ft.icon 
FROM pd_course_file p
INNER JOIN course ON pd_course_file.course_id = course.id 
CROSS JOIN fileType ft
where ft.type = 'pdf'

Answer 2

You could add a subquery as part of the select. Like so...

SELECT filename
     , course.cname
     , DATE_FORMAT(pd_course_file.creationDate,"%D %M %Y") as date
     , type
     , (select icons from filetype where type = 'PDF') as icons
  FROM pd_course_file
  JOIN course 
    ON pd_course_file.course_id = course.id

But that will get you a list of all the icons from the fileType table in put them in the icons column. If there's something else to key off of in the fileType table, you'd want to add that to the selecting subquery. For example:

SELECT filename
     , course.cname
     , DATE_FORMAT(pd_course_file.creationDate,"%D %M %Y") as date
     , type
     , (select icon from filetype ft where ft.type = 'PDF' and ft.file_course_id = pd_course_file.course_id) as icon
  FROM pd_course_file
  JOIN course 
    ON pd_course_file.course_id = course.id