Question

I have the following result from the following query:

SELECT users.ID, user_registered, post_date, post_type
FROM users LEFT OUTER JOIN posts ON users.ID = post_author
ORDER BY ID, user_registered, post_date, post_type

  ID   user_registered          post_date      post_type
---+-----------------+------------------+--------------+
   1    6/18/2012 4:04     6/18/2012 4:04           page
   1    6/18/2012 4:04     6/18/2012 4:04           post
   1    6/18/2012 4:04    6/18/2012 10:32  nav_menu_item
   1    6/18/2012 4:04    6/18/2012 10:32  nav_menu_item
   1    6/18/2012 4:04    6/18/2012 10:32  nav_menu_item
   1    6/18/2012 4:04    6/18/2012 10:32  nav_menu_item
   1    6/18/2012 4:04    6/18/2012 10:32  nav_menu_item
   1    6/18/2012 4:04    6/18/2012 10:33  nav_menu_item
   1    6/18/2012 4:04    6/18/2012 10:48     attachment
   1    6/18/2012 4:04    6/19/2012 10:32     attachment
   1    6/18/2012 4:04     6/30/2012 6:49  nav_menu_item
   1    6/18/2012 4:04     6/30/2012 7:04  nav_menu_item
   1    6/18/2012 4:04      7/1/2012 7:39     attachment
   1    6/18/2012 4:04    7/13/2012 10:00           page
   1    6/18/2012 4:04    7/20/2012 18:18           page
   1    6/18/2012 4:04    7/21/2012 16:37           post
   1    6/18/2012 4:04    7/23/2012 14:15           page
   1    6/18/2012 4:04    7/31/2012 15:12           post
   1    6/18/2012 4:04      8/1/2012 8:47     attachment
   2   6/19/2012 10:05              NULL            NULL
   6    6/29/2012 4:47    6/19/2012 10:11     attachment
   6    6/29/2012 4:47    6/19/2012 10:15     attachment
   6    6/29/2012 4:47    6/19/2012 10:17     attachment
   6    6/29/2012 4:47    6/19/2012 10:20     attachment
   6    6/29/2012 4:47    6/19/2012 10:22     attachment
   6    6/29/2012 4:47     6/28/2012 0:00       employee
   6    6/29/2012 4:47     6/29/2012 0:00       employee
   6    6/29/2012 4:47      8/1/2012 0:00       employee
   6    6/29/2012 4:47      8/2/2012 0:00       employee
   6    6/29/2012 4:47      8/2/2012 0:00       employee
   6    6/29/2012 4:47      8/2/2012 0:00       employee
   7    7/7/2012 16:52    7/31/2012 14:26           post
  20   7/21/2012 14:48    7/21/2012 21:53           post
  20   7/21/2012 14:48    7/22/2012 12:50           post
  21   7/27/2012 14:56              NULL            NULL

• ID: user id. unique, primary key
• user_registered: datetime when the user became a part of the site
• post_date: datetime when the post was created by the user

• post_type: post type of post created by the user

  1. I need to get the number of days since the user last made an employee post (post_date where post_type = 'employee').
  2. If the user hasn’t made an employee post yet, then I need to get the number of days since the user registered (user_registered).

REQUIREMENT

I need to get just the ID of all users

1. whose latest employee post date is e.g. < 7 days or >= 14 days, or BETWEEN 6 AND 14 as needed; OR
2. if the user does not have an employee post, get the user who registered < 7 days or >= 14 days, or BETWEEN 6 AND 14 as in #1.

I tried the following query for a < 7 case:

SELECT users.ID
FROM users LEFT OUTER JOIN posts ON users.ID = post_author
GROUP BY ID
HAVING MIN(DATEDIFF(NOW(), IFNULL(post_date, user_registered))) < 7

and it yields the IDs 1, 6 and 7 as expected, because:

1. the user (6) who has an employee post does NOT have any other post of other post_types; and
2. the users (1, 7) who don’t have an employee post got their calculations from their user-registered date.

but then this query breaks when

1. the user who HAS an employee post ALSO HAS other post_types

because I can’t limit the date calculations to posts of employee post_type only. If a user has a post or a page or an attachment that is, say, 2 days more recent than his latest employee, the date calculation will return the result from the post or page or attachment instead.

So I tried to include WHERE post_type = 'employee', but then only ID 6 is ever returned, because all the other users don’t have an employee post. So I lose #2 of my requirement.

How do I go about this? Thanks in advance.

Answer 1

Try:

You can use a CASE expression within the aggregate function MIN to derive the minimum date difference across only post_dates where post_type is ’employee’. If there are no post_types with ’employee’ for a particular user, use his/her registered date instead for the comparison.

SELECT    a.ID
FROM      users a
LEFT JOIN posts b ON a.ID = b.post_author
GROUP BY  a.ID
HAVING    IFNULL(
              MIN(CASE b.post_type WHEN 'employee' THEN DATEDIFF(NOW(), b.post_date) END),
              DATEDIFF(NOW(), MIN(a.user_registered))
          ) < 7