6

I have a query which I was running in Microsoft SQL server and it worked just fine. It made use of CTEs because I joined three tables. The first two in an inner query then an outer one. However when I tried this on my web server it only has MySQL installed and I could not run the query. Is there any way to run a query with that structure? Here is a sample:

USE UNAMupdates;
with CTE as
( SELECT tbl_modules.module_code
FROM tbl_student
 JOIN tbl_modules ON tbl_student.registration_year = tbl_modules.registration_year
WHERE tbl_student.student_number='1002')

SELECT tbl_notifications.module_name,tbl_notifications.message,tbl_notifications.staff_username,tbl_notifications.date_time 
FROM CTE JOIN tbl_notifications
ON cte.module_code = tbl_notifications.module_code
ORDER BY tbl_notifications.date_time DESC


What I am trying to achieve is to query all modules/subjects taken by a single student and then finding out if any notifications/news items are available for that subject.

11

In this particular case the CTE could be replaced with a normal subselect (a derived table):

SELECT
  tbl_notifications.module_name,
  tbl_notifications.message,
  tbl_notifications.staff_username,
  tbl_notifications.date_time 
FROM
  (
    SELECT
      tbl_modules.module_code
    FROM
      tbl_student
      INNER JOIN tbl_modules ON tbl_student.registration_year = tbl_modules.registration_year
    WHERE
      tbl_student.student_number = '1002'
  ) AS CTE
  INNER JOIN tbl_notifications ON cte.module_code = tbl_notifications.module_code
ORDER BY
  tbl_notifications.date_time DESC
;

And that query would work both in SQL Server and MySQL (in any major SQL product, in fact).

Moreover, nesting is unnecessary here at all, and so you could have the same result with the following:

SELECT
  tbl_notifications.module_name,
  tbl_notifications.message,
  tbl_notifications.staff_username,
  tbl_notifications.date_time 
FROM
  tbl_student
  INNER JOIN tbl_modules ON tbl_student.registration_year = tbl_modules.registration_year
  INNER JOIN tbl_notifications ON tbl_modules.module_code = tbl_notifications.module_code
WHERE
  tbl_student.student_number = '1002'
ORDER BY
  tbl_notifications.date_time DESC
;

And by the way, please consider using short table aliases in your queries. Here is what the same query looks like with short aliases:

SELECT
  n.module_name,
  n.message,
  n.staff_username,
  n.date_time 
FROM
  tbl_student AS s
  INNER JOIN tbl_modules AS m ON s.registration_year = m.registration_year
  INNER JOIN tbl_notifications AS n ON m.module_code = n.module_code
WHERE
  s.student_number = '1002'
ORDER BY
  n.date_time DESC
;

It may be subjective sometimes, but I do believe that in this case readability improves greatly.

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