SQL Left join where data missing in join column - sql

Question

I'm not even sure this is possible but I'm trying to join two tables together but I'm not getting my expected results. My query is as follows:
SELECT inc.NUMBER as TICKET,
inc.UNIV_NUM,
inc.ASSIGNEE,
work.SUBMIT_DATE
work.TYPE
FROM dbo.HELP_DESK as inc
LEFT JOIN dbo.WORKLOG as work on inc.NUMBER = work.NUMBER
Where inc.ASSIGNEE = 'AB049732'
and work.SUBMIT_DATE = (Select MAX(work2.SUBMIT_DATE)
from dbo.WORKLOG as work2
where work2.NUMBER = work.NUMBER
and work2.TYPE = '16000')
My tables look like this
inc
+---------+-----------+----------+
| NUMBER | UNIV_NUM | ASSIGNEE |
+---------+-----------+----------+
| 100001 | 4321781 | AB049732 |
| 100002 | 4232756 | AB049732 |
| 100003 | 4322534 | AB049732 |
| 100004 | 4328534 | AB049732 |
+---------+-----------+----------+
work
+--------+------------+-------+
| NUMBER | DATE | TYPE |
+--------+------------+-------+
| 100001 | 23/05/2018 | 16000 |
| 100003 | 22/05/2018 | 16000 |
| 100004 | 22/05/2018 | 16000 |
+--------+------------+-------+
My expected output is:
+--------+----------+----------+------------+-------+
| NUMBER | UNIV_NUM | ASSIGNEE | DATE | TYPE |
+--------+----------+----------+------------+-------+
| 100001 | 4321781 | AB049732 | 23/05/2018 | 16000 |
| 100002 | 4232756 | AB049732 | NULL | NULL |
| 100003 | 4322534 | AB049732 | 22/05/2018 | 16000 |
| 100004 | 4328534 | AB049732 | 22/05/2018 | 16000 |
+--------+----------+----------+------------+-------+
But my actual output is:
+---------+-----------+----------+------------+-------+
| NUMBER | UNIV_NUM | ASSIGNEE | DATE | TYPE |
+---------+-----------+----------+------------+-------+
| 100001 | 4321781 | AB049732 | 23/05/2018 | 16000 |
| 100003 | 4322534 | AB049732 | 22/05/2018 | 16000 |
| 100004 | 4328534 | AB049732 | 22/05/2018 | 16000 |
+---------+-----------+----------+------------+-------+
Effectively, number 100002 isn't displaying despite being in the inc table. Am I doing something wrong or is this a case of you can't join to something that doesn't exist?

Answer 1

Your join condition is bad. Try this:
SELECT inc.NUMBER as TICKET,
inc.UNIV_NUM,
inc.ASSIGNEE,
work.SUBMIT_DATE
work.TYPE
FROM dbo.HELP_DESK as inc
LEFT JOIN dbo.WORKLOG as work on inc.NUMBER = work.NUMBER
and work.SUBMIT_DATE = (Select MAX(work2.SUBMIT_DATE)
from dbo.WORKLOG as work2
where work2.NUMBER = work.NUMBER
and work2.TYPE = '16000')
Where inc.ASSIGNEE = 'AB049732'
See the difference? If you put the work.SUBMIT_DATE = ... condition in the Where clause (as you did) then your join becomes an inner join. But you want an outer join.

Answer 2

Use window functions!
SELECT h.NUMBER as TICKET, h.UNIV_NUM, h.ASSIGNEE,
w.SUBMIT_DATE, w.TYPE
FROM dbo.HELP_DESK h LEFT JOIN
(SELECT w.*, MAX(w2.SUBMIT_DATE) OVER (PARTITION BY w.NUMBER) as max_submit_date
FROM dbo.WORKLOG w
WHERE w.TYPE = '16000'
) w
ON h.NUMBER = w.NUMBER AND w.submit_date = w.max_submit_date
WHERE h.ASSIGNEE = 'AB049732';
This is subtly different from your query, but I think it is the logic you actually want. Your query will find records that have the maximum submit date for type '16000' regardless of type. I presume that you really want the types to align to the submit date.
If this interpretation is wrong, it is easy to adjust the query:
SELECT h.NUMBER as TICKET, h.UNIV_NUM, h.ASSIGNEE,
w.SUBMIT_DATE, w.TYPE
FROM dbo.HELP_DESK h LEFT JOIN
(SELECT w.*,
MAX(CASE WHEN w.TYPE = '16000' THEN w2.SUBMIT_DATE END) OVER (PARTITION BY w.NUMBER) as max_submit_date
FROM dbo.WORKLOG w
) w
ON h.NUMBER = w.NUMBER AND w.submit_date = w.max_submit_date
WHERE h.ASSIGNEE = 'AB049732';
These versions are not only simpler, but they should have better performance as well.

Answer 3

You can use a CTE like this:
WITH WorkDates
AS (SELECT SUBMIT_DATE,TYPE
From WORKLOG work
Where SUBMIT_DATE = (Select MAX(work2.SUBMIT_DATE)
from dbo.WORKLOG as work2
where work2.NUMBER = work.NUMBER
and work2.TYPE = '16000'))
SELECT inc.NUMBER as TICKET,
inc.UNIV_NUM,
inc.ASSIGNEE,
WorkDates.SUBMIT_DATE
WorkDates.TYPE
FROM dbo.HELP_DESK as inc
LEFT JOIN WorkDates on inc.NUMBER = WorkDates.NUMBER
Where inc.ASSIGNEE = 'AB049732'