左外连接在同一张桌子上

Question

I am using this code to sort any 'name' that appears in the last 70 days with the Master Symptom of 'Problem' I have open and closed columns and so am only interested in any problem that is still open.

select 
    name, LastEDT 
from 
    vw_UserView_ME_Open
where 
    received between dateadd(day, datediff(day, 0, getdate()) - 70, 0) 
                 and dateadd(day, datediff(day, 0, getdate()), 0)
    and MasterSymptom = 'Problem' 
    and closed is null
order by
    'LastEDT' asc;

What I looking to do is create an outer left join on the same table to reference any additional master symptoms that are logged against the 'name'

I have very limited (read no) experience of joins on the same table and so I am a little lost and I cant figure it out from the posts that I have read here.

Can anyone help?

Answer 1

This might get you started? Basically a self-JOIN is just the same as any other JOIN if you are careful with your aliases. My example simply looks for any entries with the same name where the Master Symptom = "Problem", then COUNTs these up. You may well want to do something different with the results?

SELECT 
    o.name, 
    o.LastEDT,
    COUNT(r.*) AS related_problems
FROM 
    vw_UserView_ME_Open o
    LEFT JOIN vw_UserView_ME_Open r ON r.name = o.name AND r.MasterSymptom = 'Problem' 
WHERE 
    o.received BETWEEN CONVERT(DATE, DATEADD(DAY, -70, GETDATE())) AND CONVERT(DATE, GETDATE())
    AND o.MasterSymptom = 'Problem' 
    AND o.closed IS NULL
GROUP BY
    o.name, 
    o.LastEDT
ORDER BY 
    o.LastEDT;

I "tidied" up your SQL a bit at the same time.

Remember that because this is a LEFT JOIN you can't add any criteria for the related table into the WHERE clause, or you are essentially making the LEFT JOIN into an INNER JOIN. So if you also wanted to test that closed IS NULL then you could add:

r.closed IS NULL

to the end of the LEFT JOIN criteria, but not to the WHERE clause.

Actually that's not the best example, as you COULD actually add that extra IS NULL criteria to the WHERE clause without breaking the JOIN. However, the results would probably not be as intended, as this would now mean:

• get me records where the related entry has a NULL is closed value;
• also get me records where there are no related entries at all (as this will also return a NULL).