Solved

Convert NOT IN Sub-Select to LEFT JOIN

Posted on 2011-09-30
12
306 Views
Last Modified: 2012-05-12
When I run this query:

select distinct idQuestion from Question
where parentid is null AND idQuestion not in (select distinct parentid from Question where parentid is not null)
order by idQuestion

Open in new window


The query never finishes after waiting several minutes. I've researched the issue and the conclusion is to use a LEFT JOIN instead of a sub-select. However, I'm not sure how to properly construct the query with these limitations.
0
Comment
Question by:BCRobert
  • 5
  • 4
  • 3
12 Comments
 
LVL 142

Expert Comment

by:Guy Hengel [angelIII / a3]
ID: 36891743
please try this:

select q.idQuestion 
from Question q
where q.parentid is null 
AND NOT EXISTS ( select NULL from Question o where o.parentid = q.idQuestion )
order by idQuestion 

Open in new window


and please ensure you have an index on parentid
0
 
LVL 5

Expert Comment

by:eridanix
ID: 36891750
Hi,

I mean you have little nonsense in your query, becouse you compare idQuestion with parentid

The correct query should only be:
select distinct idQuestion
from Question
where parentid is not null
order by idQuestion

Maybe, you need get something else. So try to explain what shloud be result of this query.
0
 

Author Comment

by:BCRobert
ID: 36891794
@angellll: parentid isn't indexed because it's not unique. This query is taking just as long.

@eridanix: That query won't work. Here's what I'm trying to find:

Find the question IDs that are not a parent ID of another question and the parentid is null.

idQuestion    |    data     |    parentid
123               |    abc      |       NULL
321               |    def       |        123
444               |    qqq       |    NULL

Open in new window


The query should return just '444' as its parentid is null AND is not a parentid of another question.
0
 
LVL 5

Expert Comment

by:eridanix
ID: 36891882
select distinct idQuestion
from Question
where parentid is null and idQuestion NOT IN (select idQuestion from Question where parentid is not null)
order by idQuestion
0
 
LVL 142

Accepted Solution

by:
Guy Hengel [angelIII / a3] earned 500 total points
ID: 36891919
>parentid isn't indexed because it's not unique. T

indexes can be on non-unique fields.
please create such an index.
0
 
LVL 142

Expert Comment

by:Guy Hengel [angelIII / a3]
ID: 36891930
edit: you might not know the a primary key is a UNIQUE index, under the hood, but you can created non-unique indexes.
actually, a index is non-unique by default, unless you specify it to add the restriction that each value can only be present once.
0
Complete VMware vSphere® ESX(i) & Hyper-V Backup

Capture your entire system, including the host, with patented disk imaging integrated with VMware VADP / Microsoft VSS and RCT. RTOs is as low as 15 seconds with Acronis Active Restore™. You can enjoy unlimited P2V/V2V migrations from any source (even from a different hypervisor)

 

Author Comment

by:BCRobert
ID: 36891937
@eridanix: That will not return the results I need. I need to make sure the the idQuestion IS NOT the parentid of another question.
0
 

Author Comment

by:BCRobert
ID: 36891968
@angellll: The parentid is already an index, apparently (I didn't create the database/tables):

parentIdIdx      BTREE      No      No      parentId      41002      A      YES      
0
 
LVL 5

Expert Comment

by:eridanix
ID: 36892014
select idQuestion
from dbo.Questions
where idQuestion IN (select idQuestion from Question where parentid is null) AND idQuestion NOT IN  (select parentid from Question where parentid is not null)
0
 

Author Comment

by:BCRobert
ID: 36892103
@angellll: It seems that re-creating the index helped tremendously. Not sure why, but the query finished after about 45 seconds, which is still slow, but it's not a query we'll be running often.
0
 

Author Closing Comment

by:BCRobert
ID: 36892108
Creating/recreating the index helped the initial query finished. Converting it to LEFT JOIN/NOT IN isn't needed.
0
 
LVL 142

Expert Comment

by:Guy Hengel [angelIII / a3]
ID: 36892237
please try to remove the 2 DISTINCT in the query.

IN ( SELECT DISTINCT  ... ) will be the same, but slower normally, as
IN ( SELECT ... )

the NOT EXISTS () version I posted should be fastest ...
0

Featured Post

IT, Stop Being Called Into Every Meeting

Highfive is so simple that setting up every meeting room takes just minutes and every employee will be able to start or join a call from any room with ease. Never be called into a meeting just to get it started again. This is how video conferencing should work!

Join & Write a Comment

Foreword In the years since this article was written, numerous hacking attacks have targeted password-protected web sites.  The storage of client passwords has become a subject of much discussion, some of it useful and some of it misguided.  Of cou…
Popularity Can Be Measured Sometimes we deal with questions of popularity, and we need a way to collect opinions from our clients.  This article shows a simple teaching example of how we might elect a favorite color by letting our clients vote for …
This video gives you a great overview about bandwidth monitoring with SNMP and WMI with our network monitoring solution PRTG Network Monitor (https://www.paessler.com/prtg). If you're looking for how to monitor bandwidth using netflow or packet s…
This video demonstrates how to create an example email signature rule for a department in a company using CodeTwo Exchange Rules. The signature will be inserted beneath users' latest emails in conversations and will be displayed in users' Sent Items…

758 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

20 Experts available now in Live!

Get 1:1 Help Now