Solved

SQL select query

Posted on 2011-09-09
8
267 Views
Last Modified: 2012-05-12
I have the following very simply query
Select PropertyId, contractorid, max(InspectionDate)
From InspectionReport
group by PropertyId

This gives me a simple list.  But when I try to add additional columns in the select statement I get an error about grouping.  This is a simple table with some properties, with some dates, and some other columns.  You will notice the max(inspectionDate).  Each property has a bunch of date related events recorded and this query gets the record with the most recent event.  But why can't I get the rest, or some more of the columns to show?  I am sure this is simple. I am just learning SQL queries.  Thanks.  Hope its not too dump a question.
0
Comment
Question by:hconant
  • 5
  • 3
8 Comments
 
LVL 59

Expert Comment

by:Kevin Cross
ID: 36514356
MySQL often doesn't complain about that as evidenced by the contractorid not being in the aggregate; however, I would advise you to put all the non-aggregated columns into the group by or into an aggregate unless you can find a way to make the data unique.

For example, if you are trying to bring back data that corresponds to the MAX(InspectionDate), then this is an alternative.
SELECT PropertyId, contractorid, InspectionDate
FROM InspectionReport ir
JOIN (
   SELECT PropertyId
        , MAX(InspectionDate) AS MAX_InspectionDate
   FROM InspectionReport
   GROUP BY PropertyId
) ir_max ON ir_max.propertyid = ir.propertyid
   AND ir_max.MAX_InspectionDate = ir.InspectionDate
;

Open in new window

0
 

Author Comment

by:hconant
ID: 36514374
Why would I need a JOIN?  Its all in one table?
0
 
LVL 59

Accepted Solution

by:
Kevin Cross earned 500 total points
ID: 36514385
Look at what the JOIN is doing. You can do this in a WHERE clause. Just my personal preference to use a derived table. Typically works with the caveat that your dates are unique. If they are not, then you would get duplicate rows matching MAX(). If that is the case, then you can do this using ranking.

I wrote an Article on that here:
http://www.experts-exchange.com/Programming/Languages/SQL_Syntax/A_1555-Analytical-SQL-Where-do-you-rank.html

You would rank the data by the InspectionDate descending and then select the top 1 row for each. You will have to use some secondary column(s) in the ranking to determine how to resolve conflicts or collisions in max date.
0
Free learning courses: Active Directory Deep Dive

Get a firm grasp on your IT environment when you learn Active Directory best practices with Veeam! Watch all, or choose any amount, of this three-part webinar series to improve your skills. From the basics to virtualization and backup, we got you covered.

 

Author Comment

by:hconant
ID: 36522330
I have one smalll problem with your query.  In the first SELECT, PropertyId is showing an ambiguous column name.  It is fine in the lines below using same name.  The result set does not have the PropertyId showing, only contractorId and Inspection date.  I can list the PropertyId in a simple query.
SELECT PropertyId From InspectionReport.  I've trible checked for typos and even picked the column name from the list.
??
0
 
LVL 59

Expert Comment

by:Kevin Cross
ID: 36522541
Forgot the aliases.
SELECT ir.PropertyId, ir.contractorid, ir.InspectionDate
FROM InspectionReport ir
JOIN (
   SELECT PropertyId
        , MAX(InspectionDate) AS MAX_InspectionDate
   FROM InspectionReport
   GROUP BY PropertyId
) ir_max ON ir_max.propertyid = ir.propertyid
   AND ir_max.MAX_InspectionDate = ir.InspectionDate
;

Open in new window

0
 
LVL 59

Expert Comment

by:Kevin Cross
ID: 36522554
Another way to fix it is to name the PropertyId in the derived table something different.
SELECT PropertyId, contractorid, InspectionDate
FROM InspectionReport ir
JOIN (
   SELECT PropertyId AS ID
        , MAX(InspectionDate) AS MAX_InspectionDate
   FROM InspectionReport
   GROUP BY PropertyId
) ir_max ON ir_max.id = ir.propertyid
   AND ir_max.MAX_InspectionDate = ir.InspectionDate
;

Open in new window

0
 

Author Comment

by:hconant
ID: 36522569
OK. That worked.  Thanks again.
0
 
LVL 59

Expert Comment

by:Kevin Cross
ID: 36522581
You are most welcome!
Best regards and happy coding,
Kevin
0

Featured Post

Free Tool: SSL Checker

Scans your site and returns information about your SSL implementation and certificate. Helpful for debugging and validating your SSL configuration.

One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Foreword This is an old article.  Instead of using the MySQL extension that was used in the original code examples, please choose one of the currently supported database extensions instead.  More information is available here: MySQLi / PDO (http://…
Load balancing is the method of dividing the total amount of work performed by one computer between two or more computers. Its aim is to get more work done in the same amount of time, ensuring that all the users get served faster.
A short tutorial showing how to set up an email signature in Outlook on the Web (previously known as OWA). For free email signatures designs, visit https://www.mail-signatures.com/articles/signature-templates/?sts=6651 If you want to manage em…

829 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