Solved

I need assistance using UNION in my SQL Query

Posted on 2013-11-18
4
3,314 Views
Last Modified: 2013-12-11
Hi Experts,
I am writing a query using UNION to join two tables in SQL Server 2012.  One table has 37,550 records, and the other table has 940,400 records, for a combined total of 977,950.  When I run my query using UNION, the result set only returns 885,067.  Why am I getting less records?

my query syntax:
SELECT [Column 0] AS SSN, [Column 1] AS fee,
[Column 2] AS DATE, 
[Column 4] AS OrderNo
FROM TABLE1
UNION
SELECT [Column 3] AS SSN, [Column 4] AS fee,
[Column 0] AS DATE, 
[Column 2] AS OrderNo
FROM TABLE2
ORDER BY OrderNo

Open in new window


Thanks in advance for your help,
mrotor..
0
Comment
Question by:mainrotor
4 Comments
 
LVL 24

Accepted Solution

by:
chaau earned 500 total points
ID: 39658324
You need to use UNION ALL. UNION on its own omit the duplicate entries. So, the correct query will be:
SELECT [Column 0] AS SSN, [Column 1] AS fee,
[Column 2] AS DATE, 
[Column 4] AS OrderNo
FROM TABLE1
UNION ALL
SELECT [Column 3] AS SSN, [Column 4] AS fee,
[Column 0] AS DATE, 
[Column 2] AS OrderNo
FROM TABLE2
ORDER BY OrderNo

Open in new window

0
 
LVL 65

Expert Comment

by:Jim Horn
ID: 39658337
UNION ALL should be used when you're absolutely certain there will be no duplicates, and is just a set A + set B as a single return set.
SELECT name FROM whorehouses UNION ALL SELECT name FROM burgerjoints

Open in new window

UNION eliminates duplicates (as chaau stated above), and there is an extra cost to that as the query processor has to match up and remove rows from the final set, therefore should be avoided if possible.
SELECT name from customers UNION SELECT name FROM vendors

Open in new window

0
 
LVL 69

Expert Comment

by:Scott Pletcher
ID: 39660050
>> UNION ALL should be used when you're absolutely certain there will be no duplicates <<

Not true.  There's no problem at all with returning duplicate rows using UNION ALL.  If you don't care about and/or want duplicates in the result set, use UNION ALL:

SELECT 'A' AS col1 UNION ALL SELECT 'A' UNION ALL SELECT 'A' UNION ALL SELECT 'A' UNION ALL SELECT 'A' --...


Otoh, UNION should be used only when you explicitly want/need to prevent duplicate rows from being returned by the query, because of the extra overhead to SQL of removing any potential duplicate rows.
0
 
LVL 32

Expert Comment

by:awking00
ID: 39660109
Since the difference between the total rows and the result set is less than all of the rows in the other table, there are obviously duplicate values for columns 0, 1, 2, and 4 in the first table by itself as well.
0

Featured Post

Windows Server 2016: All you need to know

Learn about Hyper-V features that increase functionality and usability of Microsoft Windows Server 2016. Also, throughout this eBook, you’ll find some basic PowerShell examples that will help you leverage the scripts in your environments!

Question has a verified solution.

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

Suggested Solutions

JSON is being used more and more, besides XML, and you surely wanted to parse the data out into SQL instead of doing it in some Javascript. The below function in SQL Server can do the job for you, returning a quick table with the parsed data.
International Data Corporation (IDC) prognosticates that before the current the year gets over disbursing on IT framework products to be sent in cloud environs will be $37.1B.
This video shows how to set up a shell script to accept a positional parameter when called, pass that to a SQL script, accept the output from the statement back and then manipulate it in the Shell.
Via a live example, show how to shrink a transaction log file down to a reasonable size.

863 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