Comments on: SQL SERVER – Introduction and Example of UNION and UNION ALL

By: Aparna

Aparna — Tue, 02 Jun 2009 07:58:46 +0000

I was short of time with lots of work in hand so did not really look at the execution plan. I will post my findings in this forum. Thanks Imran for your suggestion. wondering if anyone had similar issues.

FYI Table 1 is not distint by itself and Table 2 is not distinct by itself but there are dateid’s in Table1 and Table2 that makes their union distinct

TABLE1
DATEID INVESTOR
20090530 1
20090530 2
20090530 1

TABLE 2

DATEID INVESTOR
20090630 1
20090630 2
20090630 1

Any one has thoughts until I look at the execution plan

By: Imran Mohammed

Imran Mohammed — Tue, 02 Jun 2009 04:59:59 +0000

@Aparna,

Why don’t you look at Execution plan. Look at cost of each step and each query. You will get an answer.

Also, please post your observation.

~ IM.

By: Aparna

Aparna — Tue, 02 Jun 2009 02:28:42 +0000

I was looking at performance gaining and a friend of mine doing a peer review of my code suggested that we use union all instead of union, I was not sure how I will gain performance and while researching came across your article.

I do need a distinct list of records and therefore used union
but she has an opinion that I do a union all and then group by the columns individually before applying union all.

Which is best?
get the distinct list of table 1 (by using group by clause) and distinct list of table2 and then do union all

or select list from table 1 and select list from table 2 and do union there by eliminating duplicates in individual tables

By: Ria

Ria — Fri, 29 May 2009 11:11:55 +0000

Is it possible to use ‘union all’ in hibernate, Since I am using Java and Hibernate for my application, I need to convert the query using ‘union all’ to hibernate.Please Help me to solve this issue.

By: chris

chris — Tue, 31 Mar 2009 06:00:46 +0000

sorry for the dumb question, i should have included tcol3 in my inner query…

By: chris

chris — Tue, 31 Mar 2009 05:55:17 +0000

is ther a way to use a union all statement and then issue a where at the end to filter the entire query?

example:

table 1
tcol1 tcol2 tcol3
1 1 1
2 2 1

table 2
tcol1 tcol2 tcol3
3 3 1
4 4 2

SELECT tcol1,tcol2 FROM (
SELECT tcol1,tcol2
UNION ALL
SELECT tcol1,tcol2
) AS temptable
WHERE tcol3=’1′ <—

thanks

By: Siuli Basu

Siuli Basu — Wed, 31 Dec 2008 08:43:40 +0000

I want to know the difference between UNION and INSERT INTO

By: Vani K Patil

Vani K Patil — Tue, 11 Nov 2008 09:34:05 +0000

First time i am going through this site.It’s simply the best.
U have all the details without redundant details. Thanks tons for this.

By: Joggee

Joggee — Wed, 15 Oct 2008 03:32:05 +0000

Its a hobby to read your blog’s article on daily basis to refresh my knowledge.

Some tricky part which mostly ignore, Catch it from your articles.

Keep up the good work.

God Bless you

Joggee.

By: Imran Mohammed

Imran Mohammed — Wed, 15 Oct 2008 02:04:23 +0000

Hello,

I wish I had read this article one day before.

Yesterday when I was working on one production issue, I saw a strange case, in which we create two temporary table on the fly, and then we do a select statement with a union. Every time we use to get an error… You wont believe we spent 2-3 hours on it. At last we saw the temporary tables that were created with select * into command was creating different datatype, one with int and another with char, and union was not able to perform its action and our process was failing. I learnt a good lesson, it was worth it :)

Thanks,
Imran.