Combining union and union all

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Combining union and union all

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OzYbOi d(-_-)b SSCarpal Tunnel Points: 4054 More actions · Answer 1

thanks for the question and the input from everyone today.

Ken SSCoach Points: 17191 More actions · Answer 2

Good question and discussion. I always enjoy order of precedence questions.

Keld Laursen (SEGES) SSC Eights! Points: 843 More actions · Answer 3

Thank you for the question.

I deduced the result using operator precedence and parsing the input by hand (or head?).

Typed it into my local, friendly SSMS, and got the same result.

Although I hadn't really thought about it, the "operator precedence" combined with the "test data might thwart you" nature of this question really made the discussion pay off.

Keep your databases clean.

lightyun SSC Rookie Points: 39 More actions · Answer 4

Really a nice question, make me learn more about this basic knowledge.

However, in real case, we really seldom use "Union" and "Union all" together just like the questions. always use any one of it to make the script consistence

anyway, now I am much more clearer about these two commands and the sequence effect

TomThomson SSC Guru Points: 104773 More actions · Answer 5

Nice question, and the explanation has provoked an interesting discussion.

Now there's maybe a problem with the explanation adopted as the result of that discussion: is it anywhere documented that UNION strings associate left to right unless overruled by parantheses, or could the optimiser decide (for whatever reason) to go right to left, or some arbitrary order? I guess this doesn't happen in existing versions (but haven't done extensive testing to verify that, it is purely a guess), but is that part of the T-SQL definition or something that a future quick fix could change? If it could change, then the right answer would presumably be "it depends what the optimiser chooses to do".

Tom

R.P.Rozema SSChampion Points: 12307 More actions · Answer 6

L' Eomot Inversé (3/9/2012)
Nice question, and the explanation has provoked an interesting discussion.
Now there's maybe a problem with the explanation adopted as the result of that discussion: is it anywhere documented that UNION strings associate left to right unless overruled by parantheses, or could the optimiser decide (for whatever reason) to go right to left, or some arbitrary order? I guess this doesn't happen in existing versions (but haven't done extensive testing to verify that, it is purely a guess), but is that part of the T-SQL definition or something that a future quick fix could change? If it could change, then the right answer would presumably be "it depends what the optimiser chooses to do".

I have not yet found any official docu stating that it is always evaluated left-to-right. So, I think you are right: "it may depend". BUT, if in a future version of SQL server this changes, then I am going to ask MS to re-evaluate a lot of change requests and bug reports on Connect that have been rejected because "the change could break existing applications" :cool:. Waiting for the day! 😀

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Peter Trast SSCarpal Tunnel Points: 4392 More actions · Answer 7

Koen Verbeeck (3/8/2012)
UNION queries are evaluated from left to right. If the last query contains duplicates and is preceded with UNION ALL, you will have duplicates in your result set.

Well put. Great question!

Peter Trast
Microsoft Certified ...(insert many literal strings here)
Microsoft Design Architect with Alexander Open Systems

ldorian81 SSCommitted Points: 1694 More actions · Answer 8

FYI. Doesn't work on SQL 2005

Error showing on line "Insert Into #t1(col) Values (1),(1);"

Msg 102, Level 15, State 1, Line 5

Incorrect syntax near ','.

I believe this function is not available until 2008!

Regards
ld

Stoke-on-Trent
United Kingdom

If at first you don't succeed, go to the pub and drink away your current thought plan.

R.P.Rozema SSChampion Points: 12307 More actions · Answer 9

Yes, you're right. Forgot about that. It's just for setting up the sample data, the union will still behave the same given the same data. i.e. if you insert the data using insert() select-statements you'll still be able to run it and get the same results.

As I said, it was tested on SQL 2008R2 only.

Posting Data Etiquette - Jeff Moden[/url]
Posting Performance Based Questions - Gail Shaw[/url]
Hidden RBAR - Jeff Moden[/url]
Cross Tabs and Pivots - Jeff Moden[/url]
Catch-all queries - Gail Shaw[/url]

If you don't have time to do it right, when will you have time to do it over?

Britt Cluff SSCertifiable Points: 5083 More actions · Answer 10

Good question. Thanks for submitting.

http://brittcluff.blogspot.com/

yannick.dinger SSC Enthusiast Points: 196 More actions · Answer 11

got it right and learn a behaviour great love it 🙂

tibrize Old Hand Points: 347 More actions · Answer 12

Answer of the question is wrong !!

select @@VERSION

create table #t1 (col int not null)

create table #t2 (col int not null)

create table #t3 (col int not null)

insert #t1 values (1), (1)

insert #t2 values (2)

insert #t3 values (3)

select COL from #t1

UNION

select COL from #t2

union all

select COL from #t3

select COL from #t1

UNION ALL

select COL from #t2

union all

select COL from #t3

Result:

Microsoft SQL Server 2008 R2 (RTM) - 10.50.1617.0 (X64) Apr 22 2011 19:23:43 Copyright (c) Microsoft Corporation Data Center Edition (64-bit) on Windows NT 6.1 <X64> (Build 7600: )

COL

1

2

3

COL

1

2

3

Sean Lange SSC Guru Points: 286573 More actions · Answer 13

tibrize (7/20/2012)
Answer of the question is wrong !!

Nope, you typed it wrong. 😛 The very last query is a UNION and you have UNION ALL

select @@VERSION

create table #t1 (col int not null)

create table #t2 (col int not null)

create table #t3 (col int not null)

insert #t1 values (1), (1)

insert #t2 values (2)

insert #t3 values (3)

select COL from #t1

UNION

select COL from #t2

union all

select COL from #t3

select COL from #t1

UNION ALL

select COL from #t2

union all

select COL from #t3

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BudaCli Hall of Fame Points: 3384 More actions · Answer 14

not confused @ all

What you don't know won't hurt you but what you know will make you plan to know better

demonfox SSCertifiable Points: 6289 More actions · Answer 15

great question, although I got it right, but I am glad i read the posts .Otherwise , I may have ended with wrong idea.

~ demonfox
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Wondering what I would do next , when I am done with this one :ermm: