Beefy Boxes and Bandwidth Generously Provided by pair Networks
Clear questions and runnable code
get the best and fastest answer
 
PerlMonks  

Re^2: T-SQL problem - Perl solution?

by Win (Novice)
on Oct 25, 2004 at 13:52 UTC ( [id://402215]=note: print w/replies, xml ) Need Help??


in reply to Re: T-SQL problem - Perl solution?
in thread T-SQL problem - Perl solution?

Should the following work?
WHILE (exists(select 1 from INFORMATION_SCHEMA.tables where table_name + = 'Experimental_parameters') ...

Replies are listed 'Best First'.
Re^3: T-SQL problem - Perl solution?
by monktim (Friar) on Oct 25, 2004 at 14:17 UTC
    That should work Win. What's nice about using information_schema is that it's ANSI standard so it will work on compliant dBs. Your question pertains directly to T-SQL but I like to code generically when there isn't a performance hit. When I work on other dBs I have to remember less vendor specific stuff. You don't need the SELECT 1 here. SELECT * with an EXISTS can yield internal performance optimizations with MS SQL Server. In this case it's probably trival but I thought I'd mention it.

      In this case SQL 2000 produces the exact same execution plan for "SELECT 1" and "SELECT *" -

      D:\>copy con: select1.sql set showplan_text on go if exists (select 1 from sysobjects where xtype = 'U' and name = 'jobs +') print 'exists' else print 'doesn''t exist' ^Z 1 file(s) copied. D:\>copy con: selectstar.sql set showplan_text on go if exists (select * from sysobjects where xtype = 'U' and name = 'jobs +') print 'exists' else print 'doesn''t exist' ^Z 1 file(s) copied. D:\>osql -dpubs -n -i selectstar.sql > selectstar.txt D:\>osql -dpubs -n -i select1.sql > select1.txt D:\>diff select1.txt selectstar.txt 3c3 < if exists (select 1 from sysobjects where xtype = 'U' and name = 'j +obs') --- > if exists (select * from sysobjects where xtype = 'U' and name = 'j +obs')

      And while Win's SQL does compile without error, it may be a disaster in production. For example, this code (including the code you sanction) starts an infinite loop -

      use pubs go create table Experimental_parameters (c char (1) primary key) insert into Experimental_parameters (c) values ('a') go create table another_table (c char (1) references Experimental_paramet +ers (c)) insert into another_table (c) values ('a') go WHILE (exists(select 1 from INFORMATION_SCHEMA.tables where table_name + = 'Experimental_parameters')) begin drop table Experimental_parameters -- fails end go print 'finished' -- never gets here

       

        Good point on the execution plans EdwardG. This example may be trivial enough that SELECT * is not better than SELECT 1. I should have stated more clearly that it is good practice to use SELECT * in an EXISTS because it can give performance improvements. Your mileage may vary.

        There is no inherent problem with using the WHILE clause just as I posted it. I certainly didn't sanction the code you posted but it is a good example and well worth noting. You never want to get stuck in an infinite loop. Error checking should also always be done.

        DROP TABLE Experimental_parameters IF @@ERROR <> 0 BEGIN --DO SOMETHING HERE (i.e. RAISERROR & RETURN) END
        Update: I just read the OP. I was replying to Re^2: T-SQL problem - Perl solution? which didn't indicate a temp table or code inside the WHILE. My sanctioned code could yield the behavior EdwardG posted based on the OP. The OP had
        if (SELECT COUNT(*) FROM #Experimental_parameters) <> 0 begin DROP TAB +LE Experimental_parameters end
        Error hanling code needs to be used inside the WHILE.

        Good call EdwardG. Thanks
Re^3: T-SQL problem - Perl solution?
by EdwardG (Vicar) on Oct 25, 2004 at 13:59 UTC

    That's a hard question to give a meaningful answer to; it could work in a limited set of circumstances, but I don't know your intention, or your schema for that matter. For instance, is Experimental_parameters a temporary table?

     

Log In?
Username:
Password:

What's my password?
Create A New User
Domain Nodelet?
Node Status?
node history
Node Type: note [id://402215]
help
Chatterbox?
and the web crawler heard nothing...

How do I use this?Last hourOther CB clients
Other Users?
Others contemplating the Monastery: (6)
As of 2024-03-28 22:30 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?

    No recent polls found