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From:Michael Widenius Date:January 10 2000 8:37pm
Subject:Where can user variables be used?
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>>>>> "Paul" == Paul DuBois <paul@stripped> writes:

Paul> At 9:53 AM +1100 2000-01-10, Albert Ng wrote:
>> Thanks Paul.
>> Well I was hoping that I can use variables to
>> find the size of a table so that I can display only
>> the last few rows of the table by using limit since
>> I can't do a limit -10 to signify I want it to go from
>> the bottom of the records. And without sub-query,
>> looks like my only choice is to do 2 sql's  and
>> use a local variable in my program. sigh....oh,
>> I can't upgrade to 3.23 for various reasons.

Paul> Well, I just tried doing what you want to do under 3.23.8,
Paul> and it doesn't appear to be possible anyway.  I tried
Paul> setting a variable to the number of rows in a table like this:

Paul> SELECT @a:=COUNT(*) FROM tbl_name;

Paul> That worked.  Then I tried getting the last few rows like this:

Paul> SELECT * FROM tbl_name LIMIT @a-10, 10;

Paul> That didn't work (syntax error at '@a').

Paul> Then I tried setting @a directly with a SET @a:=num statement,
Paul> followed by:

Paul> SELECT * FROM tbl_name LIMIT @a, 10;

Paul> Still get a syntax error at '@a'.

Paul> So my question is: in what contexts can you use user variables.
Paul> In a SET statement, clearly.  Also in the selection list for a
Paul> SELECT statement.  And I've been able to use them in the WHERE
Paul> clause as well.  But what are the actual rules governing where you
Paul> can use them?


You can use variables in all cases where mysqld doesn't currently
force you to use an integer (like in LIMIT).

I have added to the TODO to allow variables with LIMIT.


user variablesAlbert Ng7 Jan
  • Re: user variablesPaul DuBois7 Jan
  • Re: user variables -> calculating limitAlbert Ng10 Jan
    • Re: user variables -> calculating limitBenjamin Pflugmann10 Jan
    • Re: user variables -> calculating limitPaul DuBois10 Jan
    • Where can user variables be used?Paul DuBois10 Jan
      • Where can user variables be used?Michael Widenius10 Jan
  • Re: Where can user variables be used?Albert Ng10 Jan
  • Re: Where can user variables be used?Sasha Pachev10 Jan