PL/SQL check if query returns empty

Question

I'm writing a procedure, and i need to check whether my select query returned an empty record or not. (In this example whether there is no x,y shelf)

How can i do that?

I tried this:

temp shelves.loadability%TYPE := NULL;
BEGIN

select loadability into temp from shelves where rownumber = x and columnnumber = y;
IF temp IS NOT NULL THEN
/* do something when it's not empty */
ELSE
/* do the other thing when it's empty */
END IF;

But the second branch of the if never works...

EDIT:

Oh, it was so easy...

temp shelves.loadability%TYPE;
BEGIN

select count(*) into temp from shelves where rownumber = x and columnnumber = y;
IF temp != 0 THEN
/* do something when it's not empty */
ELSE
/* do the other thing when it's empty */
END IF;

END;

select count(*) is nor a very good way to check for rows. See my answer below for a more optimal approach — Rene
– Rene, Commented May 11, 2011 at 9:42

Rene · Accepted Answer · 2011-05-11 09:41:00Z

21

Use an exception handler

Begin
  select column
  into variable
  from table
  where ...;

  -- Do something with your variable

exception
 when no_data_found then
    -- Your query returned no rows --

 when too_many_rows
    -- Your query returned more than 1 row --

end;

answered May 11, 2011 at 9:41

Rene

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8 Comments

Yehuda Shapira Over a year ago

Is that good practice? You're using an exception like a condition.

alisianoi Over a year ago

Not, it is not good practice, see the "Tip" in the Error Trapping section: postgresql.org/docs/current/static/…

Rene Over a year ago

It's called exception handling. It's the way plsql works.

Romeo Sierra Over a year ago

I don't think that this is a good approach. Because, by definition exceptional condition is a case that the program is unaware of how to handle. At such a case, the safest thing to do is gracefully terminate the process/function/procedure. Implementing business rules inside the graceful shutdown routine wouldn't be a really good idea, in terms of good programming practices!

Rene Over a year ago

When you execute a select ... into in PLSQL you must expect 0, 1, or more results. And you need to handle these cases. Of course you can do a select count(*) first, but there is no 100% garantee that the data hasn't changed when you execute the select into, a moment later. Handling the no_data_found and too_many_rows exceptions is a very standard way of working with select .. into queries in PLSQL.

|

schwarz · Accepted Answer · 2013-09-25 15:25:34Z

7

Exception handling would be the first thing I think of too, but if you don't want to burden yourself with handling all the different cases, I tend to use a select count(*) from. The nice thing with count(*) is that it ALWAYS returns something (assuming your query is legal). In this case you could count to see if it returns 0 (no matches) or more (in which case you can do something.

You could get something like this:

declare
  v_count number := 0;
begin
  select count(*) into v_count from table where condition;

  if v_count = 0 then
      --do something
  else
      --do something else
  end if;
end;

answered Sep 25, 2013 at 15:25

schwarz

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Comments

MJB · Accepted Answer · 2011-05-10 19:04:14Z

3

It is generally more SQL-like to just do the work for the records that exist. In other words, you could perform your task for each occurrence of the match, and if there are no occurrences you don't do it. So you would not even need the IF-ELSE construct.

I won't recommend using a cursor to do the work, because that would be counter to my first suggestion that you do it more SQL-like. But if you have to do it this way, then a cursor might do what you want.

And yes, I realize that does not answer your question directly.

answered May 10, 2011 at 19:04

MJB

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3 Comments

WonderCsabo Over a year ago

But i have to do a task, if there is no such a shelf... I have to insert a tuple with the new shelf. Sorry if it wasn't clear.

WonderCsabo Over a year ago

So, any tips how can i do that?

MJB Over a year ago

@WonderCsabo: Sorry, I didn't get back to the question in time. Glad you resolved it.

Strauteka · Accepted Answer · 2014-01-08 09:59:35Z

catch first not wanted condition and use count(1) because count(*) actually trying to count something and add rownum=1, you need only one first not matching condition. i use this statement.

       declare
      v_check number;
    begin
      select count(1) into v_check 
from table
 where condition(something not wanted) AND rownum=1;

      if v_check = 0 then 
           --do something else
      elsif v_check = 1 --dont want theat
         rise some error or more..
      end if;
    end;

For you just

select count(1) into v_check from dual where exists (select count(1) 
    from table
     where condition AND rownum=1);

if v_check = 0 then --nothing found
                 something...
          elsif v_check = 1 --found something
            something...
          end if;
        end;

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