Returning a default value in SQL when no rows found

cubmar

Hi

How can change my SQL to return a default value if no rows are found?

Eg:
select text from text_file where criteria=met

If no rows are found, I would like text to contain '**Empty' for example


Thanks

105967

You have to code it yourself. Check if there is no row returned and if yes set the variable to your desired value.
(just think about it, if there is NO ROW, how can a column be filled ie there is no column to be filled)

MichaelS

e.g.

SQL> var met number
SQL>
SQL> exec :met := 1

PL/SQL procedure successfully completed.

SQL>
SQL> with text_file as
  2  (
  3    select 1 criteria, 'some_text' text from dual
  4  )
  5  select nvl(text,'***empty') text from text_file t1, (select :met met from dual) where t1.criteria(+) = met
  6  /

TEXT
---------
some_text

SQL>
SQL> exec :met := 2

PL/SQL procedure successfully completed.

SQL>
SQL> /

TEXT
---------
***empty

cubmar

Well, I would perhaps do something like this

select text from text_file where criteria=met
UNION
select '** No data' text from dual

But of course only when no data is returned.

cubmar

Soemthing like this works, I guess:

  1  with text_file as

  2  (select 'This' text, 'met' criteria from dual)

  3  select nvl(text,dfttext) text from

  4  (select 1 lnk, text from text_file where criteria='met') qry

  5  ,

  6  (select 1 lnk, '** No data' dfttext from dual) dft

  7* where qry.lnk(+)=dft.lnk

  8  /


TEXT

----------

This

  1  with text_file as

  2  (select 'This' text, 'notmet' criteria from dual)

  3  select nvl(text,dfttext) text from

  4  (select 1 lnk, text from text_file where criteria='met') qry

  5  ,

  6  (select 1 lnk, '** No data' dfttext from dual) dft

  7* where qry.lnk(+)=dft.lnk

SQL> /


TEXT

----------

** No data

It might not look too pretty!

Keith Jamieson

As leo said, you have to code it.

if sql%rowcount = 0
<perform action>
end if;

or a local pl/sql block.

begin
<select statement >
exception
when no_data_found
then
perform action>
end;

cubmar

Keith,

There are two (very similar) examples of how to do it without the need for PL/SQL

Thanks

Keith Jamieson

Try this and see what you get:

select nvl(dummy,'A NULL') from dual where dummy='Y'

The other solutions "Cheat" in that they actually force a row to be found.

But maybe if you posted the problem you're trying to solve, ie the question behind your question we could help further.

105967

... and where you easily can end up with a full table scan, ie a disastrous performance.

BluShadow

And just for fun without using NVL or outer joins...

SQL> ed
Wrote file afiedt.buf

  1  with t as (select 'fred' as nm from dual union all
  2             select 'bob' from dual union all
  3             select 'jim' from dual)
  4  -- end of test data
  5  select nm from (
  6    select rownum rn, nm from t where nm like '&name%'
  7    union
  8    select 0, '~~ NO DATA ~~' from dual
  9    order by 1 desc
 10    )
 11* where (rn > 0 or (rownum = 1 and rn = 0))
SQL> /
Enter value for name: will
old   6:   select rownum rn, nm from t where nm like '&name%'
new   6:   select rownum rn, nm from t where nm like 'will%'

NM
-------------
~~ NO DATA ~~

SQL> /
Enter value for name: fred
old   6:   select rownum rn, nm from t where nm like '&name%'
new   6:   select rownum rn, nm from t where nm like 'fred%'

NM
-------------
fred

SQL>

MichaelS

... and where you easily can end up with a full table scan, ie a disastrous performance

Why should it be so? Of course a proper Index layout is assumed:

SQL>  create table text_file as select object_id criteria, object_name text from all_objects
/
Table created.

SQL>  create unique index text_file_idx on text_file (criteria)
/
Index created.

SQL>  set autotrace on explain

SQL>  select nvl(text,'***empty') text from text_file t1, (select 15111 met from dual) where t1.criteria(+) = met
/
TEXT                          
------------------------------
/df245789_JavaToSQLMapMapClass
1 row selected.

Execution Plan
----------------------------------------------------------
           SELECT STATEMENT Optimizer Mode=ALL_ROWS (Cost=3 Card=1 Bytes=30)
   1         NESTED LOOPS OUTER (Cost=3 Card=1 Bytes=30)
   2    1      FAST DUAL (Cost=2 Card=1)
   3    1      TABLE ACCESS BY INDEX ROWID MICHAEL.TEXT_FILE (Cost=1 Card=1 Bytes=30)
   4    3        INDEX UNIQUE SCAN MICHAEL.TEXT_FILE_IDX (Cost=0 Card=1)


SQL>  select nvl(text,'***empty') text from text_file t1, (select 106276 met from dual) where t1.criteria(+) = met
/
TEXT                          
------------------------------
***empty                      
1 row selected.


Execution Plan
----------------------------------------------------------
           SELECT STATEMENT Optimizer Mode=ALL_ROWS (Cost=3 Card=1 Bytes=30)
   1         NESTED LOOPS OUTER (Cost=3 Card=1 Bytes=30)
   2    1      FAST DUAL (Cost=2 Card=1)
   3    1      TABLE ACCESS BY INDEX ROWID MICHAEL.TEXT_FILE (Cost=1 Card=1 Bytes=30)
   4    3        INDEX UNIQUE SCAN MICHAEL.TEXT_FILE_IDX (Cost=0 Card=1)

SQL>  drop table text_file
/
Table dropped.