What s the most efficient way to check if a record exists in Oracle

Question

A    select decode count     0   N    Y   rec exists from  select  X        from dual       where exists  select  X                      from sales                     where sales type    Accessories       B   select decode count     0   N    Y   rec exists from  select  X        from sales       where sales type    Accessories       C  Something else  specify   EDIT  It was hard to pick the  correct  answer  as the best approach depends on what you want to do after checking if the value exists  as pointed out by APC  I ended up picking the answer by RedFilter  since I had originally envisioned this check as a function by itself

User · Answer

Simply get a count of the record s  you re looking for   If count   0 then record s  exist        DECLARE     rec count NUMBER    0       BEGIN      select count         into rec count       from EMPLOYEETABLE       WHERE employee id   inEMPLOYEE ID       AND department nbr   inDEPARTMENT NBR          if rec count   0 then           UPDATE EMPLOYEETABLE      else           INSERT INTO EMPLOYEETABLE        end if         END

User · Answer

The most efficient and safest way to determine if a row exists is by using a FOR-LOOP    You won t even have a difficult time if you are looking to insert a row or do something based on the row NOT being there but  this will certainly help you if you need to determine if a row exists   See example code below for the ins and outs     If you are only interested in knowing that 1 record exists in your potential multiple return set  than you can exit your loop after it hits it for the first time     The loop will not be entered into at all if no record exists   You will not get any complaints from Oracle or such if the row does not exist but you are bound to find out if it does regardless   Its what I use 90  of the time  of course dependent on my needs      EXAMPLE    DECLARE  v exist varchar2 20    BEGIN    FOR rec IN     SELECT LOT  COMPONENT     FROM TABLE    WHERE REF DES    SELECT REF DES FROM TABLE2 WHERE ORDER    1234       AND ORDER    1234     LOOP        v exist     IT EXISTS      INSERT INTO EAT SOME SOUP TABLE  LOT  COMPONENT      VALUES  rec LOT  rec COMPONENT         --Since I don t want to do this for more than one iteration  just in case there may have been more than one record returned  I will EXIT      EXIT     END LOOP     IF v exist  IS NULL      THEN              --do this  END IF   END    --This is outside the loop right here  The IF-CHECK just above will run regardless  but then you will know if your variable is null or not right     If there was NO records returned  it will skip the loop and just go here to the code you would have next     If  in our case above   4 records were returned  I would exit after the first iteration due to my EXIT      If that wasn t there  the 4 records would iterate through and do an insert on all of them   Or at least try too   By the way  I m not saying this is the only way you should consider doing this     You can   SELECT COUNT    INTO v counter WHERE         etc      Then check it like   if v counter  gt  0       THEN          --code goes here    END IF    There are more ways    Just determine it when your need arises   Keep performance in mind  and safety

User · Answer

select CASE  when exists  SELECT U USERID U USERNAME U PASSWORDHASH FROM  TBLUSERS U WHERE U USERID  U USERID  AND U PASSWORDHASH U PASSWORDHASH   then  OLD PASSWORD EXISTS   else  OLD PASSWORD NOT EXISTS   end as OUTPUT from DUAL

User · Answer

What is the underlying logic you want to implement   If  for instance  you want to test for the existence of a record to determine to insert or update then a better choice would be to use MERGE instead   If you expect the record to exist most of the time  this is probably the most efficient way of doing things  although the CASE WHEN EXISTS solution is likely to be just as efficient     begin     select null into dummy     from sales     where sales type    Accessories      and rownum   1       --  do things here when record exists                   exception     when no data found then         -- do things here when record doesn t exists               end    You only need the ROWNUM line if SALES TYPE is not unique   There s no point in doing a count when all you want to know is whether at least one record exists

User · Answer

select decode count     0   N    Y   rec exists        from sales        where sales type    Accessories

User · Answer

SELECT  Y  REC EXISTS              FROM SALES                        WHERE SALES TYPE    Accessories    The result will either be  Y  or NULL  Simply test against  Y

User · Answer

here you can check only y   n if we need to select a name as well that whether this name exists or not   select name   decode count name  0   N    Y     from table  group by name    Here when it is Y only then it will return output otherwise it will give null always  Whts ths way to get the records not existing with N like in output we will get Name   N  When name is not existing in table

User · Answer

select count 1  into existence     from sales where sales type    Accessories  and rownum 1    Oracle plan says that it costs 1 if seles type column is indexed

User · Answer

select case              when exists  select 1                           from sales                           where sales type    Accessories                then  Y               else  N           end as rec exists from dual

User · Answer

select NVL   select  Y  from  dual where exists     select  1 from sales where sales type    Accessories     N   as rec exists from dual   1 Dual table will return  Y  if record exists in sales type table 2 Dual table will return null if no record exists in sales type table and NVL will convert that to  N

[sql] What's the most efficient way to check if a record exists in Oracle?

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