Proper way of checking if row exists in table in PL/SQL block - sql

Question

I was writing some tasks yesterday and it struck me that I don't really know THE PROPER and ACCEPTED way of checking if row exists in table when I'm using PL/SQL.
For examples sake let's use table:
PERSON (ID, Name);
Obviously I can't do (unless there's some secret method) something like:
BEGIN
IF EXISTS SELECT id FROM person WHERE ID = 10;
-- do things when exists
ELSE
-- do things when doesn't exist
END IF;
END;
So my standard way of solving it was:
DECLARE
tmp NUMBER;
BEGIN
SELECT id INTO tmp FROM person WHERE id = 10;
--do things when record exists
EXCEPTION
WHEN no_data_found THEN
--do things when record doesn't exist
END;
However I don't know if it's accepted way of doing it, or if there's any better way of checking, I would really apprieciate if someone could share their wisdom with me.

Answer 1

I wouldn't push regular code into an exception block. Just check whether any rows exist that meet your condition, and proceed from there:
declare
any_rows_found number;
begin
select count(*)
into any_rows_found
from my_table
where rownum = 1 and
... other conditions ...
if any_rows_found = 1 then
...
else
...
end if;

Answer 2

IMO code with a stand-alone SELECT used to check to see if a row exists in a table is not taking proper advantage of the database. In your example you've got a hard-coded ID value but that's not how apps work in "the real world" (at least not in my world - yours may be different :-). In a typical app you're going to use a cursor to find data - so let's say you've got an app that's looking at invoice data, and needs to know if the customer exists. The main body of the app might be something like
FOR aRow IN (SELECT * FROM INVOICES WHERE DUE_DATE < TRUNC(SYSDATE)-60)
LOOP
-- do something here
END LOOP;
and in the -- do something here you want to find if the customer exists, and if not print an error message.
One way to do this would be to put in some kind of singleton SELECT, as in
-- Check to see if the customer exists in PERSON
BEGIN
SELECT 'TRUE'
INTO strCustomer_exists
FROM PERSON
WHERE PERSON_ID = aRow.CUSTOMER_ID;
EXCEPTION
WHEN NO_DATA_FOUND THEN
strCustomer_exists := 'FALSE';
END;
IF strCustomer_exists = 'FALSE' THEN
DBMS_OUTPUT.PUT_LINE('Customer does not exist!');
END IF;
but IMO this is relatively slow and error-prone. IMO a Better Way (tm) to do this is to incorporate it in the main cursor:
FOR aRow IN (SELECT i.*, p.ID AS PERSON_ID
FROM INVOICES i
LEFT OUTER JOIN PERSON p
ON (p.ID = i.CUSTOMER_PERSON_ID)
WHERE DUE_DATA < TRUNC(SYSDATE)-60)
LOOP
-- Check to see if the customer exists in PERSON
IF aRow.PERSON_ID IS NULL THEN
DBMS_OUTPUT.PUT_LINE('Customer does not exist!');
END IF;
END LOOP;
This code counts on PERSON.ID being declared as the PRIMARY KEY on PERSON (or at least as being NOT NULL); the logic is that if the PERSON table is outer-joined to the query, and the PERSON_ID comes up as NULL, it means no row was found in PERSON for the given CUSTOMER_ID because PERSON.ID must have a value (i.e. is at least NOT NULL).
Share and enjoy.

Answer 3

Many ways to skin this cat. I put a simple function in each table's package...
function exists( id_in in yourTable.id%type ) return boolean is
res boolean := false;
begin
for c1 in ( select 1 from yourTable where id = id_in and rownum = 1 ) loop
res := true;
exit; -- only care about one record, so exit.
end loop;
return( res );
end exists;
Makes your checks really clean...
IF pkg.exists(someId) THEN
...
ELSE
...
END IF;

Answer 4

select nvl(max(1), 0) from mytable;
This statement yields 0 if there are no rows, 1 if you have at least one row in that table. It's way faster than doing a select count(*). The optimizer "sees" that only a single row needs to be fetched to answer the question.
Here's a (verbose) little example:
declare
YES constant signtype := 1;
NO constant signtype := 0;
v_table_has_rows signtype;
begin
select nvl(max(YES), NO)
into v_table_has_rows
from mytable -- where ...
;
if v_table_has_rows = YES then
DBMS_OUTPUT.PUT_LINE ('mytable has at least one row');
end if;
end;

Answer 5

If you are using an explicit cursor, It should be as follows.
DECLARE
CURSOR get_id IS
SELECT id
FROM person
WHERE id = 10;
id_value_ person.id%ROWTYPE;
BEGIN
OPEN get_id;
FETCH get_id INTO id_value_;
IF (get_id%FOUND) THEN
DBMS_OUTPUT.PUT_LINE('Record Found.');
ELSE
DBMS_OUTPUT.PUT_LINE('Record Not Found.');
END IF;
CLOSE get_id;
EXCEPTION
WHEN no_data_found THEN
--do things when record doesn't exist
END;

Answer 6

You can do EXISTS in Oracle PL/SQL.
You can do the following:
DECLARE
n_rowExist NUMBER := 0;
BEGIN
SELECT CASE WHEN EXISTS (
SELECT 1
FROM person
WHERE ID = 10
) THEN 1 ELSE 0 INTO n_rowExist END FROM DUAL;
IF n_rowExist = 1 THEN
-- do things when it exists
ELSE
-- do things when it doesn't exist
END IF;
END;
/
Explanation:
In the query nested where it starts with SELECT CASE WHEN EXISTS and after the parenthesis (SELECT 1 FROM person WHERE ID = 10) it will return a result if it finds a person of ID of 10. If the there's a result on the query then it will assign the value of 1 otherwise it will assign the value of 0 to n_rowExist variable. Afterwards, the if statement checks if the value returned equals to 1 then is true otherwise it will be 0 = 1 and that is false.

Answer 7

Select 'YOU WILL SEE ME' as ANSWER from dual
where exists (select 1 from dual where 1 = 1);
Select 'YOU CAN NOT SEE ME' as ANSWER from dual
where exists (select 1 from dual where 1 = 0);
Select 'YOU WILL SEE ME, TOO' as ANSWER from dual
where not exists (select 1 from dual where 1 = 0);

Answer 8

select max( 1 )
into my_if_has_data
from MY_TABLE X
where X.my_field = my_condition
and rownum = 1;
Not iterating through all records.
If MY_TABLE has no data, then my_if_has_data sets to null.