I am trying to evaluate IF a value exists in the results of a SELECT statement. I know you can do something like:
IF v_var IN ('A', 'B')
THEN
DBMS_OUTPUT.PUT_LINE('True');
END IF;
However, I would like to do something like:
IF v_var IN (SELECT x FROM DUAL)
THEN
DBMS_OUTPUT.PUT_LINE('True');
END IF;
Is there a syntax legal way to accomplish this?
I would typically handle this sort of logic by running a count(*) query on the table to check for a row that matches the value you are looking for. Then count > 0 tells you the value exists, and count = 0 tells you it does not. So the code would look like:
select count(*) into v_cnt from tbl where x = v_var and rownum < 2;
IF v_cnt > 0 THEN
DBMS_OUTPUT.PUT_LINE('True');
END IF;
Notice that the optimizer is free to use relevant access structures (e.g. indexes, partitioning, zone maps) to efficiently execute this query. And the rownum < 2 predicate stops the query as soon as it finds one match for v_var; we do this because EXISTS means we don't care exactly how many matches we have, we just need to distinguish zero matches vs non-zero matches.
Related
CREATE OR REPLACE PROCEDURE test_max_rows (
max_rows IN NUMBER DEFAULT 1000
)
IS
CURSOR cur_test ( max_rows IN number ) IS
SELECT id FROM test_table
WHERE user_id = 'ABC'
AND ROWNUM <= max_rows;
id test_table.id%TYPE;
BEGIN
OPEN cur_test(max_rows) ;
LOOP
FETCH cur_test INTO id;
EXIT WHEN cur_test%NOTFOUND;
DBMS_OUTPUT.PUT_LINE('ID:' || id);
END LOOP;
END;
My requirement is to modify the above code so that when I pass -1 for max_rows, the proc should return all the rows returned by the query. Otherwise, it should limit the rows as per max_rows.
For example:
EXECUTE test_max_rows(-1);
This command should return all the rows returned by the SELECT statement above.
EXECUTE test_max_rows(10);
This command should return only 10 rows.
You can do this with a OR clause; change:
AND ROWNUM <= max_rows;
to:
AND (max_rows < 1 OR ROWNUM <= max_rows);
Then passing zero, -1, or any negative number will fetch all rows, and any positive number will return a restricted list. You could also replace the default 1000 clause with default null, and then test for null instead, which might be a bit more obvious:
AND (max_rows is null OR ROWNUM <= max_rows);
Note that which rows you get with a passed value will be indeterminate because you don't have an order by clause at the moment.
Doing this in a procedure also seems a bit odd, and you're assuming whoever calls it will be able to see the output - i.e. will have done set serveroutput on or the equivalent for their client - which is not a very safe assumption. An alternative, if you can't specify the row limit in a simple query, might be to use a pipelined function instead - you could at least then call that from plain SQL.
CREATE OR REPLACE FUNCTION test_max_rows (max_rows IN NUMBER DEFAULT NULL)
RETURN sys.odcinumberlist PIPELINED
AS
BEGIN
FOR r IN (
SELECT id FROM test_table
WHERE user_id = 'ABC'
AND (max_rows IS NULL OR ROWNUM <= max_rows)
) LOOP
PIPE ROW (r.id);
END LOOP;
END;
/
And then call it as:
SELECT * FROM TABLE(test_max_rows);
or
SELECT * FROM TABLE(test_max_rows(10));
Here's a quick SQL Fiddle demo. But you should still consider if you can do the whole thing in plain SQL and PL/SQL altogether.
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.
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;
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.
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;
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;
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;
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.
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);
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.
I'm writing a function in PL/pgSQL, and I'm looking for the simplest way to check if a row exists.
Right now I'm SELECTing an integer into a boolean, which doesn't really work. I'm not experienced with PL/pgSQL enough yet to know the best way of doing this.
Here's part of my function:
DECLARE person_exists boolean;
BEGIN
person_exists := FALSE;
SELECT "person_id" INTO person_exists
FROM "people" p
WHERE p.person_id = my_person_id
LIMIT 1;
IF person_exists THEN
-- Do something
END IF;
END; $$ LANGUAGE plpgsql;
Update - I'm doing something like this for now:
DECLARE person_exists integer;
BEGIN
person_exists := 0;
SELECT count("person_id") INTO person_exists
FROM "people" p
WHERE p.person_id = my_person_id
LIMIT 1;
IF person_exists < 1 THEN
-- Do something
END IF;
Simpler, shorter, faster: EXISTS.
IF EXISTS (SELECT FROM people p WHERE p.person_id = my_person_id) THEN
-- do something
END IF;
The query planner can stop at the first row found - as opposed to count(), which scans all (qualifying) rows regardless. Makes a big difference with big tables. The difference is small for a condition on a unique column: only one row qualifies and there is an index to look it up quickly.
Only the existence of at least one qualifying row matters. The SELECT list can be empty - in fact, that's shortest and cheapest. (Some other RDBMS don't allow an empty SELECT list on principal.)
Improved with #a_horse_with_no_name's comments.
Use count(*)
declare
cnt integer;
begin
SELECT count(*) INTO cnt
FROM people
WHERE person_id = my_person_id;
IF cnt > 0 THEN
-- Do something
END IF;
Edit (for the downvoter who didn't read the statement and others who might be doing something similar)
The solution is only effective because there is a where clause on a column (and the name of the column suggests that its the primary key - so the where clause is highly effective)
Because of that where clause there is no need to use a LIMIT or something else to test the presence of a row that is identified by its primary key. It is an effective way to test this.
My problem isn't overly complicated, but I am a newbie to PL/SQL.
I need to make a selection from a COMPANIES table based on certain conditions. I then need to loop through these and convert some of the fields into a different format (I have created functions for this), and finally use this converted version to join to a reference table to get the score variable I need. So basically:
select id, total_empts, bank from COMPANIES where turnover > 100000
loop through this selection
insert into MY_TABLE (select score from REF where conversion_func(MY_CURSOR.total_emps) = REF.total_emps)
This is basically what I am looking to do. It's slightly more complicated but I'm just looking for the basics and how to approach it to get me started!
Here's the basic syntax for cursor loops in PL/SQL:
BEGIN
FOR r_company IN (
SELECT
ID,
total_emps,
bank
FROM
companies
WHERE
turnover > 100000
) LOOP
INSERT INTO
my_table
SELECT
score
FROM
ref_table
WHERE
ref.total_emps = conversion_func( r_company.total_emps )
;
END LOOP;
END;
/
You don't need to use PL/SQL to do this:
insert into my_table
select score
from ref r
join companies c
on r.total_emps on conversion_func(c.total_emps)
where c.turnover > 100000
If you have to do this in a PL/SQL loop as asked, then I'd ensure that you do as little work as possible. I would, however, recommend bulk collect instead of the loop.
begin
for xx in ( select conversion_func(total_emps) as tot_emp
from companies
where turnover > 100000 ) loop
insert into my_table
select score
from ref
where total_emps = xx.tot_emp
;
end loop;
end;
/
For either method you need one index on ref.total_emps and preferably one on companies.turnover
LV_id number;
Cursor CR_test Is
select t.id
from table1 t
where t.foo = p_foo
order by t.creation_date;
Open CR_test;
Fetch CR_test
Into LV_id;
Close CR_test;
or this one :
select x.id
from(select t.id
from table1 t
where t.foo=p_foo
order by t.creation_date) x
where rownum = 1
Both above make similar result but i need known about which one is more efficient!
This is Tom Kyte's mantra:
You should do it in a single SQL statement if at all possible.
If you cannot do it in a single SQL Statement, then do it in PL/SQL.
If you cannot do it in PL/SQL, try a Java Stored Procedure.
If you cannot do it in Java, do it in a C external procedure.
If you cannot do it in a C external routine, you might want to seriously think about why it is you need to do it…
http://tkyte.blogspot.com/2006/10/slow-by-slow.html
Easiest way to find out in this case is to test your queries.
Make sure to test this yourself, indexes and data in your table may produce different results with your table.
Without any index, it looks like there is a better approach using analytic function DENSE_RANK:
SELECT MIN(id) KEEP (DENSE_RANK FIRST ORDER BY creation_date)
INTO lv_id
FROM table1
WHERE foo = p_foo;
I used the following code to test the time consumed by your queries (execute this block several times, results may vary):
DECLARE
p_foo table1.foo%TYPE := 'A';
lv_id table1.id%TYPE;
t TIMESTAMP := SYSTIMESTAMP;
BEGIN
FOR i IN 1 .. 100 LOOP
-- Query here
END LOOP;
dbms_output.put_line(SYSTIMESTAMP - t);
END;
Results:
Using cursor, fetching first row:
2.241 s
Using query with ROWNUM:
1.483 s
Using DENSE_RANK:
1.168 s