Trigger that restricts by date - sql

Question

Im trying to create a trigger that restricts the amount a reader can read in a given month.
CREATE OR REPLACE trigger Readings_Limit
Before update or insert on reading
for each row declare
readingcount integer;
max_read integer := 5;
Begin
Select count(*) into readingcount
from (select *
from Reading
where to_char(DateRead, 'YYYY-MM') = to_char(DateRead, 'YYYY-MM'))
where employeeid = :new.employeeid;
if :old.employeeid = :new.employeeid then
return;
else
if readingcount >= max_read then
raise_application_error (-20000, 'An Employee can only read 5 per month');
end if;
end if;
end;
This restricts the reader to 5 max in total no matter the month, i can't seem to get it to be 5 max each month. any ideas greatly appreciated!

Answer 1

Try to rewrite your trigger like this:
CREATE OR REPLACE trigger Readings_Limit
Before update or insert on reading
for each row
declare
readingcount integer;
max_read integer := 5;
Begin
Select count(*) into readingcount
from Reading
where DateRead between trunc(sysdate,'MM') and last_day(sysdate)
and employeeid = :new.employeeid;
if :old.employeeid = :new.employeeid then
return;
else
if readingcount >= max_read then
raise_application_error (-20000, 'An Employee can only read 5 per month');
end if;
end if;
end;
You add actual month into yout select and you avoid unnecessary date conversion.
I don't understand the condition
if :old.employeeid = :new.employeeid then
Does it mean, the trigger should not fire on updates? In this case it is better to make trigger only for insert or use clause if inserting then...

Answer 2

In order to properly create this validation using a trigger a procedure should be created to obtain user-specified locks so the validation can be correctly serialized in a multi-user environment.
PROCEDURE request_lock
(p_lockname IN VARCHAR2
,p_lockmode IN INTEGER DEFAULT dbms_lock.x_mode
,p_timeout IN INTEGER DEFAULT 60
,p_release_on_commit IN BOOLEAN DEFAULT TRUE
,p_expiration_secs IN INTEGER DEFAULT 600)
IS
-- dbms_lock.allocate_unique issues implicit commit, so place in its own
-- transaction so it does not affect the caller
PRAGMA AUTONOMOUS_TRANSACTION;
l_lockhandle VARCHAR2(128);
l_return NUMBER;
BEGIN
dbms_lock.allocate_unique
(lockname => p_lockname
,lockhandle => p_lockhandle
,expiration_secs => p_expiration_secs);
l_return := dbms_lock.request
(lockhandle => l_lockhandle
,lockmode => p_lockmode
,timeout => p_timeout
,release_on_commit => p_release_on_commit);
IF (l_return not in (0,4)) THEN
raise_application_error(-20001, 'dbms_lock.request Return Value ' || l_return);
END IF;
-- Must COMMIT an autonomous transaction
COMMIT;
END request_lock;
To have the least impact on scalability the serialization should be done at the finest level, which for this constraint is per employeeid and month. Types may be used in order to create variables to store this information for each row before the constraint is checked after the statement has completed. These types can be either defined in the database or (from Oracle 12c) in package specifications.
CREATE OR REPLACE TYPE reading_rec
AS OBJECT
(employeeid NUMBER(10) -- Note should match the datatype of reading.employeeid
,dateread DATE);
CREATE OR REPLACE TYPE readings_tbl
AS TABLE OF reading_rec;
The procedure and types can then be used in a compound trigger (assuming at least Oracle 11, this will need to be split into individual triggers in earlier versions)
CREATE OR REPLACE TRIGGER too_many_readings
FOR INSERT OR UPDATE ON reading
COMPOUND TRIGGER
-- Table to hold identifiers of inserted/updated readings
g_readings readings_tbl;
BEFORE STATEMENT
IS
BEGIN
-- Reset the internal readings table
g_readings := readings_tbl();
END BEFORE STATEMENT;
AFTER EACH ROW
IS
BEGIN
-- Store the inserted/updated readings
IF ( INSERTING
OR :new.employeeid <> :old.employeeid
OR :new.dateread <> :old.dateread)
THEN
g_readings.EXTEND;
g_readings(g_readings.LAST) := reading_rec(:new.employeeid, :new.dateread);
END IF;
END AFTER EACH ROW;
AFTER STATEMENT
IS
CURSOR csr_readings
IS
SELECT DISTINCT
employeeid
,trunc(dateread,'MM') monthread
FROM TABLE(g_readings)
ORDER BY employeeid
,trunc(dateread,'MM');
CURSOR csr_constraint_violations
(p_employeeid reading.employeeid%TYPE
,p_monthread reading.dateread%TYPE)
IS
SELECT count(*) readings
FROM reading rdg
WHERE rdg.employeeid = p_employeeid
AND trunc(rdg.dateread, 'MM') = p_monthread
HAVING count(*) > 5;
r_constraint_violation csr_constraint_violations%ROWTYPE;
BEGIN
-- Check if for any inserted/updated readings there exists more than
-- 5 readings for the same employee in the same month. Serialise the
-- constraint for each employeeid so concurrent transactions do not
-- affect each other
FOR r_reading IN csr_readings LOOP
request_lock('TOO_MANY_READINGS_'
|| r_reading.employeeid
|| '_' || to_char(r_reading.monthread, 'YYYYMM'));
OPEN csr_constraint_violations(r_reading.employeeid, r_reading.monthread);
FETCH csr_constraint_violations INTO r_constraint_violation;
IF csr_constraint_violations%FOUND THEN
CLOSE csr_constraint_violations;
raise_application_error(-20001, 'Employee ' || r_reading.employeeid
|| ' now has ' || r_constraint_violation.readings
|| ' in ' || to_char(r_reading.monthread, 'FMMonth YYYY'));
ELSE
CLOSE csr_constraint_violations;
END IF;
END LOOP;
END AFTER STATEMENT;
END;

Answer 3

You need to set the month you are looking at, so if you are considering the current month, make the inner query read like this:
( select * from Reading
where to_char(DateRead,'YYYY-MM') = to_char(DateRead,'YYYY-MM')
and to_char(sysdate,'YYYY-MM') = to_char(DateRead,'YYYY-MM'))
that way it will always compare to the current month and should move as your date moves.