Question

I got a table that I am allowing identical entries (duplicates, triplecates etc.) but I also got a column that I want everytime that an entry is being made to be updated with the how many times that entry exists.

So I thought to write a trigger, I can already find the duplicate entries by doing

select count(pid) from items group by pid having count(*);

but the thing is that this query returns less columns that the orinal table (cause there are many duplicates)

so there is no 1 to 1 relation between the query and the table so I can use update. How could I modify this to get the desired result

thank you in advanced.

Answer 1

The main problem that you’ll face here is that MySQL will not allow you to modify the items table using an AFTER INSERT ... trigger following a modification to the items table itself (think of how this could lead to a circular reference).

One solution is to store the counts in a separate table altogether (say items_pid_info). The primary key of this table would be pid and it is this table that would be updated by the triggers on the main items table. When you need to access the pidCount for a given pid simply join onto this table and you will have up-to-date pid counts for your given pid. Hence:

create table items_pid_info    
(pid int unsigned not null primary key,
pidCount int unsigned not null);

Now create INSERT, UPDATE and DELETE triggers on your items table to update the items_pid_info table:

DELIMITER &&

CREATE TRIGGER items_pid_count_ins_trg
AFTER INSERT ON items

FOR EACH ROW BEGIN
DECLARE c int;

set c := (select count(*) from items im where im.pid = NEW.pid);

insert into items_pid_info values (NEW.pid,c) on duplicate key update pidCount = c;

END&&


CREATE TRIGGER items_pid_count_upd_trg
AFTER UPDATE ON items
FOR EACH ROW BEGIN

DECLARE c int;

set c := (select count(*) from items im where im.pid = NEW.pid);

insert into items_pid_info values (NEW.pid,c) on duplicate key update pidCount = c;
END&&


CREATE TRIGGER items_pid_count_del_trg
AFTER DELETE ON items
FOR EACH ROW BEGIN

DECLARE c int;

set c := (select count(*) from items im where im.pid = OLD.pid);

insert into items_pid_info values (OLD.pid,c) on duplicate key update pidCount = c;
END&&

DELIMITER ;

Hope this helps.