Mysql triggers - capture each column change - mysql

Question

I am trying to create trigger, that capture changes in database after update.
Table my_table I am watching:
Table my_table_log where I am writing changes to log them
And here is trigger so far:
CREATE TRIGGER `log_update`
AFTER UPDATE ON `my_table`
FOR EACH ROW
BEGIN
INSERT INTO
`my_table_log`
(
`id`,
`action`,
`column_name`,
`value_before`,
`value_after`,
`who`,
`ts`
)
VALUES
(
NEW.id,
'u',
'name',
OLD.name,
NEW.name,
user(),
NOW()
);
END
Question: How to log each change of column ?
Problem: I am curently watching only if column name changed in my_table. And I have another trigger for column age. How to set trigger for each row and each column that was changed?
Thank you for your suggestions/code/inspirations

Answer 1

You might use ifs for every column you'd like to watch in your trigger:
create trigger `log_update`
after update on `my_table`
for each row
begin
if (old.name <> new.name) then
insert into `my_table_log`
(
`id`,
`action`,
`column_name`,
`value_before`,
`value_after`,
`who`,
`ts`
)
values
(
new.id,
'u',
'name',
old.name,
new.name,
user(),
now()
);
end if;
if (old.age <> new.age) then
insert into `my_table_log`
(
`id`,
`action`,
`column_name`,
`value_before`,
`value_after`,
`who`,
`ts`
)
values
(
new.id,
'u',
'age',
old.age,
old.age,
user(),
now()
);
end if;
end
But better make the insert a stored procedure to avoid redudancy:
create procedure `log_insert`
(
id int(11),
`action` char,
column_name varchar(255),
value_before varchar(255),
value_after varchar(255)
)
begin
insert into `my_table_log`
(
`id`,
`action`,
`column_name`,
`value_before`,
`value_after`,
`who`,
`ts`
)
values
(
id,
`action`,
column_name,
value_before,
value_after,
user(),
now()
);
end
And call it in your trigger:
create trigger `log_update`
after update on `my_table`
for each row
begin
if (old.name <> new.name) then
call log_insert
(
new.id,
'u',
'name',
old.name,
new.name
);
end if;
if (old.age <> new.age) then
call log_insert
(
new.id,
'u',
'age',
old.age,
new.age
);
end if;
end
You can re-use the stored procedure to log events in your insert and delete triggers.
Make shure to use a composite primary key in your my_table_log to allow updates over several columns. I'd use at least:
primary key(id,column_name,who,ts).
Or use dedicated single column primary key to avoid varchars in your primary key for better performance.

Answer 2

One alternative is to just log the new values together with user() and now():
create table my_table_log
( id ...
, name ...
, age ...
, action ...
, who ...
, ts ... )
To determine what was changed, compare with the previous row.
It is however rather expensive to determine what a row looked like at a certain point in time, you will have to find the last version before that point in time. Another model that makes this a lot easier is to keep track of begin_ts and end_ts for each row:
create table my_table_log
( id ...
, name ...
, age ...
, action ...
, who ...
, begin_ts ...
, end_ts ...)
The insert trigger adds a copy of the row with begin_ts = now() and end_ts = null. The update trigger updates end_ts = now() where end_ts is null and inserts a row like the insert trigger. The delete trigger updates end_ts and might add a copy together with who deleted the row. Determining what a row looked like at ts t is just a matter of where t between start_ts and end_ts