Update first occurrence of value in a time interval - mysql

Question

I'm trying to set the value of another column on the first occurrence of any value in a username column in monthly intervals, if there's another column with an specific value.
create table table1
(
username varchar(30) not null,
`date` date not null,
eventid int not null,
firstFlag int null
);
insert table1 (username,`date`, eventid) values
('john','2015-01-01', 1)
, ('kim','2015-01-01', 1)
, ('john','2015-01-01', 1)
, ('john','2015-01-01', 1)
, ('john','2015-03-01', 2)
, ('john','2015-03-01', 1)
, ('kim','2015-01-01', 1)
, ('kim','2015-02-01', 1);
This should result in:
| username | date | eventid | firstFlag |
|----------|------------|---------|-----------|
| john | 2015-01-01 | 1 | 1 |
| kim | 2015-01-01 | 1 | 1 |
| john | 2015-01-01 | 1 | (null) |
| john | 2015-01-01 | 1 | (null) |
| john | 2015-03-01 | 2 | 1 |
| john | 2015-03-01 | 1 | (null) |
| kim | 2015-01-01 | 1 | (null) |
| kim | 2015-02-01 | 1 | 1 |
I've tried using joins as described here, but it updates all rows:
update table1 t1
inner join
( select username,min(`date`) as minForGroup
from table1
group by username,`date`
) inr
on inr.username=t1.username and inr.minForGroup=t1.`date`
set firstFlag=1;

Answer 1

As a1ex07 points out, it would need another per row unique constrain to update the rows I need to:
update table1 t1
inner join
( select id, username,min(`date`) as minForGroup
from table1
where eventid = 1
group by username,month(`date`)
) inr
on inr.id=t1.id and inr.username=t1.username and inr.minForGroup=t1.`date`
set firstFlag=1;
Add an Id column, and use it on the join on constrains.
To allow only those that satisfies a specific condition on another column you need the where clause inside the subquery, otherwise it would try to match different rows as the subquery would return rows with eventid=2 while the update query would return only those with eventid=1.
To use yearly intervals instead of monthly, change the group by statement to use years.