Insert value into specific column from another table with groupby - mysql

Question

I am using MySQL. I want to insert value's result from groupby of datetime to specific column (using where, maybe). Let say:
I have two tables (a, b). In table a, I want to get how many total records during a hour (which I have datetime column), then the result will insert into table b, but in specific ID (there is already exist ID's value).
This is my error code:
INSERT INTO b(value)
WHERE ID=15
SELECT DAY COUNT(*)
FROM a
WHERE date >= '2015-09-19 00:00:00' AND date < '2015-09-19 00:59:59'
GROUP BY DAY(date),HOUR(date);";
Is that possible I make a query from this case?
Thank you very much for any reply!

Answer 1

Schema
create table tA
( id int auto_increment primary key,
theDate datetime not null,
-- other stuff
key(theDate) -- make it snappy fast
);
create table tB
( myId int primary key, -- by definition PK is not null
someCol int not null
);
-- truncate table tA;
-- truncate table tB;
insert tA(theDate) values
('2015-09-19'),
('2015-09-19 00:24:21'),
('2015-09-19 07:24:21'),
('2015-09-20 00:00:00');
insert tB(myId,someCol) values (15,-1); -- (-1) just for the heck of it
insert tB(myId,someCol) values (16,-1); -- (-1) just for the heck of it
The Query
update tB
set someCol=(select count(*) from tA where theDate between '2015-09-19 00:00:00' and '2015-09-19 00:59:59')
where tB.myId=15;
The Results
select * from tB;
+------+---------+
| myId | someCol |
+------+---------+
| 15 | 2 |
| 16 | -1 |
+------+---------+
only myId=15 is touched.