MYSQL get the first value in JOIN query - mysql

Question

I have 3 tables: A, B and C.
A has AID, B has AID and BID, and C has BID Value and Date.
I need to create a query that returns me AID and the first (according to date) Value from C.
WHAT I've tried:
SELECT A.AID, Value FROM A INNER JOIN B on A.AID = B.BID
INNER JOIN C ON C.BID = B.BID GROUP BY A.AID
It gives me the last Value and not the first.
Data example:
A:
AID:
1
2
3
B:
AID BID
1 1
1 2
2 3
3 4
3 5
3 6
C:
BID Value Date
1 15 1.1.1970
1 422 1.1.1992
2 945 1.1.1975
3 149 1.1.1994
3 147 1.1.2015
4 110 1.1.2004
5 142 1.1.2005
The output should be:
AID Value
1 15
2 149
3 110

Answer 1

If you do not have too many records with the same value and value doesn't have any commas, then the group_concat()/substring_index() trick is probably the easiest way:
select b.aid,
substring_index(group_concat(c.value order by c.date desc), ',' 1) as first_value
from c join
b
on c.bid = b.bid
group by b.aid;
Larger amounts of data require a more complicated query. Something like:
select b.aid, c.value
from c join
b
on c.bid = b.bid
where c.date = (select min(c2.date)
from c2 join
b2
on c2.bid = b2.bid
where b2.aid = b.aid
);

Answer 2

To restrict C to just those rows with the latest (minimum) date you need a subquery that will produce the minimum date, then use that to limit the rows from C
SELECT
A.AID
, C.Value
FROM A
INNER JOIN B ON A.AID = B.BID
INNER JOIN C ON b.bid = c.bid
INNER JOIN (
SELECT
bid
, MIN(date) AS mindate
FROM c
GROUP BY
bid
) AS m ON c.bid = m.bid
AND c.date = m.mindate

Answer 3

DROP TABLE IF EXISTS b;
CREATE TABLE b
(aid INT NOT NULL
,bid INT NOT NULL
,PRIMARY KEY(aid,bid)
);
INSERT INTO b VALUES
(1 ,1),
(1 ,2),
(2 ,3),
(3 ,4),
(3 ,5),
(3 ,6);
DROP TABLE IF EXISTS c;
CREATE TABLE c
(bid INT NOT NULL
,value INT NOT NULL
,date DATE
,PRIMARY KEY(bid,date)
);
INSERT INTO c VALUES
(1 ,15 ,'1970-01-01'),
(1 ,422 ,'1992-01-01'),
(2 ,945 ,'1975-01-01'),
(3 ,149 ,'1994-01-01'),
(3 ,147 ,'2015-01-01'),
(4 ,110 ,'2004-01-01'),
(5 ,142 ,'2005-01-01');
SELECT x.aid
, y.value
FROM b x
JOIN c y
ON y.bid = x.bid
JOIN
( SELECT b.aid
, MIN(c.date) min_date
FROM b
JOIN c
ON c.bid = b.bid
GROUP
BY b.aid
) z
ON z.min_date = y.date
AND z.aid = x.aid;
+-----+-------+
| aid | value |
+-----+-------+
| 1 | 15 |
| 2 | 149 |
| 3 | 110 |
+-----+-------+