Join to another table only matching specific records-CodePudding

I have a table of ports:

drop table if exists ports;
create table ports(id int, name char(20));

insert into ports (id, name ) values
  (1, 'Port hedland'), 
  (2, 'Kwinana');

And a table of tariffs connected to those ports:

drop table if exists tariffs;
create table tariffs(id int, portId int, price decimal(12,2), expiry bigint(11));

insert into tariffs (id, portId, price, expiry ) values
    (1, 2, 11.00, 1648408400), 
    (2, 2, 12.00, 1648508400), 
    (3, 2, 13.00, 1648594800), 
    (4, 2, 14.00, 1651273200),
    (5, 2, 15.00, 2250000000 );

insert into tariffs (id, portId, price, expiry ) values
    (1, 1, 21.00, 1648408400), 
    (2, 1, 22.00, 1648508400), 
    (3, 1, 23.00, 1648594800), 
    (4, 1, 24.00, 1651273200),
    (5, 1, 25.00, 2250000000 );

Each tariff has an expiry.

I can easily make a query to figure out the right tariff for as specific date for each port. For example at timestamp 1648594700 the right tariff is:

SELECT * FROM tariffs 
WHERE 1648594700 < expiry AND portId = 2
ORDER BY expiry
LIMIT 1

Result:

id  portId  price   expiry
3   2   13.00   1648594800

However, in my application I want to be able to pull in the right tariff starting from the ports record.

For one record, I can do this:

SELECT * FROM ports 
LEFT JOIN tariffs on tariffs.portId = ports.id 
WHERE 1648594700 < tariffs.expiry AND ports.id = 2
LIMIT 1

Result:

id  name    id  portId  price   expiry
2   Kwinana 3   2   13.00   1648594800

This feels a little 'dirty', especially because I am doing a lookup on a record, and then forcing only one result using LIMIT. But, OK.

What I cannot do, and can't work out how to do, is a query that will return a list of ports, and each port having a price field that matches the constraint above (that is, the record with the highest expiry compared to 1648594700 for each port).

This obviously won't work:

SELECT * FROM ports 
left join tariffs on tariffs.portId = ports.id
where 1648594700 < tariffs.expiry

Since the result of the query, testing with timestamp 1648594700, would be:

id  name    id  portId  price   expiry
2   Kwinana 3   2   13.00   1648594800
2   Kwinana 4   2   14.00   1651273200
2   Kwinana 5   2   15.00   2250000000
1   Port he 3   1   23.00   1648594800
1   Port he 4   1   24.00   1651273200
1   Port he 5   1   25.00   2250000000

Instead, the result for all ports (before further filtering) should be:

id  name    id  portId  price   expiry
2   Kwinana 3   2   13.00   1648594800
1   Port he 3   1   23.00   1648594800

Is there a clean, non-hacky way to have such a result? As an added constraint, is this possible for this to be done in ONE query, without temp tables etc.?

CodePudding user response：

You can select the lowest expiry, do your join and only take the rows having this minimum expiry:

SELECT p.id, p.name, t.id, t.portId, t.price, t.expiry
FROM ports p 
LEFT JOIN tariffs t ON p.id = t.portId
WHERE expiry = (SELECT MIN(expiry) FROM tariffs WHERE 1648594700 < expiry)
ORDER BY p.id;

This will get your desired result, please see here: db<>fiddle

CodePudding user response：

On MySQL 8 , ROW_NUMBER should work here:

WITH cte AS (
    SELECT p.id, p.name, t.price, t.expiry,
           ROW_NUMBER() OVER (PARTITION BY p.id ORDER BY t.expiry) rn
    FROM ports p
    LEFT JOIN tariffs t ON t.portId = p.id
    WHERE t.expiry > 1648594700
)

SELECT id, name, price, expiry
FROM cte
WHERE rn = 1
ORDER BY id;

This logic would return one record for each port having the nearest expiry.