Complex mysql query with conditional results - mysql

Question

I have the next structure in a MySQL database:
boats
id name
-------------
1 name1
2 name2
boat_prices
id boat_id date duration price is_default
---------------------------------------------------------------
1 1 '2018-01-01' 1 100
2 1 '2018-01-01' 2 200
3 1 null null 100 1
4 2 '2018-01-02' 2 400
5 2 '2018-01-02' 4 800
6 2 null null 200 1
7 3 '2018-01-03' 5 1500
8 3 null null 300 1
The boats have a price for a specific date and duration in days.
All boats have a default "from" price that is identified by date = null and duration = null.
But, not all boats have prices for all days.
When I search for boat prices for a specific date and duration, the query should return all rows with a price for that date and duration, and in case a boat hasn´t got a price for that date return its "from" default price.
Example: For the date = '2018-01-01 and duration = 1, the result should be:
boat_prices
id boat_id date duration price is_default
----------------------------------------------------------------
1 1 '2018-01-01' 1 100
6 2 null null 200 1
8 3 null null 300 1
I did this query example just to simplify, but please take into account apart from this, the query has some other joins with other tables.
I need help with the query.

Answer 1

I believe Rick was on the right direction having left join, but you probably need TWO. One to get the boat prices that qualify the date interested in, another explicitly for the default.
select
b.id,
b.name,
DefPrice.price as DefaultPrice,
Specials.price as SpecialsPrice,
COALESCE( Specials.price, DefPrice.price ) as DiscountOrDefaultPrice
from
( select #parmDate = '2018-01-01' ) sqlvars,
boats b
JOIN boat_prices DefPrice
on b.id = DefPrice.boat_id
AND DefPrice.date IS NULL
AND DefPrice.Duration IS NULL
LEFT JOIN boat_prices Specials
on b.id = Specials.boat_id
AND Specials.date <= #parmDate
AND #parmDate <= Date_Add( Specials.Date, INTERVAL (Specials.duration -1 ) DAY )
Now, you could always return only the one price in question by doing a COALESCE() in case there is no Specials price, it gets the default via the DiscountOrDefaultPrice column.
Take your pick version of which column(s) you want to run with. This should get ALL boats, regardless of some special price based on durations. As you change whatever your parameter date in question is -- even if you do a current date, it will work. This is because you are testing the date in question against ALL possible special boat prices and its beginning to beginning + duration end date range. If you have multiple prices that overlap dates, that will just return those multiple rows that overlap.
My Adding of the duration is subtracting 1. For example, if your date is 2018-01-01 and its good for 1 day, does that mean it is only good for that one day? or up to and including 2018-01-02. The -1 forces the qualification to just the one day. So the price on 2018-01-01 good for 1 day is ONLY 2018-01-01.
Your other example for 2018-01-02 has two day duration. To me, indicating 2 days including 01-02 through 01-03. Two actual days.
CONFIRMATION from comment about dates and range
I guess my interpretation was wrong then on your data needs. Your sample of TWO dated boat price records apparently is not enough. You stated you want ALL boats regardless of qualification of a special price record. So you must start with the boat and the join to get all possible "Default" pricing no matter what. It is only the LEFT-JOIN component that needs to be adjusted.
That being said, lets simulate more data. Assume you have the following
Boad ID Date Duration Rate
1 2018-01-01 1 x
1 2018-01-02 4 y
2 2018-01-02 2 z
2 2018-01-04 4 a
3 2018-01-03 5 b
If I provide the date 2018-01-01, what rate records should I see?
If I provide date 2018-01-03, what records?
If I provide date 2018-01-05, what records?

Answer 2

For the particular date "2018-01-01" and duration of 1, i will use an UNION clause like this:
(Note: Edited for add is_default column)
-- Get prices for particular day and duration.
(SELECT
boat_id,
date,
duration,
price,
0 AS is_default
FROM
boat_prices
WHERE
date = "2018-01-01" AND duration = 1)
UNION
-- Add defaults prices for those don't have a price on the particular day and duration
(SELECT
boat_id,
date,
duration,
price,
is_default
FROM
boat_prices
WHERE
date IS NULL
AND
duration IS NULL
AND
boat_id NOT IN (SELECT boat_id
FROM boat_prices
WHERE date ="2018-01-01" AND duration = 1))
EXAMPLE WITH STORED PROCEDURE SOLUTION
DELIMITER //
CREATE PROCEDURE GetPricesByDateAndDuration(IN pDate DATE, IN pDuration INT)
BEGIN
-- Get prices for particular day and duration.
(SELECT
boat_id,
date,
duration,
price,
0 AS is_default
FROM
boat_prices
WHERE
date = pDate AND duration = pDuration)
UNION
-- Add defaults prices for those don't have a price on the particular day and duration
(SELECT
boat_id,
date,
duration,
price,
is_default
FROM
boat_prices
WHERE
date IS NULL
AND
duration IS NULL
AND
boat_id NOT IN (SELECT boat_id
FROM boat_prices
WHERE date = pDate AND duration = pDuration))
END //
DELIMITER ;
Then you can call the procedure like this:
CALL GetPricesByDateAndDuration('2018-01-01', 1);

Answer 3

Instead of that clunky output, consider:
boat_id price default
-----------------------------
1 100
2 300 (default)
Something like this should generate that:
SELECT boat_id,
IF(b.price IS NULL, dflt.price, b.price) AS price,
IF(b.price IS NULL, '(default)', '') AS default
FROM boat_prices AS dflt
LEFT JOIN boat_prices AS b USING(boat_id)
WHERE dflt.date IS NULL
AND dflt.duration IS NULL
AND '2018-01-01' >= b.date
AND '2018-01-01' < b.date + INTERVAL b.duration DAY
GROUP BY boat_id