SELECT from multiple tables with cross-references and self-references [brain cracker] - mysql

Question

Anyone up to an SQL challenge? Cause all my efforts so far are barely enough to simplify the problem and put it in a question...
Here it goes. In the example below we need to include:
All paid flights
Flights to a country (paid or not) if a person has made another flight to a paid city in that country
It's already tricky, but there is more to it.
If a person flies to a city with no entry fee, but it is located in a
country that DOES have a fee, that flight is still considered paid
and must be included as well.
EDIT: I've added flight 110, which should help reveal unnecessarily added free fligts.
Below is the result set that should come out of the SQL query:
+--------------------------------------------------------------+
| Desired result set |
+--------------------------------------------------------------+
| FlightNumber | ID | Name | LocationID | location.Name |
+--------------------------------------------------------------+
| 102 | 2 | Tom | 500 | NL - NoFee | -> because Tom has a paid flight to Amsterdam
| 103 | 2 | Tom | 501 | Amsterdam (NL) - Fee | -> because Amsterdam is a paid location
| 105 | 4 | Bob | 501 | Amsterdam (NL) - Fee | -> because Amsterdam is a paid location
| 107 | 6 | Bill | 503 | ITA - Fee | -> because ITA is a paid location
| 108 | 7 | Ryan | 503 | ITA - Fee | -> because ITA is a paid location
| 109 | 7 | Ryan | 505 | Venice (ITA) - NoFee | -> because Venice is located inside ITA
+--------------------------------------------------------------+
Does anyone know how get this sweet result set with SQL?
A good place to start:
SELECT flights.FlightNumber, people.ID, people.Name, flights.LocationID, locations.Name
FROM flights
INNER JOIN people ON (people.ID = flights.ID)
INNER JOIN locations ON (locations.LocationID = flights.LocationID)
CREATE/INSERT
CREATE TABLE `people` (
`ID` INT NOT NULL,
`Name` VARCHAR(45) NULL,
PRIMARY KEY (`ID`) );
CREATE TABLE `locations` (
`LocationID` INT NOT NULL,
`Name` VARCHAR(45) NULL,
`EntryFee` TINYINT(1) NULL,
`ParentLocationID` INT NULL,
PRIMARY KEY (`LocationID`) );
CREATE TABLE `flights` (
`FlightNumber` INT NOT NULL,
`ID` INT NULL,
`LocationID` INT NULL,
PRIMARY KEY (`FlightNumber`) ,
INDEX `fk_purchases_buyers_idx` (`LocationID` ASC) ,
INDEX `fk_flights_people1_idx` (`ID` ASC) ,
CONSTRAINT `fk_purchases_buyers`
FOREIGN KEY (`LocationID`)
REFERENCES `locations` (`LocationID`)
ON DELETE NO ACTION
ON UPDATE NO ACTION,
CONSTRAINT `fk_flights_people1`
FOREIGN KEY (`ID`)
REFERENCES `people` (`ID`)
ON DELETE NO ACTION
ON UPDATE NO ACTION);
INSERT INTO `people` (`ID`, `Name`) VALUES
(1, 'John'),
(2, 'Tom'),
(3, 'Kate'),
(4, 'Bob'),
(5, 'Mike'),
(6, 'Bill'),
(7, 'Ryan');
INSERT INTO `locations` (`LocationID`, `Name`, `EntryFee`, `ParentLocationID`) VALUES
(500, 'NL - NoFee', 0, NULL),
(501, 'Amsterdam (NL) - Fee', 1, 500),
(502, 'Rotterdam (NL) - NoFee', 0, 500),
(503, 'ITA - Fee', 1, NULL),
(504, 'Rome (ITA) - Fee', 1, 503),
(505, 'Venice (ITA) - NoFee', 0, 503);
INSERT INTO `flights` VALUES
(100, 1, 500),
(101, 1, 502),
(102, 2, 500),
(103, 2, 501),
(104, 3, 500),
(105, 4, 501),
(106, 5, 502),
(107, 6, 503),
(108, 7, 503),
(109, 7, 505),
(110, 6, 502);
Unimportant note: I know this example is not completely logical in a sense of storing countries and cities in the same table and having flight records to a country and to a city. But this is just an example. At least it's more readable than t1.col1-like stuff.

Answer 1

Following should work :
SELECT F.FlightNumber, P.ID, P.Name, F.LocationID, LOC.Name AS Loc_Name
FROM flights F
INNER JOIN people P ON P.ID = F.ID
INNER JOIN (SELECT L1.LocationID, L1.`Name`, L1.`ParentLocationID`
FROM locations L1
LEFT JOIN locations L2 ON L1.`ParentLocationID` = L2.LocationID
WHERE L1.`EntryFee` = 1 OR L2.`EntryFee` = 1) AS LOC ON LOC.LocationID = F.LocationID
UNION
SELECT F.FlightNumber, PAID_FL.ID, PAID_FL.Name, F.LocationID, PAID_FL.Loc_Name
FROM flights F
INNER JOIN (SELECT F.FlightNumber, P.ID, P.Name, LOC.Name AS Loc_Name,LOC.`ParentLocationID` AS LocationID
FROM flights F
INNER JOIN people P ON P.ID = F.ID
INNER JOIN (SELECT L1.LocationID, L1.`ParentLocationID`, L2.`Name`
FROM locations L1
LEFT JOIN locations L2 ON L1.`ParentLocationID` = L2.LocationID
WHERE L1.`EntryFee` = 1 OR L2.`EntryFee` = 1) AS LOC ON LOC.LocationID = F.LocationID) PAID_FL ON F.ID = PAID_FL.ID AND F.LocationID = PAID_FL.LocationID

Answer 2

I didn't do any testing, but I think this will work:
SELECT F.FlightNumber, P.ID, P.Name, L.LocationID, L.Name
FROM flights F
INNER JOIN people P ON P.ID = F.ID
INNER JOIN locations L ON L.LocationID = F.LocationID
WHERE P.ID IN (SELECT F.ID FROM flights F INNER JOIN locations L ON L.LocationID = F.LocationID WHERE L.EntryFee = 1)
See it at SQLFiddle.com.

Answer 3

SELECT x.*
, COALESCE(y.entryfee,x.entryfee,0) fee
FROM locations x
LEFT
JOIN locations y
ON y.parentlocationid = x.locationid;