Tricky SQL Query about COUNT - mysql

Question

Here are two table with schema:
Ships(name, yearLaunched, country, numGuns, gunSize, displacement)
Battles(ship, battleName, result)
A typical Ships tuple would be:
('New Jersey', 1943, 'USA', 9, 16, 46000)
which means that the battleship New Jersey was launched in 1943; it belonged to the USA, carried 9 guns of size 16-inch (bore, or inside diameter of the barrel), and weighted (displaced, in nautical terms) 46,000 tons.
A typical tuple for Battles is:
('Hood', 'North Atlantic', 'sunk')
That is, H.M.S. Hood was sunk in the battle of the North Atlantic. The other possible results are 'ok' and 'damaged'.
And here is the tricky query:
For the battle of Surigao Strait, for each country engaged in that battle (had one or more battleships participating), give the number of its battleships that were sunk. Note: this question is very tricky. In particular, you need to deal with the (historical) case that a country engaged in the battle but did not have any ships sunk.
What I've tried so far.
SELECT country,COUNT(name)
FROM ships RIGHT JOIN battles
ON ships.name=battles.ship
WHERE battleName='Battle1' AND result='sunk'
GROUP BY country

Answer 1

country engaged in that battle (had one or more battleships
participating)
It means has at least one record in the battles where the battleName is 'Surigao Strait'. It means an INNER JOIN.
give the number of its battleships that were sunk
It is a conditional count, and here is the "trick". You can use a SUM with a condition end then you can have the count of the ships sunk.
SELECT country,SUM(CASE WHEN result = 'sunk' THEN 1 ELSE 0 END) AS TotalShipSunk
FROM ships
INNER JOIN battles
ON ships.name=battles.ship
WHERE battleName='Surigao Strait'
GROUP BY country

Answer 2

You need to join the tables and I guess the relationship with is other is by name of the ship. try this one,
SELECT a.country,
SUM(CASE WHEN b.result = 'SUNK' THEN 1 ELSE 0 END) totalSunkShips,
SUM(CASE WHEN b.result = 'OK' THEN 1 ELSE 0 END) totalUNSunkShips
FROM battles a
INNER JOIN ships b
ON a.ship = b.name
WHERE b.battleName = 'Surigao Strait'
GROUP BY a.country

Answer 3

Sample Ships Record
name yearLaunched country numGuns gunSize displacement
New Jersey 1943 USA 9 16 46000
Surigao Strait Battle ship USA 1800 USA 9 16 5000
Surigao Strait Battle ship USA 1800 USA 9 16 5000
Surigao Strait Battle ship UK 1800 UK 7 16 27000
Surigao Strait Battle ship France 1800 France 9 16 5000
Surigao Strait Battle ship Urugaya 1800 Urugaya 7 16 27000
New Jersey 1943 UK 9 16 46000
Sample Battle Record
ship battleName result
Hood North Atlantic sunk
Surigao Strait Battle ship USA Surigao Strait sunk
Surigao Strait Battle ship UK Surigao Strait damaged
Surigao Strait Battle ship France Surigao Strait ok
Surigao Strait Battle ship Urugaya Surigao Strait sunk
Is this what you are looking for
SELECT s.country,Count(s.name) AS Cnt
FROM ships s
JOIN (SELECT *
FROM Battles
WHERE battleName='Surigao Strait' AND result='sunk' )b
ON s.name=b.ship
GROUP BY s.country
Result
country Cnt
Urugaya 1
USA 2