Idan Idan - 1 year ago 65
SQL Question

How do I create a SQL query that can filter on count?

Short database description "Ships":

The database of naval ships that took part in World War II is under consideration. The database has the following relations:

  • Classes(class, type, country, numGuns, bore, displacement)

  • Ships(name, class, launched)

  • Battles(name, date)

  • Outcomes(ship, battle, result)

Ships in classes are arranged to a single project. A class is normally assigned the name of the first ship in the class under consideration (head ship); otherwise, the class name does not coincide with any ship name in the database.

The Classes relation includes the class name, type (bb for a battle ship, or bc for a battle cruiser), country where the ship was built, number of main guns, gun caliber (diameter of the gun barrel, in inches), and displacement (weight in tons).

The Ships relation includes the ship name, its class name, and launch year. The Battles relation covers the name and date of a battle the ships participated; while the result of their participation in the battle (sunk, damaged, or unharmed - OK) is in the Outcomes relation. Note: the Outcomes relation may include the ships not included in the Ships relation.

Point out the battles in which at least three ships from the same country took part.

Can someone help with this query? I haven't used SQL for quite a while.


As I was told it's not allowed to ask for a broad question, I'll be more specific :

I thought about how I can do it, but eventually I need some kind of way to count duplicates?
for exmaple..for the list

if I'll say count > 2 I'll get a and c


Answer Source

Try this

FROM   Battles b
       SELECT     NULL
       FROM       Outcomes   o            
       INNER JOIN Ships      s
       ON         o.ship   =
       INNER JOIN Classes    c
       ON         s.class  = c.class
       WHERE      o.battle =
       GROUP BY
       HAVING     count(*) >= 3
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