Grundlegende MySQL-Abfragen

Sie können die Abfragen unter http://sqlfiddle.com/#!9/839d2/1 ausprobieren

Abfrage 1

Liste aller Planeten, die Kapitäne haben. Es zeigt id_planet, planet_name, die Gesamtzahl der Kapitäne auf jedem dieser Planeten sowie die Anzahl der Schlachten (falls vorhanden), die dort stattfanden.

SELECT DISTINCT aa.id_planet, aa.planet_name, _aa.captains_count, _bb.battles_count
FROM planet AS aa
INNER JOIN soldier AS bb
ON aa.id_planet = bb.planet_id
INNER JOIN (
    SELECT planet_id, COUNT(*) AS captains_count
    FROM soldier
    WHERE rank = 'captain'
    GROUP BY planet_id
) AS _aa
ON aa.id_planet = _aa.planet_id
LEFT JOIN (
    SELECT id_planet_battle, COUNT(*) AS battles_count
    FROM battle
    GROUP BY id_planet_battle
) AS _bb
ON aa.id_planet = _bb.id_planet_battle
WHERE bb.rank = 'captain';

Sie können die gleichen Ergebnisse erzielen, indem Sie Folgendes verwenden:

SELECT DISTINCT aa.id_planet, aa.planet_name,
(
    SELECT COUNT(*)
    FROM soldier AS _aa
    WHERE _aa.rank = 'captain' AND aa.id_planet = _aa.planet_id
    GROUP BY _aa.planet_id
) AS captains_count,
(
    SELECT COUNT(*)
    FROM battle AS _bb
    WHERE aa.id_planet = _bb.id_planet_battle
    GROUP BY _bb.id_planet_battle 
) AS battles_count
FROM planet AS aa
INNER JOIN soldier AS bb
ON aa.id_planet = bb.planet_id
WHERE bb.rank = 'captain';

Abfrage 3

SELECT aa.name, aa.rank, bb.planet_name AS planet_from, (
    SELECT COUNT(*) 
    FROM soldier AS _aa 
    WHERE _aa.planet_id = aa.planet_id
) AS number_of_soldiers, 
(
    SELECT COUNT(*)
    FROM battle AS _bb
    WHERE _bb.id_planet_battle = aa.planet_id
) AS number_of_battles
FROM soldier AS aa
INNER JOIN planet AS bb
ON aa.planet_id = bb.id_planet;

Hier habe ich nicht Joins verwendet, um number_of_soldiers und number_of_battles zu berechnen, wie ich es bei Abfrage 1.1 getan habe, da dies eine korrelierte Unterabfrage wäre und als solche keinen Zugriff auf die äußeren Abfragen haben könnte (https://dev.mysql.com/doc/refman/5.5/en/from-clause -subqueries.html ).

Falsche Abfrage:

SELECT DISTINCT aa.id_planet, aa.planet_name, _aa.captains_count, _bb.battles_count
FROM planet AS aa
INNER JOIN soldier AS bb
ON aa.id_planet = bb.planet_id
INNER JOIN (
    SELECT COUNT(*) AS captains_count
    FROM soldier AS _aa
    WHERE _aa.rank = 'captain' AND aa.id_planet = _aa.planet_id
    GROUP BY _aa.planet_id
) AS _aa
ON aa.id_planet = _aa.planet_id
LEFT JOIN (
    SELECT COUNT(*) AS battles_count
    FROM battle AS _bb
    WHERE aa.id_planet = _bb.id_planet_battle
    GROUP BY _bb.id_planet_battle 
) AS _bb
ON aa.id_planet = _bb.id_planet_battle
WHERE bb.rank = 'captain';

Die obige Abfrage ist also falsch und erzeugt den Fehler:Unbekannte Spalte „aa.id_planet“ in „where-Klausel“.

Was die zweite Frage angeht, um die Sie gebeten haben, hoffe ich, dass jemand anderes es versuchen könnte.