Du kan prøve forespørgslerne påhttp://sqlfiddle.com/#!9/839d2/1
Forespørgsel 1
Liste over alle planeter, der har kaptajner. Den viser id_planet, planet_name, det samlede antal kaptajner på hver af disse planeter samt antallet af kampe (hvis nogen), der fandt sted der.
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';
Du kan få samme resultater ved at bruge denne:
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';
Forespørgsel 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;
Her brugte jeg ikke Joins til at beregne antal_af_soldater og antal_kampe, som jeg gjorde ved forespørgsel 1.1, fordi det ville være en korreleret underforespørgsel, og som sådan kunne den ikke have adgang ved de ydre forespørgsler (https://dev.mysql.com/doc/refman/5.5/en/from-clause -subqueries.html ).
Forkert forespørgsel:
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';
Så ovenstående forespørgsel er forkert og producerer fejlen:Ukendt kolonne 'aa.id_planet' i 'where clause'.
Med hensyn til den anden forespørgsel, du bad om, håber jeg, at en anden kan give det en chance.