这对我来说是一个相当令人困惑的问题。我有一个充满棒球统计数据的数据库。运行此查询：
SELECT * FROM hits
JOIN stadiums ON stadiums.gameName = hits.gameName
JOIN players ON (players.gameName = hits.gameName AND players.id = hits.batter)
JOIN games ON games.gameName = hits.gameName
WHERE games.type = 'R'
LIMIT 50
返回：
/* 0 rows affected, 50 rows found. Duration for 1 query: 0.218 sec. */
但是运行这个查询：
SELECT * FROM hits
JOIN stadiums ON stadiums.gameName = hits.gameName
JOIN players ON (players.gameName = hits.gameName AND players.id = hits.batter)
JOIN games ON games.gameName = hits.gameName
WHERE games.leagueLevel = 'mlb'
LIMIT …