不确定我是否完全理解你的问题，但我想这就是你想要的。

(
  SELECT * FROM your_table
  WHERE difficulty = 'A'
  ORDER BY RAND()
  LIMIT 4
)
UNION ALL
(
  SELECT * FROM your_table
  WHERE difficulty = 'B'
  ORDER BY RAND()
  LIMIT 4
)
UNION ALL
(
  SELECT * FROM your_table
  WHERE difficulty = 'C'
  ORDER BY RAND()
  LIMIT 2
)

字符串
它由三个单独的SELECT语句组成，这些语句由UNION连接。每个SELECT语句检索对应难度类型的指定数量的随机行。

试试这样的方法：

(
  SELECT * FROM table
  WHERE difficulty='A'
  ORDER BY RAND()
  LIMIT 4
)
UNION ALL
(
  SELECT * FROM table
  WHERE difficulty='B'
  ORDER BY RAND()
  LIMIT 4
)
UNION ALL
(
  SELECT * FROM table
  WHERE difficulty='C'
  ORDER BY RAND()
  LIMIT 2
);

字符串