mysql - MySQL 选择数据对

Question

我有两张桌子：

CREATE TABLE `digits` (
  `digit` tinyint(3) unsigned NOT NULL,
  `group` tinyint(3) unsigned NOT NULL
) ENGINE=InnoDB  DEFAULT CHARSET=utf8;

INSERT INTO `digits` (`digit`, `group`) VALUES
(1, 1),
(2, 1),
(3, 1),
(4, 2),
(5, 2);

和

CREATE TABLE `scores` (
  `digit1` tinyint(3) unsigned NOT NULL,
  `digit2` tinyint(3) unsigned NOT NULL,
  `score` tinyint(3) unsigned NOT NULL
) ENGINE=InnoDB  DEFAULT CHARSET=utf8;

INSERT INTO `scores` (`digit1`, `digit2`, `score`) VALUES
(1, 2, 5),
(1, 4, 9),
(2, 3, 3);

我想做的是选择所有带有分数的数字组合（不重复），如果没有分数，则选择 null

示例答案（WHERE group <=1）：

digit1 | digit2 | score
-----------------------
1      | 2      | 5
1      | 3      | null
2      | 3      | 3

示例答案（WHERE group <=2）：

digit1 | digit2 | score
-----------------------
1      | 2      | 5
1      | 3      | null
1      | 4      | 9
1      | 5      | null
2      | 3      | 3
2      | 4      | null
2      | 5      | null
3      | 4      | null
3      | 5      | null
4      | 5      | null

Answer 1

尝试这个：

select d1.digit, d2.digit, s.score
from digits d1 join
     digits d2
     on d1.group <= <groupnumber> and
        d2.group <= <groupnumber> and
        d1.digit < d2.digit left outer join
     score s
     on s.digit1 = d1.digit and
        s.digit2 = d2.digit