如何使用SQL统计组的出现次数，并将两个不同的列视为未排序的集合？

根据表格：

CREATE TABLE foo (
    thing_a VARCHAR2(15),
    thing_b VARCHAR2(15)
);

那排呢

INSERT INTO foo (thing_a, thing_b) VALUES 'red', 'green';
INSERT INTO foo (thing_a, thing_b) VALUES 'red', 'green';
INSERT INTO foo (thing_a, thing_b) VALUES 'green', 'red';
INSERT INTO foo (thing_a, thing_b) VALUES 'red', 'blue';

我想知道结果

green,red: 3
blue,red: 1

换言之，我想对每一组的发生情况进行统计，但我想把“红色”和“绿色”看作“绿色”和“红色”。 我找到的最接近“解决方案”是：

因此，它正确地进行分组，但不计算任何内容

---

关于如何解决这个问题，有什么建议吗？

你就快到了。只需将

案例

查询包装在派生表中，然后

按其结果分组：

select thing, count(*)
from
(
    SELECT CASE WHEN thing_a < thing_b THEN thing_a || thing_b
           ELSE thing_b || thing_a
           END as thing
    FROM foo
) dt
group by thing

select ta, tb, count(*)
from
(
    select thing_a as ta, thing_b as tb from foo where thing_a <= thing_b
    union all
    select thing_b, thing_a from foo where thing_a > thing_b
) dt
group by ta, tb

符合ANSI SQL标准且可移植
 这可以在单个
SELECT
语句中完成-按最小值、最大值分组。注意：如果任一列中可能存在
null
，则此操作无效；如果这是可能的，那么在编写查询时必须更加小心地处理它

with
     foo ( thing_a, thing_b ) as (
       select 'red'  , 'green' from dual union all
       select 'red'  , 'green' from dual union all
       select 'green', 'red'   from dual union all
       select 'red'  , 'blue'  from dual union all
       select 'ab'   , 'cde'   from dual union all
       select 'abcd' , 'e'     from dual union all
       select 'cde'  , 'ab'    from dual
     )
-- end of test data; SQL query begins BELOW THIS LINE
select least(thing_a, thing_b) as thing_a, greatest(thing_a, thing_b) as thing_b,
       count(*) as cnt
from   foo
group by least(thing_a, thing_b), greatest(thing_a, thing_b)
order by thing_a, thing_b   --   if needed
;

THING_A THING_B CNT
------- ------- ---
ab      cde       2
abcd    e         1
blue    red       1
green   red       3

4 rows selected.

@我看到你把这个答案标记为“正确”，考虑到你的经验，这是令人惊讶的。假设您的数据中有以下无序集：
（'ab'，'cde'）
和
（'abcd'，'e'）
。这个解决方案将把它们当作一样对待。@mathguy，哎呀。。。我能说什么。。。在a和b之间添加逗号似乎是一个简单的编辑。请稍后再试。如果字符串本身包含逗号，这将不起作用。@mathguy，我知道，但OP在示例数据中没有逗号或其他奇数字符。无论如何，上一个UNION ALL/GROUP BY查询没有逗号问题。很好！我忘了提到我的数据符合
^A-Z$
（它们都是三个字母的大写代码），而我的实际解决方案最终实现了jarlh在这里提出的建议——添加逗号来分隔它们。这很有趣，我不知道你可以使用分组方式中的函数！我的数据中没有空值，所以这可以正常工作。
select thing, count(*)
from
(
    SELECT CASE WHEN thing_a < thing_b THEN thing_a || ', ' || thing_b
           ELSE thing_b || ', ' || thing_a
           END as thing
    FROM foo
) dt
group by thing

select ta, tb, count(*)
from
(
    select thing_a as ta, thing_b as tb from foo where thing_a <= thing_b
    union all
    select thing_b, thing_a from foo where thing_a > thing_b
) dt
group by ta, tb

with
     foo ( thing_a, thing_b ) as (
       select 'red'  , 'green' from dual union all
       select 'red'  , 'green' from dual union all
       select 'green', 'red'   from dual union all
       select 'red'  , 'blue'  from dual union all
       select 'ab'   , 'cde'   from dual union all
       select 'abcd' , 'e'     from dual union all
       select 'cde'  , 'ab'    from dual
     )
-- end of test data; SQL query begins BELOW THIS LINE
select least(thing_a, thing_b) as thing_a, greatest(thing_a, thing_b) as thing_b,
       count(*) as cnt
from   foo
group by least(thing_a, thing_b), greatest(thing_a, thing_b)
order by thing_a, thing_b   --   if needed
;

THING_A THING_B CNT
------- ------- ---
ab      cde       2
abcd    e         1
blue    red       1
green   red       3

4 rows selected.