删除包含两列可能包含相同数据的行

Question

鉴于以下数据为例，

Letter One | Letter Two    
         A | B
         B | A
         A | C

我怎样才能选择任何具有相同数据的行，所以在这种情况下我希望输出删除第二行，因为这两个字母是相同的。所以我的输出是：

Letter One | Letter Two
         A | B 
         A | C

Answer 1

假设没有确切的重复，你可以这样做：

select letterA, letterB
from t
where letterA <= letterB
union all
select letterA, letterB
from t
where letterA > letterB and
      not exists (select 1 from t t2 where t2.letterA = t.letterB and t2.letterB = t.letterA);

如果您不关心最终列的顺序，您可以：

select distinct (case when letterA <= letterB then letterA else letterB end) as letterA,
       (case when letterA > letterB then letterA else letterB end) as letterB
from t;

Answer 2

SELECT 
         l1.LetterOne , l1.LetterTwo
FROM
         letters l1
LEFT JOIN
         letters l2 ON l1.LetterOne = l2.LetterTwo AND l1.LetterTwo = l2.LetterOne
WHERE
         l2.LetterOne IS NULL OR l1.LetterOne < l2.LetterOne

Answer 3

UNION将过滤掉重复项。最低值将在第一列中返回：

SELECT LetterOne, LetterTwo
FROM <table>
WHERE LetterOne < LetterTwo
UNION 
SELECT LetterTwo, LetterOne
FROM <table>
WHERE LetterTwo<=LetterOne