mysql - sql自连接表删除重复行

Question

我有下表:

╔════════╦════════════╗
║ USERID ║ LANGUAGEID ║
╠════════╬════════════╣
║      1 ║          2 ║
║      1 ║          7 ║
║      1 ║          8 ║
║      2 ║         10 ║
║      2 ║          3 ║
╚════════╩════════════╝

现在我想为每个用户创建所有可能的语言对这意味着我希望结果集是:对于用户 1:(2,7), (7,8), (2,8)

对于用户 2:(10,3)

为此，我完成了以下查询:

SELECT a.userId , a.LanguageId, b.LanguageId
FROM knownlanguages a, knownlanguages b  
WHERE a.userID=b.userID  
AND a.LanguageId<>b.LanguageId

我得到的结果是对于用户 1:(2,7), (7,8), (2,8) , (7,2), (8,7), (8,2)

对于用户 2:(10,3), (3,10)

(10,3) 和 (3,10) 对我来说没有区别

如何删除重复的行？

tnx

Answer 1

使用您的标识符:

SELECT a.userId , a.LanguageId, b.LanguageId
  FROM knownlanguages a inner join knownlanguages b  
    on a.userID=b.userID and a.LanguageId < b.LanguageId

测试:Fot表:

create table t ( u int, l int);

insert into t values 
(    1,               2),
(    1,               7),
(    1,               8),
(    2,               10),
(    2,               3);

查询是:

select t1.u, t1.l as l1, t2.l as l2
from t t1 inner join t t2
   on t1.u = t2.u and t1.l < t2.l

(Results)