sql - 查询以显示一列相乘

Question

我有以下3张桌子；

表名：

user_id   Name  
------------------
  1       Mark
  2       Tom
  3       Ana

表_语言：

language_id    Language
-----------------------
    1          English
    2          German

Table_Name_Lang

id   user_id   language_id
---------------------------
 1     1          1
 2     1          2
 3     2          1
 4     3          2

如何创建查询以显示如下所示的预期结果？

Name    Expr_1_Eng   Expr_1_Ger
---------------------------------
Mark    English      German
Tom     English
Ana                  German

谢谢托克

Answer 1

我会做：

select n.name,
       max(case when l.language = 'English' then l.language end) as has_English,
       max(case when l.language = 'German' then l.language end) as has_German
from names n join
     name_lang nl
     on nl.user_id = n.user_id join
     lang l
     on nl.language_id = l.language_id
group by n.name

Answer 2

我认为你应该使用枢轴：

   select n.name,[0] as language_one,[1] as language_two,[2],[3] from 
    (select n.name,tl.language
    tablename n
    left join Table_Name_Lang tnl on n.userid=tnl.userid
    left join table_language tl on tl.id=tnl.id
    group by n.name,tl.language
    )T
    pivot
    (
    language
    for name in [0],[1],[2],[3]
    )AS PivotTable;

Answer 3

您可以尝试使用JOIN条件聚合函数

SELECT Name,
    MAX(CASE WHEN tnl.language_id = 1 then tl.Language end),
    MAX(CASE WHEN tnl.language_id = 2 then tl.Language end)
FROM 
Table_Name_Lang tnl 
JOIN Table_Names tn on tnl.language_id = tn.language_id
JOIN Table_Language tl on tl.user_id = tnl.user_id
GROUP BY Name