SQL用於連接不同的表

Question

我有以下表格：

我想寫一個選擇Statment，顯示以下結果

我試圖加入/聯盟/聯盟所有的兩個表，但我沒有得到desird輸出。任何人都可以幫助我嗎？

Answer 1

select 
'group_a' as groupname 
, max(case classname when 'C123' then group_a else null end) as c123 
, max(case classname when 'C456' then group_a else null end) as c456 
from table1 
union all 
select 
'group_b' as groupname 
, max(case classname when 'C123' then group_b else null end) as c123 
, max(case classname when 'C456' then group_b else null end) as c456 
from table1 
union all 
select 
'group_d' as groupname 
, max(case classname when 'C123' then group_d else null end) as c123 
, max(case classname when 'C456' then group_d else null end) as c456 
from table1 
union all 
select 
'group_e' as groupname 
, max(case classname when 'C123' then group_e else null end) as c123 
, max(case classname when 'C456' then group_e else null end) as c456 
from table1