Question

我正在尝试对以下两个表进行联接：

table 1 
id   addr
1    Mumbai
2    delhi
3    Orissa

table 2
flight  add
y       mum
y       del
n       ori

我的SQL查询是：

select 
b.flight,
a.id
from table2 b
join table1 a 
on b.add = (select case when a.addr = 'Mumbai' then 'mum'
                   when a.addr = 'Delhi' then 'del
                   end);

在Netezza上运行时，出现以下错误：

This form of correlated query is not supported - consider rewriting

有人可以告诉我正确的方法吗？

Answer 1

case是一个表达式，就像其他任何表达式一样。您在那里不需要相关查询，只需直接使用表达式即可：

select 
b.flight,
a.id
from table2 b
join table1 a 
on b.add = case when a.addr = 'Mumbai' then 'mum'
                when a.addr = 'Delhi' then 'del
           end;

Answer 2

您可以根据需要使用SUBSTRING和LOWER

select 
b.flight,
a.id
from table2 b
join table1 a 
on b.add = SUBSTRING(LOWER(a.addr), 1, 3)

Answer 3

我无法确定Netezza是否支持这种联接，但是您可以在以下子查询中移动case语句：

select 
  b.flight,
  a.id
from table2 b join (
  select id, 
    case addr 
      when 'Mumbai' then 'mum'
      when 'Delhi' then 'del'
    end addr
  from table1 
) a on b.add = a.addr;

Answer 4

如果您要查找表1中以表2中的值addr开头的add，写此联接的一种简单方法是使用Like

select  b.flight
        , a.id
from    table1 as a
join    table2 as b on a.addr like b.[add] + '%'

SQL Fiddle

Answer 5

如果您只想将'Mumbai'与'mum'结合在一起，并将'delhi'与'del'结合起来，则只需检查配对：

select b.flight, a.id
from table2 b
join table1 a on (b.add = 'mum' and a.addr = 'Mumbai')
              or (b.add = 'del' and a.addr = 'delhi')

如您所见，在CASE WHEN和WHERE条件下不需要ON，只需使用AND和OR。

我不认识Netezza。也许它甚至支持元组：

select b.flight, a.id
from table2 b
join table1 a on (b.add, a.addr) in (('mum', 'Mumbai'), ('del', 'delhi'))

在联接中使用案例语句

5 个答案: