DB2 SQL-如何在另一个表的结果字段上联接表

Question

I'm trying to join two tables but I can't join field1 of table1 with field2 of table2 which is the result of a DISTINCT instruction.

eg

Select f1
From   table1 
join (Select distinct(f2) as "secondField"
      From table2
     ) b on f1 = *X*

What can I set as X ? Maybe b.f2 or b.secondField ?

If X is a specific field I've got no problem, if it is given by a DISTINCT, SUM or similar functions I'm not able to manage it.

Can someone give me a tip?

Answer 1

SELECT f1 
FROM table1 a
JOIN (SELECT DISTINCT(f2) as "secondField" FROM table2) b
  ON a.f1 = b.secondField

Will get what you're looking for. Since you gave a specific name inside the sub-query, that's what you'd use (and what you would have to use, since you're using the DISTINCT ).

Answer 2

Have you tried joining on secondfield ? Please see the following:

Select f1
From   table1 
join (Select distinct(f2) as "secondField"
      From table2
    ) b on b.secondField = table1.f1

Simply if you are using an aggregate function such as Distinct, Count, Max, etc. in a sub-query, give the aggregate an alias and use this alias in the join statement.

Answer 3

It looks like what you are trying to do might be written simpler as:

SELECT f1 
FROM table1 a
WHERE  f1 IN (SELECT f2 as "secondField" FROM table2)

Or

SELECT f1 
FROM table1 a
WHERE  f1 IN (SELECT DISTINCT(f2) as "secondField" FROM table2)

One of these may be faster than your JOIN