在DB2中按order by子句关联子查询

Question

I had a query similar to the following and was wondering that DB2 complained about the correlation use in the ORDER BY clause. It errored with something like

[42703][-206] "A.ID" is not valid in the context where it is used.. SQLCODE=-206, SQLSTATE=42703

I was able to rewrite the query to avoid the correlation usage but I couldn't find a reference in the documenation about this. Is this a bug or am I just not able to find details on the expected behavior?

SELECT a.id
FROM A a
ORDER BY (
  SELECT COUNT(*)
  FROM   B b
  WHERE  b.id = a.id
)

Answer 1

You can't use correlated query in order by clause. However there is many ways to get same result, for example

select count(*) as count_num ,a.ID 
from
   a join  b on  a.ID=b.ID
GROUP BY a.ID
order by 1 DESC

Answer 2

solution 1:

SELECT a.id, (select count(*) from B where B.id=a.id) nbOFB 
FROM A 
order by 2

Answer 3

solution 2:

select * from (
SELECT a.id, (select count(*) from B where B.id=a.id) nbOFB 
FROM A
) tmp 
order by nbOFB

Answer 4

Solution 3:

SELECT a.id, c.nb 
FROM A 
inner join lateral
 (
   select count(*) nb from B where B.id=a.id
 ) c on 1=1
order by c.nb 

Answer 5

Solution 4 :

SELECT a.id, ifnull(c.nb, 0) nb 
FROM A 
left outer join 
(
   select b.id, count(*) nb from B group by b.id
) c on a.id=c.id
order by ifnull(c.nb, 0)

Answer 6

Solution 5:

with c as (
select b.id, count(*) nb from B group by b.id
)
SELECT a.id, ifnull(c.nb, 0) nb 
FROM A left outer join c on a.id=c.id
order by ifnull(c.nb, 0)