Question

In a SQL script which is due to end up feeding a SSRS RDL document, I’m trying to leverage CTEs to recursively fetch my data.

The actual SQL code is way too big to copypaste here but I made up this short repro:

declare @temp table
(
id uniqueidentifier not null default(newid()),
name varchar(100) not null default('new'),
value varchar(100) not null default('newValue'),
parentid uniqueidentifier null
)

insert into @temp (id, name, value, parentid) values ('12312312-1234-1234-1234-123412341234', 'o1', 'val1', null)
insert into @temp (id, name, value, parentid) values ('12312312-1234-1234-1234-123412341235', 'o2', 'val2', '12312312-1234-1234-1234-123412341234')
insert into @temp (id, name, value, parentid) values ('12312312-1234-1234-1234-123412341236', 'o3', 'val3', '12312312-1234-1234-1234-123412341234')

;with
    cte(id,name, value, pid, pname, pvalue) as (
        select id, name, value, null, null, null from @temp where parentid is null
        union all
        select r.id, r.name, r.value, a.id, a.name, a.value 
        from @temp r inner join cte a
        on a.id = r.parentid
    )
    select * from cte

The CTE errors out like this (tested in SSMS 2012):

Msg 240, Level 16, State 1, Line 13
Types don't match between the anchor and the recursive part in column "pid" of recursive query "cte".
Msg 240, Level 16, State 1, Line 13
Types don't match between the anchor and the recursive part in column "pname" of recursive query "cte".
Msg 240, Level 16, State 1, Line 13
Types don't match between the anchor and the recursive part in column "pvalue" of recursive query "cte".

I’m pretty sure this is due to the NULL values being selected in the anchor part as opposed to the NOT NULL fields fetched in the recursive part… Can this issue be worked around ?

(I’m open to alternative ways of accomplishing this task.)

Answer 1

Cast the literal null values to match.

;with
cte(id,name, value, pid, pname, pvalue) as (
    select id, name, value, cast(null as uniqueidentifier),
        cast(null as varchar(100)), cast(null as varchar(100))
    from @temp where parentid is null
    union all
    select r.id, r.name, r.value, a.id, a.name, a.value 
    from @temp r inner join cte a
    on a.id = r.parentid
)

Otherwise SQL will assume the default type int for each null which won’t match the second part of the union.