Question

I have a poorly normalized set of tables and I am trying to correct this issue. The DB is MS SQL 2005.

Table1 has an id field (record ID), a mainID field (person ID) and a sequence field (and int that determines the sort order)

Table2 has its own id field and a copy of the id for the first record by a person (sequence = 1).

I’ve added a new field to table2 call table2_id and I would like to populate this field with the ID from table2 so that I can do away with table1.mainID. table2 only has a record for one of the records for each person and mainId is set to the id where table1.sequence = 1.

This is the update query I thought would do the job by I’m getting errors

update table1 as a  
set regInfoID = (select b.id 
                 from table2 as b 
                 where b.ref1 = (select c.id 
                                 from table1 as c 
                                 where c.mainID = a.mainID 
                                       and sequence = 1))  

I believe I’m on the right track her since the following query works fine

select regInfoID = (select b.id 
                    from table2 as b 
                    where b.ref1 = (select c.id 
                                    from table1 as c 
                                    where c.mainID = a.mainID 
                                          and sequence = 1)), a.*  
from table1 as a  

Answer 1

I think your query is equivalent to this:

update a
set regInfoID = b.id
-- select a.*, b.id
from table2 b 
inner join table1 c on c.id = b.ref1
inner join table1 a on c.mainID = a.mainID and c.sequence = 1

From this query, I think you will potentially have indeterminate results because table2 (b) is not guaranteed to be a single row result. So regInfoID will be set to one of the resulting b.id values.