访问在联合查询中删除重复项的某些部分

Question

I'm working in Access 2007 and know nothing about SQL and very, very little VBA. I am trying to do a union query to join two tables, and delete the duplicates.

BUT, a lot of my duplicates have info in one entry that's not in the other. It's not a 100% exact duplicate.

Example,
Row 1: A, B, BLANK
Row 2: A, BLANK, C

I want it to MERGE both of these to end up as one row of A, B, C.

I found a similar question on here but I don't understand the answer at all. Any help would be greatly appreciated.

Answer 1

I would suggest a query like this:

select
  coalesce(t1.a, t2.a) as a,
  coalesce(t1.b, t2.b) as b,
  coalesce(t1.c, t2.c) as c
from
  table1 t1
  inner join table2 t2 on t1.key = t2.key

Here, I have used the keyword coalesce . This will take the first non null value in a list of values. Also note that I have used key to indicate the column that is the same between the two rows. From your example it looks like A but I cannot be sure.

Answer 2

If your first table has all the key values, then you can do:

select t1.a, nz(t1.b, t2.b), nz(t1.c, t2.c) as c
from table1 as t1 left join
     table2 as t2
     on t1.a = t2.a;

If this isn't the case, you can use this rather arcane looking construct:

select t1.a, nz(t1.b, t2.b), nz(t1.c, t2.c) as c
from table1 as t1 left join
     table2 as t2
     on t1.a = t2.a
union all
select t2.a, t2.b, t2.c
from table2 as t2
where not exists (select 1 from table1 as t1 where t1.key = t2.key)

The first part of the union gets the rows where there is a key value in the first table. The second gets the rows where the key value is in the second but not the first.

Note this is much harder in Access than in other (dare I say "real") databases. MS Access doesn't support common table expressions (CTEs), union s in subqueries, or full outer join -- all of which would help simplify the query.