Question

I have two tables which have the exact same structure. Both tables can store the same data with different primary keys (autoincremented integers). Therefore, there is a third table which lists which two primary keys list the same data. However, there also exist rows which don’t exist in the other. Therefore, a simple join won’t work since you will have two rows with the same primary key but different data. Therefore, is there a way of reassigning primary keys to unused values in the view?

Table1

 ID    name
 1     Adam
 2     Mark
 3     David
 4     Jeremy

Table2

 ID    name
 1     Jessica
 2     Jeremy
 3     David
 4     Mark

Table3

 T1ID  T2ID
 2     4
 3     3
 4     2

I am looking for a result table like the following:

Result

 ID    name
 1     Adam
 2     Mark
 3     David
 4     Jeremy
 5     Jessica

The real heart of the question is how i can assign the temporary fake id of 5 to Jessica and not just some random number. The rule I want for the ids is that if the row exists in the first table, then use its own id. Otherwise, use the next id that an insert statement would have generated (the column is on autoincrement).

Answer 1

Answer to edited question

select id, name from table1
union all
select X.offset + row_number() over (order by id), name
from (select MAX(id) offset from table1) X
cross join table2
where not exists (select * from table3 where t2id = table2.id)

The MAX(id) is used to “predict” the next identity that would occur if you merged the data from the 2nd table into the first. If Table3.T2ID exists at all, it means that it is already included in table1.

Using the test data below

create table table1 (id int identity, name varchar(10))
insert table1 select 'Adam' union all
select 'Mark' union all
select 'David' union all
select 'Jeremy'

create table table2 (id int identity, name varchar(10))
insert table2 select 'Jessica' union all
select 'Jeremy' union all
select 'David' union all
select 'Mark'

create table table3 (t1id int, t2id int)
insert table3 select 2,4 union all
select 3,3 union all
select 4,2

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Answer to original question below

So the 3rd table is the one you want to build (a view instead of a table)?

select newid=row_number() over (order by pk_id), *
from
(
    select a.*
    from tblfirst a
    UNION ALL
    select b.*
    from tblsecond b
) X

The data will contain a unique newid value for each record, whether from first or second table. Change pk_id to your primary key column name.