Question

I have two tables that I would like to display as a single result, using UNION or other technique.

The ID field relates both tables.

The second table has one field missing, so that missing value should be assumed from the first table.

The sample code below works, but is very slow for large datasets.

Is there any solution more efficient?

T1:                       T2:
+----+-------+--------+   +----+------+
| id | name  | town   |   | id | name |
+----+-------+--------+   +----+------+
|  1 | Alice | London |   |  1 | Bob  |
|  2 | Alan  | Zurich |   +----+------+
+----+-------+--------+   

Desired result:

+----+-------+--------+
| id | name  | town   |
+----+-------+--------+
|  1 | Alice | London |
|  2 | Alan  | Zurich |
|  1 | Bob   | London |
+----+-------+--------+   

Sample code:

with T1 as
(
    select * from 
    (
        values
            (1,'Alice','London') ,
            (2,'Alan','Zurich') 
    ) as t (id,name,town)
), T2 as
(
    select * from 
    (
        values
            (1,'Bob') 
    ) as t (id,name)
), T2WithTown as
(
    select t2.id,t2.name,t1.town from T2
    inner join T1 on t2.id=t1.id
)

select id,name,town from T1
union
select id,name,town from T2WithTown

Answer 1

The sample data shows both tables have distinct values. So I will prefer UNION ALL

Select ID, Name, Town From T1
UNION ALL
select T2.ID, T2.Name, T1.Town
from T1
INNER JOIN T2 on T2.id = T1.id