在FULL OUTER JOIN中选择非空列

Question

Consider the following tables:

Id     |   Name     |   Family
1      |   name1    |   family1
2      |   name2    |   family2

And:

Id     |   Orderr   |   Countt
1      |   order1   |   17
1      |   order2   |   18
3      |   order3   |   16

And the following query:

select table1.id,table1.name,table1.family,table2.orderr,table2.countt
from table1 FULL OUTER JOIN table2
on table1.id = table2.id

It returns:

Id     |   Name     |   Family    |   Orderr   |   Countt
1      |   name1    |   family1   |   order1   |    17
1      |   name1    |   family1   |   order2   |    18
2      |   name2    |   family2   |   NULL     |    NULL
NULL   |   NULL     |   NULL      |   order3   |    16

As you see in the last row it doesn't show the Id column. How can I change my query to return the Id column in the last row? I don't want to include table2.id in my select query because in this way I will have tow Id column.

Answer 1

Use COALESCE or ISNULL

COALESCE "returns the first non-null expression among its arguments," and ISNULL "replaces NULL with the specified replacement value."

Analysis in details at http://sqlmag.com/t-sql/coalesce-vs-isnull

SELECT ISNULL(table1.id, table2.id) AS id,
table1.name,table1.family,table2.orderr,table2.countt
FROM table1 FULL OUTER JOIN table2
ON table1.id = table2.id

OR

SELECT COALESCE(table1.id, table2.id) AS id,
table1.name,table1.family,table2.orderr,table2.countt
FROM table1 FULL OUTER JOIN table2
ON table1.id = table2.id

Answer 2

Use coalesce

select coalesce(table1.id, table2.id) as id 

It returns the first non-null value in the list.