Merge two tables in SQL, with one common column

Question

I'm trying to merge to tables in MSSQL, as in the sample below. My situation is similiar to this question. T-SQL merging two tables with common and uncommon columns

I have tried using the following query:

select coalesce(t1.a, t2.a), t1.b, t1.c, t2.d, t2.k
from Table1_test t1
full outer join Table2_test t2 on t2.a = t1.a

But it results in this, where the rows that have the same value in column a, merges in the same row

a   b   c      d    k
1   2   3      5    6
7   8   9     null  null
4 null  null   6    7
9 null  null   0    2

Table1_test:

a b c
1 2 3
7 8 9 

Table2_test:

a d k
4 6 7
1 0 2

Merged Table I want:

a   b   c      d    k
1   2   3     null  null
7   8   9     null  null
4 null  null   6    7
1 null  null   0    2

Answer 1

You can use the following using UNION ALL :

SELECT a, b, c, NULL AS d, NULL AS k FROM table1_test
UNION ALL
SELECT a, NULL, NULL, d, k FROM table2_test

You can also use the FULL OUTER JOIN with a false match condition ( 1 = 2 ):

SELECT COALESCE(t1.a, t2.a) AS a, t1.b, t1.c, t2.d, t2.k
FROM Table1_test t1 FULL OUTER JOIN Table2_test t2 ON 1 = 2

You can also generate the above SQL query (in case there are a lot of columns, or you don't want to pick the common / uncommon column names yourself):

DECLARE @sql VARCHAR(MAX) = 'SELECT ' + (
  SELECT STUFF((SELECT ',' + + col FROM (
    SELECT CASE WHEN i1.t IS NOT NULL AND i2.t IS NOT NULL THEN 'COALESCE(' + i1.t + '.' + i1.COLUMN_NAME + ', ' + i2.t + '.' + i2.COLUMN_NAME + ') AS ' + i1.COLUMN_NAME ELSE COALESCE(i1.COLUMN_NAME, i2.COLUMN_NAME) END AS col, COALESCE(i1.COLUMN_NAME, i2.COLUMN_NAME) cname FROM (
    SELECT 't1' AS t, COLUMN_NAME FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME = 'Table1_test'
    ) i1 FULL OUTER JOIN (
    SELECT 't2' AS t, COLUMN_NAME FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME = 'Table2_test'
    ) i2 ON i1.COLUMN_NAME = i2.COLUMN_NAME

  ) cols ORDER BY cname FOR XML PATH(''), TYPE).value('.', 'NVARCHAR(MAX)'), 1, 1, '')
) + ' FROM Table1_test t1 FULL OUTER JOIN Table2_test t2 ON 1 = 2'

EXECUTE(@sql)

You can put the above script into a stored procedure with two parameters to make the script more flexible. You can find an example of the stored procedure on the demo.

demo on dbfiddle.uk