Combining Columns from different tables - sql

Question

I've write a SQL code to combine several columns from different tables.
SELECT *
FROM
(
SELECT PD_BARCODE
FROM docsadm.PD_BARCODE
WHERE SYSTEM_ID = 11660081
) t,
(
SELECT A_JAHRE
FROM docsadm.A_PD_DATENSCHUTZ
WHERE system_ID = 2066
) t2,
(
SELECT PD_PART_NAME
FROM docsadm.PD_FILE_PART
WHERE system_id = 11660082
) t3;
code works fine but if one of my where clause is not found in a table,the result is null even the other columns have value. How you can solve this problem?

Answer 1

It looks like you are doing a cross join between the three subquery tables. This would probably only yield output which makes sense if each subquery return a single value. I might suggest instead that you use a UNION ALL here:
SELECT ISNULL(PD_BARCODE, 'NA' AS value
FROM docsadm.PD_BARCODE WHERE SYSTEM_ID = 11660081
UNION ALL
SELECT ISNULL(A_JAHRE, 'NA')
FROM docsadm.A_PD_DATENSCHUTZ WHERE system_ID = 2066
UNION ALL
SELECT ISULL(PD_PART_NAME, 'NA')
FROM docsadm.PD_FILE_PART WHERE system_id = 11660082
The above union query might require a slight modification if the three columns being select don't all have the same type (which I assume to be varchar in my query).
If you really need these three points of data as separate columns, then you can just include the three subqueries as items in an outer select:
SELECT
(SELECT ISNULL(PD_BARCODE, 'NA')
FROM docsadm.PD_BARCODE WHERE SYSTEM_ID = 11660081) AS PD_BARCODE,
(SELECT ISNULL(A_JAHRE, 'NA')
FROM docsadm.A_PD_DATENSCHUTZ WHERE system_ID = 2066) AS A_JAHRE,
(SELECT ISNULL(PD_PART_NAME, 'NA')
FROM docsadm.PD_FILE_PART WHERE system_id = 11660082) AS PD_PART_NAME;
Note that as the above is written we simply including the subqueries as values in the select statement. But as you wrote your original query, you are joining the subqueries as separate tables.

Answer 2

Here is Query.
You can replace the word 'empty' by your required word or value
SELECT isnull(
(
SELECT PD_BARCODE
FROM docsadm.PD_BARCODE
WHERE SYSTEM_ID = 11660081
), 'Empty') AS PD_BARCODE,
isnull(
(
SELECT A_JAHRE
FROM docsadm.A_PD_DATENSCHUTZ
WHERE system_ID = 2066
), 'Empty') AS A_JAHRE,
isnull(
(
SELECT PD_PART_NAME
FROM docsadm.PD_FILE_PART
WHERE system_id = 11660082
), 'Empty') AS PD_PART_NAME;

Answer 3

This is a special case since you would be getting only one value per query of yours which you are trying to put up as column. In this case, you can use SQL PIVOT clause with UNION ALL of your queries like shown below. MIN can be used for aggregation in this case.
This would mean, you can get your data row-wise as you like, even for multiple different fields and then pivot it into columns in one go.
SELECT * FROM
(
SELECT 'PD_BARCODE' as KeyItem, PD_BARCODE Name from docsadm.PD_BARCODE
where SYSTEM_ID=11660081
UNION ALL
SELECT 'A_JAHRE', A_JAHRE from docsadm.A_PD_DATENSCHUTZ
where system_ID=2066
UNION ALL
select 'PD_PART_NAME', PD_PART_NAME from docsadm.PD_FILE_PART
where system_id=11660082
) VTABLE
PIVOT(MIN(Name)
FOR KeyItem IN ([PD_BARCODE], [A_JAHRE], [PD_PART_NAME])) as pivotData;