SQL Join two tables with different number of rows get all rows from one table - sql

Question

I'm trying to join two tables with a different number of rows. Client Number is an identifying field in both tables.
The first table includes the client number for all clients.
so either could be referenced). To complicate things further, the ClientNumber2 field is a text field but I need it to be a number like the other client number fields. There is also a region field in the second table that I need to limit to certain region numbers.
The second table includes the client number only for clients who meet certain specifications. Some of the clients in the second table have a second client number.
I want to create a new column that shows the client number from the first table for all clients who do not exist in the second table, shows the second client number in the second table if it exists, otherwise, the client number from the first table (which is the same as client number in the first column of the second table
I've included the syntax I'm using below. It runs without errors. The OriginalCN field returns the desired value for those with a value in ClientNumber2 of Table 2 but is returning null values for all others. I cannot figure out how to get it to work correctly. I've also included sample tables and my desired table. Any help is greatly appreciated!
CLIENT TABLE 1
CLIENT NUMBER
1
2
3
4
5
6
7
8
CLIENT TABLE 2
CLIENT NUMBER 2ND CLIENT NUMBER REGION
2 14 1
6 2
8 15 2
DESIRED RESULT
1
14
3
4
5
6
7
15
Here is the syntax I am using:
SELECT
TABLE2.CLIENTNUMBER
TABLE1.CLIENTNUMBER
CASE
WHEN TABLE2.CLIENTNUMBER IS NULL THEN TABLE1.CLIENTNUMBER
WHEN TABLE2.CLIENTNUMBER2 IS NULL THEN TABLE2.CLIENTNUMBER
WHEN TABLE2.CLIENTNUMBER2 = ' ' THEN TABLE2.CLIENTNUMBER
ELSE
CAST(TABLE2.CLIENTNUMBER2 AS INT)
END AS OriginalCN
FROM DSS.DBO.TABLE1
LEFT OUTER JOIN
RPTO.DBO.TABLE2
ON
DSS.DBO.TABLE1.CLIENTNUMBER = RPTO.DBO.TABLE2.CLIENTNUMBER
WHERE TABLE2.REGION IN (1,2,3)

Answer 1

Try this:
SELECT COALESCE(CAST(NULLIF(t2.ClientNumber2,' ') As Int), t1.ClientNumber) As ClientNumber
FROM DSS.DBO.TABLE1 t1
LEFT JOIN RPTO.DBO.TABLE2 t2 ON t1.CLIENTNUMBER = t2.CLIENTNUMBER
WHERE COALESCE(t2.REGION, 1) IN (1,2,3)

Answer 2

Try this..
CREATE TABLE #CLIENT_TABLE_1
(CLIENTNUMBER INT)
INSERT #CLIENT_TABLE_1
VALUES (1),(2),(3),(4),(5),(6),(7),(8)
CREATE TABLE #CLIENT_TABLE_2
(CLIENTNUMBER INT,scNDCLIENTNUMBER varchar(10),REGION INT)
INSERT #CLIENT_TABLE_2
VALUES( 2,'14',1),(6,' ',2),(8,'15',2)
SELECT CASE
WHEN b.CLIENTNUMBER IS NOT NULL
AND len(b.scNDCLIENTNUMBER)>0 THEN b.scNDCLIENTNUMBER
ELSE a.CLIENTNUMBER
END Result
FROM #CLIENT_TABLE_1 a
LEFT JOIN #CLIENT_TABLE_2 b
ON a.CLIENTNUMBER = b.CLIENTNUMBER
Output :
+------+
|Result|
+------+
| 1 |
| 14 |
| 3 |
| 4 |
| 5 |
| 6 |
| 7 |
| 15 |
+------+