Join on second table if value not found in first table - sql

Question

I would like to join on a second table only if the results of the first join are blank. Below is a subsection of Table A data:
ID Metro Submarket
1 NYC Manhattan
2 NYC Brooklyn
3 NYC Queens
4 NYC Bronx
5 NYC Newark
The tables I'm using for the joins are:
Table B Table C
Metro Submarket A.Price B.Price C.Price Metro A.Price B.Price C.Price
NYC Manhattan 54 32 48 NYC 50 49 69
NYC Queens 35 39 59 Philly 49 48 37
NYC Brooklyn 20 49 58 Chicago 20 48 36
NYC Bronx 49 30 20
NYC Newark 49 50 -
I'm adding the Price columns from Table B to Table A based on a Metro and Submarket match. However, Table B doesn't have all the prices. If I can't find a match in Table B then I want to look into Table C for a match only on Metro.
For ID 5, we can find the A and B prices in Table B. However, the C price is blank. In that case, I want it to retrieve the C price from Table C (69 is what it would choose).
I'm using SAS 9.4. SQL, macros, or anything else SAS can handle is welcome!

Answer 1

You can left join both tables to the main table and simply use COALESCE(). This will give you the value if present in Table B, otherwise it will give you the value in Table C:
PROC SQL;
CREATE TABLE Output AS
SELECT
ta.ID,
ta.Metro,
ta.Submarket,
COALESCE(tb.A_Price,tc.A_Price) AS A_Price,
COALESCE(tb.B_Price,tc.B_Price) AS B_Price,
COALESCE(tb.C_Price,tc.C_Price) AS C_Price
FROM
tablea ta
LEFT JOIN
tableb tb
ON (tb.Metro = ta.Metro)
AND (tb.Submarket = ta.Submarket)
LEFT JOIN
tablec tc
ON (tc.Metro = ta.Metro);
QUIT;