[sql] PLS-00428: an INTO clause is expected in this SELECT statement

I am wanting to store a Rownum as a variable rather than use a costly Join. I need to get this from a Select statement as the Rownum will be different on various environments so it cannot be a literal string in the code.

For context, this query is executed on Oracle Siebel CRM schema and it retrieves some products of specific type and attributes.

I tried using the following SQL code in Toad and Oracle SQL Developer, however I am getting the following error:

PLS-00428: an INTO clause is expected in this SELECT statement

Here is the code

    DECLARE
    PROD_ROW_ID varchar(10) := NULL;
BEGIN
    SELECT ROW_ID INTO VIS_ROW_ID FROM SIEBEL.S_PROD_INT WHERE PART_NUM = 'S0146404';

    BEGIN
        SELECT rtrim(VIS.SERIAL_NUM) || ',' || rtrim(PLANID.DESC_TEXT) || ',' ||
               CASE
               WHEN PLANID.HIGH = 'TEST123'
               THEN
                   CASE
                   WHEN to_date(PROD.START_DATE) + 30 > sysdate
                   THEN 'Y'
                   ELSE 'N'
                   END
               ELSE 'N'
               END
               || ',' || 'GB' || ',' ||
               rtrim(to_char(PROD.START_DATE, 'YYYY-MM-DD'))
        FROM SIEBEL.S_LST_OF_VAL PLANID
            INNER JOIN SIEBEL.S_PROD_INT PROD
                ON PROD.PART_NUM = PLANID.VAL
            INNER JOIN SIEBEL.S_ASSET NETFLIX
                ON PROD.PROD_ID = PROD.ROW_ID
            INNER JOIN SIEBEL.S_ASSET VIS
                ON VIS.PROM_INTEG_ID = PROD.PROM_INTEG_ID
            INNER JOIN SIEBEL.S_PROD_INT VISPROD
                ON VIS.PROD_ID = VISPROD.ROW_ID
        WHERE PLANID.TYPE = 'Test Plan'
              AND PLANID.ACTIVE_FLG = 'Y'
              AND VISPROD.PART_NUM = VIS_ROW_ID
              AND PROD.STATUS_CD = 'Active'
              AND VIS.SERIAL_NUM IS NOT NULL;
    END;
END;
/