“ORA-01427 single-row subquery returns more than one row” without having a subquery in procedure

Question

I have a procedure that throws the error "ORA-01427: single-row subquery returns more than one row". When I use the statemt as a simple SELECT I get 1 row per ID.
I researched the ORA-01427, however I couldn't really apply the answers to my error, since from what I understand I don't have a subquery.

CREATE OR REPLACE PROCEDURE CALC_SLOPE (TBL_NAME IN VARCHAR2) AS 
BEGIN

  EXECUTE IMMEDIATE 'ALTER TABLE '||TBL_NAME||' ADD (SLOPE_MEDIAN NUMBER(2,2),
                                                     SLOPE_75 NUMBER(2,2),
                                                     SLOPE_90 NUMBER(2,2))';

  EXECUTE IMMEDIATE 'UPDATE '||TBL_NAME||' a1 SET(SLOPE_MEDIAN, SLOPE_75, SLOPE_90)
  =(
    SELECT ROUND(MEDIAN(b.SLOPE),2) AS SLOPE_MEDIAN, 
    ROUND(PERCENTILE_CONT(0.75) WITHIN GROUP (ORDER BY b.slope DESC),2) AS SLOPE_75,
    ROUND(PERCENTILE_CONT(0.9) WITHIN GROUP (ORDER BY b.slope DESC),2) AS SLOPE_90
    FROM '||TBL_NAME||' a2,
    bbx_slope b
    WHERE SDO_CONTAINS(a2.GEOMETRY, b.POINT) = ''TRUE''
    GROUP BY a2.ID
    )';

END CALC_SLOPE;  

Where lies the reason for the error and how can it be fixed?

I am working on WINDOWS 10 on ORACLE 12c Enterprise Edition.

Answer 1

Right now You trying to update single row:

 SET(SLOPE_MEDIAN, SLOPE_75, SLOPE_90)

with your select result. If your SELECT returns 50 rows, this 50 rows ORACLE tries to put into this single row. Your missing in WHERE clause some conditions on ID column

Answer 2

I think you wanted a correlated update

..
FROM '
 || tbl_name || ' a2 JOIN bbx_slope b
ON SDO_CONTAINS(a2.GEOMETRY, b.POINT) = ''TRUE''
WHERE a2.ID = a1.id --This one
GROUP BY a2.ID
..