I have created a interactive grid in oracle apex application using two tables (dhj_employee_master and st_tni_master) and st_tni_master has no data in it.
SQL query for interactive grid:

    DHJ_EMPLOYEE_MASTER.EMPH_CD as EMPH_CD,
    DHJ_EMPLOYEE_MASTER.EMPH_EMP_NAME as EMPH_EMP_NAME,
    DHJ_EMPLOYEE_MASTER.SEC_DESC as SEC_DESC,
    ST_TNI_MASTER.TNI_DETAILS as TNI_DETAILS,
    ST_TNI_MASTER.YEAR as YEAR
FROM 
    DHJ_EMPLOYEE_MASTER
LEFT JOIN 
    ST_TNI_MASTER 
ON 
    DHJ_EMPLOYEE_MASTER.EMPH_CD = ST_TNI_MASTER.EMP_CODE
WHERE 
    DHJ_EMPLOYEE_MASTER.SEC_DESC = :P18_DEPARTMENT

In this EMPH_CD,EMPH_EMPH_NAME,SEC_DESC are columns of dhj_employee_master. Now TNI_DETAILS and YEAR columns have no data and can be edited.
So, to save the all the data in st_tni_master i wrote a PL/SQL code:

    -- Merge statement to update or insert based on EMP_CODE and YEAR
    MERGE INTO ST_TNI_MASTER t
    USING (
        SELECT :EMPH_CD AS emp_code,
               :EMPH_EMPH_NAME AS emp_name,
               :YEAR AS year,
               :TNI AS tni_details
        FROM dual
    ) d
    ON (t.EMP_CODE = d.emp_code AND t.YEAR = d.year)
    WHEN MATCHED THEN
        UPDATE SET
            t.EMP_NAME = d.emp_name,
            t.YEAR = d.year,
            t.TNI_DETAILS = d.tni_details
    WHEN NOT MATCHED THEN
        INSERT (EMP_CODE, EMP_NAME, YEAR, TNI_DETAILS)
        VALUES (d.emp_code, d.emp_name, d.year, d.tni_details);

    -- Handling different row statuses (Create, Update, Delete)
    CASE :APEX$ROW_STATUS
        WHEN 'C' THEN
            INSERT INTO ST_TNI_MASTER (EMP_CODE, EMP_NAME, YEAR, TNI_DETAILS)
            VALUES (:EMPH_CD, :EMPH_EMPH_NAME, :YEAR, :TNI);
        
        WHEN 'U' THEN
            UPDATE ST_TNI_MASTER
            SET 
                EMP_NAME = :EMPH_EMPH_NAME,
                YEAR = :YEAR,
                TNI_DETAILS = :TNI
            WHERE ROWID = :ROWID;

        WHEN 'D' THEN
            DELETE FROM ST_TNI_MASTER
            WHERE ROWID = :ROWID;
    END CASE;

    -- Commit changes
    COMMIT;

EXCEPTION
    WHEN OTHERS THEN
        -- Raise application error with SQL error message
        RAISE_APPLICATION_ERROR(-20010, SQLERRM);
END;

So that when EMP_CODE of st_tni_master has no match to EMPH_CD of dhj_employee_master then it will create the data entry in the st_tni_master. After clicking save button the columns which edited vanishes and page also says changes saved but in the table st_tni_master no data entry is made. Can someone guide me how to do it??

Trying to sava data from dhj_employee_master and edited columns of st_tni-Master in st_tni_master finally.