ALTER TABLE tablename
ALTER [COLUMN] gencolname [TYPE <datatype>]
{ GENERATED ALWAYS AS | COMPUTED [BY]} (<col_expr>)

/* Empty SP_GET_OWNERS for drop REGISTRATION(F_BYDATE) */
ALTER PROCEDURE SP_GET_OWNERS(CODE_HORSE INTEGER)
 RETURNS(FARM VARCHAR(100),
FARM_EN VARCHAR(100),
ADDRESS VARCHAR(100),
ADDRESS_EN VARCHAR(100),
EMAIL VARCHAR(50),
PHONE VARCHAR(50),
FAX VARCHAR(50),
PASSPORT VARCHAR(70),
COUNTRY VARCHAR(50),
COUNTRY_EN VARCHAR(50),
OWNERSHIP VARCHAR(50),
BYDATE CHAR(10),
REMARK BLOB SUB_TYPE 1)
 AS
 BEGIN SUSPEND; END
^

SET TERM ; ^

ALTER TABLE REGISTRATION DROP F_BYDATE;

ALTER TABLE REGISTRATION ADD F_BYDATE COMPUTED BY (
CASE
    WHEN BYDATE_MASK = 'XX.XX.XXXX' THEN NULL
    WHEN BYDATE_MASK = 'XX.XX.YYYY' THEN EXTRACT(YEAR FROM BYDATE)
    WHEN BYDATE_MASK = 'XX.YY.YYYY' THEN LPAD(EXTRACT(MONTH FROM BYDATE), 2, '0') || '.' ||
                                  CAST(EXTRACT(YEAR FROM BYDATE) AS CHAR(4))
    WHEN BYDATE_MASK = 'YY.YY.YYYY' THEN LPAD(EXTRACT(DAY FROM BYDATE), 2, '0') || '.' ||
                                  LPAD(EXTRACT(MONTH FROM BYDATE), 2, '0') || '.' ||
                                  CAST(EXTRACT(YEAR FROM BYDATE) AS CHAR(4))
END);

ALTER TABLE REGISTRATION ALTER F_BYDATE COMPUTED BY (
CASE
    WHEN BYDATE_MASK = 'XX.XX.XXXX' THEN NULL
    WHEN BYDATE_MASK = 'XX.XX.YYYY' THEN EXTRACT(YEAR FROM BYDATE)
    WHEN BYDATE_MASK = 'XX.YY.YYYY' THEN LPAD(EXTRACT(MONTH FROM BYDATE), 2, '0') || '.' ||
                                  CAST(EXTRACT(YEAR FROM BYDATE) AS CHAR(4))
    WHEN BYDATE_MASK = 'YY.YY.YYYY' THEN LPAD(EXTRACT(DAY FROM BYDATE), 2, '0') || '.' ||
                                  LPAD(EXTRACT(MONTH FROM BYDATE), 2, '0') || '.' ||
                                  CAST(EXTRACT(YEAR FROM BYDATE) AS CHAR(4))
END);