If you ever get lucky to find the answer, please, post it here.
Most of us are rarely faced those specific features, but it may be useful to better know them.
CREATE TABLE SCH0.T020
( T20C01 SMALLINT NOT NULL PRIMARY KEY
, T20C02 CHAR(8) NOT NULL
, T20C03 DATE NOT NULL
, T20C04 INTEGER NOT NULL
, T20C05 SMALLINT NOT NULL
, UNIQUE (T20C04, T20C05)
)
;
CREATE UNIQUE INDEX SCH0.UT020
ON SCH0.T020 (T20C04 ASC, T20C05 ASC)
USING STOGROUP
[. . .]
;
CREATE TABLE SCH0.T021
( T21C01 SMALLINT NOT NULL PRIMARY KEY
, T21C02 INTEGER
, T21C03 SMALLINT
, T21C04 DATE NOT NULL
, FOREIGN KEY FK21AK20(T21C02, T21C03)
REFERENCES SCH0.T020(T20C04, T20C05)
ON DELETE SET NULL
)
;