Postgres 13

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CREATE TABLE tbl_ab(ab text PRIMARY KEY);

CREATE TABLE tbl (
  a text REFERENCES tbl_ab ON UPDATE CASCADE ON DELETE CASCADE
, b text REFERENCES tbl_ab ON UPDATE CASCADE ON DELETE CASCADE
);

-- function to insert
CREATE OR REPLACE FUNCTION f_tbl_insert(_a text, _b text)
 RETURNS void
 LANGUAGE sql AS
$func$
WITH ins_ab AS (
   INSERT INTO tbl_ab(ab)
   SELECT _a WHERE _a IS NOT NULL  -- NULL is allowed (?)
   UNION ALL
   SELECT _b WHERE _b IS NOT NULL
   )
INSERT INTO tbl(a,b)
VALUES (_a, _b);
$func$;
SELECT f_tbl_insert('x', 'y');


f_tbl_insert


null


-- NULL values are allowed
SELECT f_tbl_insert(NULL, NULL);


f_tbl_insert


null


TABLE tbl;


a
b


x
y

null
null


TABLE tbl_ab;


ab


x

y


-- not allowed
SELECT f_tbl_insert('x', 'y');
ERROR:  duplicate key value violates unique constraint "tbl_ab_pkey"
DETAIL:  Key (ab)=(x) already exists.
CONTEXT:  SQL function "f_tbl_insert" statement 1


-- not allowed
SELECT f_tbl_insert('y', NULL)
ERROR:  duplicate key value violates unique constraint "tbl_ab_pkey"
DETAIL:  Key (ab)=(y) already exists.
CONTEXT:  SQL function "f_tbl_insert" statement 1


-- not allowed
SELECT f_tbl_insert(NULL, 'x')
ERROR:  duplicate key value violates unique constraint "tbl_ab_pkey"
DETAIL:  Key (ab)=(x) already exists.
CONTEXT:  SQL function "f_tbl_insert" statement 1
a	b
x	y
null	null