Postgres 12

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select version();


version


PostgreSQL 12.8 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.4.1 20200928 (Red Hat 8.4.1-1), 64-bit


 create table vals( a text, b text, c text, d text);
 insert into vals (a,b,c,d)
        values ('A','B','C','D')  
             , ('A',null,'C','A')  
             , ('A',null,'C','D');            
 select * from vals;
3 rows affected



a
b
c
d


A
B
C
D

A
null
C
A

A
null
C
D


 delete 
   from vals 
  where b is null; 
2 rows affected

create or replace function do_nothing_on_vals()
  returns trigger 
  language plpgsql
as $$
begin 
	if exists ( select null 
	              from vals 
	             where (new.a, new.b, new.c) 
	                   is not distinct from 
	                   (a,b,c)
	          ) 
	then 
        return null;
    end if; 
   
    return new;
end ; 
$$;     
 
create trigger vals_bir
  before insert on vals 
  for each row 
  execute function do_nothing_on_vals(); 
 
insert into vals(a,b,c,d) 
       values ('A',null,'C','A'); 
select * from vals; 
1 rows affected



a
b
c
d


A
B
C
D

A
null
C
A


insert into vals(a,b,c,d) 
       values ('A',null,'C','A');
select * from vals;   


a
b
c
d


A
B
C
D

A
null
C
A

a	b	c	d
A	B	C	D
A	null	C	A
A	null	C	D

a	b	c	d
A	B	C	D
A	null	C	A

a	b	c	d
A	B	C	D
A	null	C	A