Composite foreign key - null values

We had a strange problem. Obviously our tables are designed with composite primary keys, foreign keys in referencing tables are sometimes null allowed. Unfortunately we updated several rows with bad (incomplete) FK , we were surprised that we could do it without error - RI violation. So we made some tests example table A column1 PK column2 PK column3 PK table B column1 null allowed FK --> references table A column2 null allowed FK column3 null allowed FK we run several statements insert into tableB values (null,"xxxx",null),... etc even update tableB set B.column1=null where column2 is not null without any error. The last one was the most serious and dangerous it destroyed data. We were rather confused because we thought that even incomplete foreign key is considered as referenced value. Primary key can't be null so FK written in such a way (part of FK null, the rest not null) must ( we thought) evoke error -691...missing key in referenced table... We tested it in Informix On line V 7.20 DSA Informix On line V 7.22 WGS Informix On line V 5.01 OS Sinix primary keys were created in all possible ways -- create index ... then alter table ... add constraint primary key -- alter table ( without previous create index) -- create index ... alter table add constraint unique ... -- alter table add constraint unique ( without explicit. created index) It seems to be a feature. We appreciate any information,hints how RI in Informix works as far as composite foreign key with several null columns concerned. katarina -- Katarina Hamzova Voice : +42 7 5384324 SWH s.r.o Bratislava Slovakia Katarina.Hamzova@swh.sk