Hi, I've found a trouble and I've tried to avoid it without success: ------- create table companies ( id serial not null primary key, firm_name text not null, activity text ); create table customers ( id int not null references companies, seller_id int not null references companies, note text, primary key (id,seller_id) ); create table invoices ( seller_id int4 not null references companies, /* who send invoice */ customer_id int4 not null, /* who receive the invoice and pay for it */ invoice_no int4 not null unique, invoice_date date, primary key (seller_id,invoice_no,invoice_date), foreign key (seller_id, customer_id) references customers ); INSERT INTO "companies" ("firm_name","activity") VALUES ('NonSoLoSoft','ASP'); INSERT INTO "companies" ("firm_name","activity") VALUES ('MyFavouriteCustomer','Buy and pay'); INSERT INTO "customers" ("id","seller_id","note") VALUES (2,1,'customer since 1966'); INSERT INTO "invoices" (seller_id,customer_id,invoice_no,invoice_date) values (1,2,1,'now'); ERROR: <unnamed> referential integrity violation - key referenced from invoices not found in customers select * from customers; id | seller_id | note ----+-----------+--------------------- 2 | 1 | customer since 1816 (1 row) ------- Why have I found this ERROR about referential integrity violation, if the record to reference is in the customer table? Thank you in advance, \fer