where oracle foreign key constraint named SYS_C comming from - oracle

Question

my database is oracle 10.2 and my create table sql is like this:
create table T_EP_SYS_COMPANY
(
company_id NUMBER not null,
company_name VARCHAR2(30),
company_address VARCHAR2(100),
is_in_use VARCHAR2(1),
is_canceled VARCHAR2(1),
is_headquarter VARCHAR2(1),
account_id NUMBER not null
)
tablespace USERS
pctfree 10
maxtrans 255
storage
(
initial 64K
minextents 1
maxextents unlimited
);
alter table T_EP_SYS_COMPANY
add constraint PK_ESHOP_SYS_COMPANY primary key (COMPANY_ID)
using index
tablespace USERS
pctfree 10
initrans 2
maxtrans 255
storage
(
initial 64K
minextents 1
maxextents unlimited
);
alter table T_EP_SYS_COMPANY
add constraint FK_SYS_COMPANY_PAY_ACCOUNT foreign key (ACCOUNT_ID)
references T_EP_PAY_ACCOUNT (ACCOUNT_ID);
but in my database,I have to 2 FKs on column "account_id":
owner constraint_name table_name column_name position
ESHOPV2 SYS_C009725 T_EP_SYS_COMPANY ACCOUNT_ID null
ESHOPV2 FK_SYS_COMPANY_PAY_ACCOUNT T_EP_SYS_COMPANY ACCOUNT_ID 1
why there are 2 FKs? I did rename the table name,dose the rename ddl has something to do with this?

Answer 1

They aren't both foreign key constraints. The SYS_C is a system-generated name for a constraint you didn't explicitly name; in this case for your not-null check. You can see those immediately after the create:
create table T_EP_SYS_COMPANY
(
company_id NUMBER not null,
company_name VARCHAR2(30),
company_address VARCHAR2(100),
is_in_use VARCHAR2(1),
is_canceled VARCHAR2(1),
is_headquarter VARCHAR2(1),
account_id NUMBER not null
);
select uc.constraint_name, uc.constraint_type, ucc.column_name, ucc.position
from user_constraints uc
join user_cons_columns ucc on ucc.constraint_name = uc.constraint_name
where uc.table_name = 'T_EP_SYS_COMPANY';
CONSTRAINT_NAME CONSTRAINT_TYPE COLUMN_NAME POSITION
------------------------------ --------------- --------------- --------
SYS_C0093988 C COMPANY_ID
SYS_C0093989 C ACCOUNT_ID
The constraint type is C, showing it's a check constraint. You can name those by explicitly adding check constraints rather than specifying them as 'not null' but there is no real benefit - you don't ever need to refer to them by name, e.g. to temporarily disable them.
After you add the primary and foreign key you see those too:
alter table T_EP_SYS_COMPANY
add constraint PK_ESHOP_SYS_COMPANY primary key (COMPANY_ID);
alter table T_EP_SYS_COMPANY
add constraint FK_SYS_COMPANY_PAY_ACCOUNT foreign key (ACCOUNT_ID)
references T_EP_PAY_ACCOUNT (ACCOUNT_ID);
select uc.constraint_name, uc.constraint_type, ucc.column_name, ucc.position
from user_constraints uc
join user_cons_columns ucc on ucc.constraint_name = uc.constraint_name
where uc.table_name = 'T_EP_SYS_COMPANY';
CONSTRAINT_NAME CONSTRAINT_TYPE COLUMN_NAME POSITION
------------------------------ --------------- --------------- --------
SYS_C0093988 C COMPANY_ID
SYS_C0093989 C ACCOUNT_ID
PK_ESHOP_SYS_COMPANY P COMPANY_ID 1
FK_SYS_COMPANY_PAY_ACCOUNT R ACCOUNT_ID 1
These have constraint type P and R, for 'primary key' and 'referential integrity'.
Read more about types of integrity constraints and how they are shown in the data dictionary.

Answer 2

owner constraint_name table_name column_name position
ESHOPV2 SYS_C009725 T_EP_SYS_COMPANY ACCOUNT_ID null
ESHOPV2 FK_SYS_COMPANY_PAY_ACCOUNT T_EP_SYS_COMPANY ACCOUNT_ID 1
why there are 2 FKs?
SYS_C009725 is not a foreign key constraint. It is a CHECK constraint for the NOT NULL condition. If you see the CONSTRAINT_TYPE then you would see it as C. And the name as SYS_C you see is because it is system-generated name.
Too keep it simple, let's see a small example:
SQL> CREATE TABLE t(
2 ID NUMBER NOT NULL
3 );
Table created.
SQL>
SQL> SELECT constraint_name,
2 constraint_type,
3 table_name,
4 search_condition
5 FROM user_constraints
6 WHERE table_name ='T';
CONSTRAINT_NAME CONSTRAINT_TYPE TABLE_NAME SEARCH_CONDITION
--------------- --------------- ---------- ----------------
SYS_C0010726 C T "ID" IS NOT NULL
SQL>