what is the differene between referential key and foreign key with some sql query examples detaily plssss

In the context of relational databases, a foreign key is a referential constraint between two tables.[1] The foreign key identifies a column or a set of columns in one (referencing) table that refers to a column or set of columns in another (referenced) table. The columns in the referencing table must form a primary key or unique key. The values in one row of the referencing columns must occur in a single row in the referenced table. Thus, a row in the referencing table cannot contain values that don't exist in the referenced table. This way references can be made to link information together and it is an essential part of database normalization. Multiple rows in the referencing table may refer to the same row in the referenced table. Most of the time, it reflects the one (master table, or referenced table) to many (child table, or referencing table) relationship.

1.Reference key is another name for foreign key. Both are same.

2.Examples for foreign key
You may define foreign key at
Table level
Column Level

Example for table level constraint

Create table emp
( Empno Varchar2(6),
ename varchar2(30),
deptno number (2) ,
constriant dept_emp foreign key(deptno) references table(deptno));

Example for column level

Create table emp
( Empno Varchar2(6),
ename varchar2(30),
deptno number (2) references dept(deptno)
);

what are column level constraint and table level constraint?

Column level constraints are those, which we can specify while specifying the column itself.

Table level constrains are those which we can specify after all columns have been specified.

You can't define a NOT NULL constraint at table level in oracle.