For example , suppose there are 5 columns says A,B,C,D,E and combination of the columns A and B constitute the Primary key for this table.
Suppose, there is no transitive dependencies are existed in this table, but column C can determine the value of column B. In other words, the value of Column B is determined by the Column C then this table can not be in the Boyce - Codd Normal Form. To make this table in BCNF we needs to divide this table into two tables "table 1" and "table2" . The table1 will contains the following columns A, B, D,E where primary key is the combination of the column A and B and table2 will contains the two columns B , C where C will be the primary key.
Table 1 (Primary key :-A,B) Table2 (Primary Key :- C)
Thus every table satisfying the 3NF condition cannot be said to be in BCNF normal form but a table in BCNF will always be in 3NF form.