constraints in sql

Constraints are used to prevent in valid data entry into our tables.Constraints are created on table columns.Oracle server supports following types

1)not null

2)unique

3)primary key

4)foreign key

5)check

All the above constraints are defined in 2 levels

i)column level

ii)table level

I)All constraints information stored under user_constraints(data dictionary)

SQL>desc user_constraints

Query:select constraint_type,constraint_name from user_constraints where table_name='EMP';(EMP should be capital)

II)If we want to view constraint names along with column names we are using USER_CONS_COLUMNS.

SQL>desc user_cons_columns

Query:select constraint_type,column_name from user_cons_columns where table_name='EMP';

III)If we want to view logical condition of the check constraint we are using  search_condition property from user_contraints

SQL>desc user_constraints;

Query:select constraint_name,constraint_type,search_condition  from user_constraints where table_name='EMP';