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How to retrieve constraints in oracle :

SELECT t.constraint_name, t.constraint_type, ct.column_name
FROM user_constraints t
INNER JOIN user_cons_columns ct on ct.constraint_name = t.constraint_name
WHERE t.table_name = '?' AND t.constraint_type = 'U' AND ct.column_name = '?';

How to Delete all tables, views, constraints and sequences in Oracle SQL :

BEGIN 
--Bye Views!  
FOR i IN (SELECT view_name FROM USER_VIEWS) LOOP 
    EXECUTE IMMEDIATE 'drop view'|| i.view_name ||'';  
END LOOP;  
--Bye Sequences! 
FOR i IN (SELECT sequence_name FROM USER_SEQUENCES) LOOP
    EXECUTE IMMEDIATE 'drop sequence '|| i.sequence_name ||''; 
END LOOP; 

--Bye Constraints!  
FOR i IN (SELECT table_name, constraint_name FROM USER_CONSTRAINTS WHERE constraint_type='r') LOOP 
    EXECUTE IMMEDIATE 'alter table '|| i.table_name ||' DROP CONSTRAINT '||i.constraint_name; 
--Bye Tables! 
FOR i IN (SELECT table_name FROM USER_TABLES) LOOP
    EXECUTE IMMEDIATE 'drop table '|| i.table_name ||' CASCADE CONSTRAINTS '; 
END LOOP; 

END;