Oracle conditions: 1=1 or 1=0, adding conditions dynamically ____oracle

Http://www.cnblogs.com/brolanda/p/4548612.html

See conditions in the WHERE statement: where 1=1 and 1=2 or 1<>1 uses: 1=1: for use with and in conjunction with other conditions when adding conditions (dynamic connection subsequent conditions) such as:

              String sql = "SELECT * from emp where 1=1";
              if (username!=null)
                    sql+= "and username=" +username;
              if (password!=null)
                    sql+= "and password=" +password;

1=2 or 1<>1 discards data in order to get the structure of the table, so that the tables created are the same as the fields in the original table, and others do not guarantee the same (for example, nullable and the original table). Like what:

CREATE TABLE Emp_tmp as SELECT * from EMP where 1=2;

Then import the data again:

INSERT INTO EMP_TMP SELECT * from EMP;