Importing into Existing Tables
This section describes factors to take into account when you import data into existing tables. 

Manually Creating Tables Before Importing Data
When you choose to create tables manually before importing data into them from an export file, you should use either the same table definition previously used or a compatible format. For example, although you can increase the width of columns and change their order, you cannot do the following: 

Add NOT NULL columns 

Change the datatype of a column to an incompatible datatype (LONG to NUMBER, for example) 

Change the definition of object types used in a table 

Change DEFAULT column values 

Disabling Referential Constraints
In the normal import order, referential constraints are imported only after all tables are imported. This sequence prevents errors that could occur if a referential integrity constraint existed for data that has not yet been imported. 

These errors can still occur when data is loaded into existing tables. For example, if table emp has a referential integrity constraint on the mgr column that verifies that the manager number exists in emp, a perfectly legitimate employee row might fail the referential integrity constraint if the manager's row has not yet been imported. 

When such an error occurs, Import generates an error message, bypasses the failed row, and continues importing other rows in the table. You can disable constraints manually to avoid this. 

Referential constraints between tables can also cause problems. For example, if the emp table appears before the dept table in the export file, but a referential check exists from the emp table into the dept table, some of the rows from the emp table may not be imported due to a referential constraint violation. 

To prevent errors like these, you should disable referential integrity constraints when importing data into existing tables. 

Manually Ordering the Import
When the constraints are reenabled after importing, the entire table is checked, which may take a long time for a large table. If the time required for that check is too long, it may be beneficial to order the import manually. 

To do so, perform several imports from an export file instead of one. First, import tables that are the targets of referential checks. Then, import the tables that reference them. This option works if tables do not reference each other in a circular fashion, and if a table does not reference itself.