sqoop_export_columns

sqoop export --connect jdbc:mysql://localhost/retail_export --username root 
--export-dir /user/hive/warehouse/test_hive_import.db/order_items             // Source 
--table order_items_export                                                    // Destination table.
--update-mode allowinsert  //when the source HDFS data has any changes it will over write in destination table
--update-key 'order_item_id,order_id' ---> comma seperated.     
-m 1 
--input-fields-terminated-by '\001' 
--input-null-non-string "null" 
--input-null-string "null" 
--columns  order_item_id ,order_item_order_id, order_item_product_id , order_item_quantity , order_item_subtotal  ,order_item_product_price   
 // we are selecting only required columns.