Discuss the Extraction Methods in Data Warehouses

This function has to deal with numerous data sources. You have to employ the  appropriate technique for each data source.  Source data may be from different source machines in diverse data formats. Part of the source data may be in relational database system. some data may be on other legacy network and hierarchical data models. many  data sources may still be in flat files. You may want to include data from spreadsheets and local departmental data sets. Data extraction may become quite complex.
Tools are available on the market for data extraction. You may want to consider using outside tools suitable for certain data sources. For the other data sources, you may want to develop in-house programs to do the data extraction. Purchasing outside tools may entail high initial costs. In-house programs, on the other hand, may mean ongoing costs for development and maintenance.
After you extract the data , where do yo keep the data for further preparation? You may perform the extraction function in the legacy platform itself if that approach suits your framework. More frequently, Data warehouse implementation teams extract the source into a separate physical environment form which moving the data into the data warehouse would be easier. In the separate environment, you may extract the source data into a group of flat files, or a data-staging relational database, or a combination of both.