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A Data Connection represents the database server information required to log in (connect) to a database, and it also contains the relation for the underlying data. The relation may be a single table, a multi-table join, or a Custom SQL query that yields a result set.
A Data Source represents the view of this data connection as it appears in the 'Data' pane on the left side of the Desktop product interface. This includes any changes you make to the fields, such as their caption, aliases, geographic role, default aggregation, etc. Additionally, a Data Source represents fields which do not exist in the underlying Data Connection, such as calculated fields, ad-hoc groups, sets, and Tableau-generated fields such as 'Measure Names', 'Measure Values', 'Latitude (generated)', and 'Longitude (generated)'. Collectively, this is sometimes referred to as the data model.
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