Tableau supports below data types:

• Text values
• Date values
• Date and time values
• Numerical values
• Geographical values (latitude and LONGITUDE used for maps)
• Boolean values (true/false, 0/1 type of conditions)

Data type can be changed for a data type either on the “Data source” page or in the “Data” pane in Tableau. For example, if CURRENT data type is defined as “text value” and this needs to be changed, then data type in “Data source” page will provide dropdown list options of other data types such as “Number (decimal)”, “Number (WHOLE)”, “Data & Time”, “Date”, “String”, “Boolean”. By DEFAULT, the data type would be defined as a “Text value”.

It is important to change the data type so that it reflects correctly while we generate an extract.

When Tableau reads from data sources such as files in Microsoft excel or csv (comma separated value) formats, then it assigns data type automatically for respective values such as text/strings, date field, date and time field, Boolean field and so on. Additionally, if we source data from a csv with some mixed-value COLUMN is mapped to a single data type field in Tableau, then Tableau may assign one of the data type. For example a mixed value could be a mixture of numbers and dates, and they can be mapped to a number data type or date data type.

Suggested approach is to handle mixed value columns in a way one can format empty cells within the data source so that data type of that field can be matched and also to create additional new field that does not contain mixed values so that it is of a particular data type.