If you load .csv files through “Add vector layer”, all columns are interpreted as strings. That’s most likely not what you want, but it’s OGR’s default behaviour:
The OGR CSV driver returns all attribute columns with a type of string if no field type information file (with .csvt extension) is available.
Let’s create a .csvt file then!
The .csvt file has to have the same name as the .csv file it describes. (Same concept used in shapefiles.) It enables definition of the following data types: Integer, Real, String, Date (YYYY-MM-DD), Time (HH:MM:SS+nn) and DateTime (YYYY-MM-DD HH:MM:SS+nn).
A .csvt file contains only one line and the types for each column have to be quoted and comma separated, e.g.
You can even specify width and precision of each column, e.g.
Read more at: www.gdal.org/ogr/drv_csv.html