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nvarchar uses 2 bytes per character, whereas varchar uses 1.
varchar can't store UNICODE data but, nvarchar stores UNICODE data.
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Nvarchar stores UNICODE data. If you have requirements to store UNICODE or multilingual data, nvarchar is the choice. Varchar stores ASCII data and should be your data type of choice for normal use. Regarding memory usage, nvarchar uses 2 bytes per character, whereas varchar uses 1.