To ensure that SQL*Plus insert statements properly encode special characters (for example, German umlauts like ä, ö, ü) and consequently correctly store in the database, set environment variable NLS_LANG to .AL32UTF8 on the client/server where you run SQL*Plus, as follows:
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Open the Environment Variables dialog.
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Click New to create a new variable.
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Type NLS_LANG in the Variable name box.
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Type .AL32UTF8 in the Variable value box.
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Click OK.