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The right form for a Unicode character string literal is to prefix the literal with a capital N and delimit the literal with single quotation marks; for example, N‘literal’. For a regular character string literal, you just delimit the literal with single quotation marks; for example, ‘literal’. It’s a very typical bad habit to specify a regular character string literal when the filtered column is of a Unicode type, as in the following example.

SELECT empid, firstname, lastname
FROM HR.Employees
WHERE lastname = 'Davis';

From

Cover of Training Kit (Exam 70-461): Querying Microsoft® SQL Server® 2012

Note

When working in unicode data types, the N prevents SQL Server from using resources to execute an implicit conversion.