string example1 = "NULL";
string query1 = "INSERT INTO [table] VALUES (" + example1 + ")"; // there are no single quotes around the value here
string example2 = "hello world!";
string query2 = "INSERT INTO [table] VALUES ('" + example2 + "')"; // there are single quotes around the value here because you are passing an actual value, and not NULL
string this_is_the_string_null = "'NULL'"; // single quotes inside of double-quotes
string this_is_DB_null = "NULL"; // double quotes only
e.g.
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