I need to change column datatypes in a database table from varchar to nvarchar in order to support Chinese characters (currently, the varchar fields that have these characters are only showing question marks).
I know how to change the values, but I want to see if it's safe to do so. Is there anything to look out for before I do the changing? Thanks!
Note that this change is a size-of-data update, see SQL Server table columns under the hood. The change will add a new NVARCHAR column, it will update each row copying the data from the old VARCHAR to the new NVARCHAR column, and then it will mark the old VARCHAR column as dropped. IF the table is large, this will generate a large log, so be prepared for it. After the update, run DBCC CLEANTABLE to reclaim the space used by the former VARCHAR column. If you can afford it , better run ALTER TABLE ... REBUILD, which will not only reclaim the space it will also completely remove physical deleted VARCHAR column. The linked article at the beginning has more details.
You may also be interested in enabling Unicode Compression for your table.