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The automatic conversion of nchar(2) or nvarchar(10) to char(8)/varchar(40) in IBM DB2 is due to the difference in the way these data types store characters.
The nchar/nvarchar data types are used for storing Unicode characters, which can take up more space than traditional ASCII characters. To accommodate this larger size, the nchar/nvarchar types use a variable-length encoding, which can result in different memory usage depending on the actual data being stored.
On the other hand, the char/varchar data types use a fixed-length encoding, where each character takes up a predetermined amount of space in memory. While this makes them less flexible than the nchar/nvarchar types, it also makes them easier and faster to store and retrieve.
To ensure compatibility and consistency in the database, IBM DB2 automatically converts nchar(2) or nvarchar(10) to char(8)/varchar(40). This ensures that the data is stored in a consistent, predictable format, regardless of the actual data being stored.