MID returns a specific number of characters from a text string, starting at the position you specify, based on the number of characters you specify. In other hand MID always counts each character, whether single-byte or double-byte, as 1, no matter what the default language setting is. 

Syntax

MID(text, start_num, num_chars) 

MIDB returns a specific number of characters from a text string, starting at the position you specify, based on the number of bytes you specify. In other hand MIDB counts each double-byte character as 2 when you have enabled the editing of a language that supports DBCS (double-byte character set) and then set it as the default language. Otherwise, MIDB counts each character as 1. 

Syntax

MIDB(text, start_num, num_bytes)