string—String expression from which characters are returned (For strings that contain Null, Null is returned.)
start—Position of the first character to be returned (If the start value is greater than the number of characters in the string, Mid returns a zero-length string.)
length—Number of characters to be returned (If length is omitted or if the value of length exceeds the number of characters between the first character and the end of the string (including the first character), all characters from the start position to the end of the string are returned.)