The SUBSTRING function extracts a substring from a source string. If the ending position you specify for the substring is past the end of the source string, the position of the last character of the source string becomes the ending position of the substring.
SUBSTRING(string, position, length)
Is the string from which to extract the substring. It can be a field, a literal in single quotation marks (‘), or a variable.
Is the starting position of the substring in string. If the position is 0, it is treated as 1. If the position is negative, the starting position is counted backward from the end of string.
Is the limit for the length of the substring. The ending position of the substring is calculated as position + length - 1. If the calculated position beyond the end of the source string, the position of the last character of string becomes the ending position.
The data type of the returned substring is AnV.
POSITION determines the position of the first letter I in LAST_NAME.
SUBSTRING(LAST_NAME, I_IN_NAME, I_IN_NAME+2)
For BANNING, the result is 5.