substr('123',1,3) и substr('123',0,3) возвращают одинаковый результат...
В DB2 вообще с этими операциями как-то... по-китайски... :(

Убил бы того индуса, который писал реализацию для SUBSTR... (
http://publib.boulder.ibm.com/infocenter/db2luw/v9r7/topic/com.ibm.db2.luw.sql.ref.doc/doc/r0000854.html

string
An expression that specifies the string from which the result is derived.

The expression must return a value that is a built-in string, numeric, or datetime data type. If the value is not a string data type, it is implicitly cast to VARCHAR before evaluating the function. If string is either a character string or a binary string, a substring of string is zero or more contiguous bytes of string. If string is a graphic string, a substring of string is zero or more contiguous double-byte characters of string.
start
An expression that specifies the position of the first byte of the result for a character string or a binary string or the position of the first character of the result for a graphic string. The expression must return a value that is a built-in numeric, CHAR, VARCHAR, GRAPHIC, or VARGRAPHIC data type. If the value is not of type INTEGER, it is implicitly cast to INTEGER before evaluating the function. The integer value must be between 1 and the length or maximum length of string, depending on whether string is fixed-length or varying-length (SQLSTATE 22011, if out of range). It must be specified as number of bytes in the context of the database code page and not the application code page.
length
An expression that specifies the length of the result. If specified, the expression must return a value that is a built-in numeric, CHAR, VARCHAR, GRAPHIC, or VARGRAPHIC data type. If the value is not of type INTEGER, it is implicitly cast to INTEGER before evaluating the function. The value of the integer must be in the range of 0 to n, where n equals (the length attribute of string) - start + 1 (SQLSTATE 22011, if out of range).

If length is explicitly specified, string is effectively padded on the right with the necessary number of blank characters (single-byte for character strings; double-byte for graphic strings) or hexadecimal zero characters (for BLOB strings) so that the specified substring of string always exists. The default for length is the number of bytes from the byte specified by the start to the last byte of string in the case of character string or binary string or the number of double-byte characters from the character specified by the start to the last character of string in the case of a graphic string. However, if string is a varying-length string with a length less than start, the default is zero and the result is the empty string. It must be specified as number of bytes in the context of the database code page and not the application code page. (For example, the column NAME with a data type of VARCHAR(18) and a value of 'MCKNIGHT' will yield an empty string with SUBSTR(NAME,10)).