Substrings return portions of character value expressions. A
    substring must have the data type CHAR, VARCHAR, LONG VARCHAR,
    NCHAR, or NCHAR VARYING.

    To specify a substring, you must specify the value expression
    and the FROM keyword, followed by the start position of the value
    expression. (The first character in the string occupies position
    1.) You can optionally add a FOR clause after the FROM clause
    to specify the length of the value expression after the start
    position.

    The start position and string length values can be a numeric
    value expression. By default, SQL expects the start position and
    the string length to be specified in octets. You can use the SET
    DIALECT or the SET CHARACTER LENGTH statements or the DIALECT
    or CHARACTER LENGTH clause of the SQL module language header and
    DECLARE MODULE statement to specify whether the length value is
    octets or characters.

    If you specify a length longer than the string, SQL returns
    only valid characters in the string and terminates the returned
    substring after the last valid character.

    If either operand of the substring is a null value, the resulting
    value is also null.

    When you use a substring with the equal (=)  conditional
    operator, the operation is case sensitive.