In oracle/PLSQL, the substr functions allows you to extract a substring from a string.
The syntax for the substr function is:
substr( string, start_position, [ length ] )
说明：
string is the source string.
start_position is the position for extraction. The first position in the string is
always 1.
length is optional. It is the number of characters to extract. Ifthis parameter is
omitted, substr will return the entire string.
For example:
substr('This is a test', 6, 2) would return 'is'
substr('This is a test', 6) would return 'is a test'
substr('TechOnTheNet', 1, 4) would return 'Tech'
substr('TechOnTheNet', -3, 3) would return 'Net'
substr('TechOnTheNet', -6, 3) would return 'The'
substr('TechOnTheNet', -8, 2) would return 'On'