mysql the substring () function characters taken

substring (parameter 1, parameter 2, parameter 3), wherein the three parameters represent: 1 represents the need to intercept parameter string parameter 2 from that represents the start position of the string taken (subscript string starting from 1), the parameters 3 shows the number of bits to be intercepted, if you do not write, represents the beginning of the rest of the interception from the location specified parameters 2 all characters.
E.g:

1. select substring ( "jason", 1,2 );
   results: ja
2. select substring ( "jason", 1 );
   results: jason
3. select substring ( "jason", 2 );
   results: ason
   These are parameters for the case of a positive number. Circumstances then when parameter 2 is negative it? Note that the argument is a 3-digit intercepted, it can not be negative, if negative, the query result will be null.

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The following is the case when parameter 2 is negative

1. select substring ( "jason", - 1);
   Results as follows: n
   taken Thought: Starting from the forward end of the string taken.
2. select substring ( "jason", - 3);
   results: son
3. select substring ( "jason", - 3,1);
   the result is: s
   interception thought: if there is 3 digits parameter specifies the need to intercept, then from left to right beginning from the first interception is to end, rather than from Last to first start.
4. select substring ( "jason", - 3,2);
   the result is: so
5. select substring ( "jason", - 3, -1);
   the result is: null
6. select substring ( "jason", - 3,0);
   the result is: null