mysql - select substring of column returns null and empty -

i'm having trouble using mysql's substr function.

my query is:

select distinct(substr(col1,0,10)) table;

the results returned null , empty row.

am using substr incorrectly, or can not use distinct or column name?

thanks

firstly, position of first character of string 1, not 0; should fix it:

select distinct(substr(col1, 1, 10))  `table` 

secondly, table contains @ least 1 row in col1 null. rows, result of substr null well.