Purpose
This function returns a substring of the length length from the position position, out of the string string.
Syntax
substring::=
                                                                         
                                                                    
Usage notes
- 
                                                                    If lengthis not specified, all of the characters to the end of the string are used.
- 
                                                                    If positionis negative, counting begins at the end of the string.
- 
                                                                    If positionis 0 or 1, the result begins from the first character of the string.
- 
                                                                    If positionis before the start or after the end of the string, the result isNULL.
- 
                                                                    For additional information, refer to the functions RIGHT, LEFT, and REGEXP_SUBSTR. 
- 
                                                                    MID is an alias for this function. 
Example
SELECT SUBSTR('abcdef',2,3) S1,
       SUBSTRING('abcdef' FROM 4 FOR 2) S2,
       SUBSTR('abcdef',-3) S3,
       SUBSTR('abcdef',7) S4,
       SUBSTR('abcdef',-7) S5
       ;Result:
| S1 | S2 | S3 | S4 | S5 | 
|---|---|---|---|---|
| bcd | de | def | NULL | NULL |