SUBSTR and INSTR SQL Oracle

Question

I\'ve started using SUBSTR and INSTR in Oracle but I got confused when I came across this.

SELECT PHONE, SUBSTR(PHONE, 1, INSTR(PHONE, \'-\') -1)
FROM DIRECTORY;         


        

Answer 1

SELECT PHONE, SUBSTR(PHONE, 1, INSTR(PHONE, '-',1,1) -1)
FROM DIRECTORY;

INSTR(String,'char_Search','position from where start searching','occurance')

If We are not giving 'starting position' and 'occurrence' it bydefault take as 1,1.

So bydefault it take as INSTR(phone,'-',1,1) --sql returns 1st occurance of string'-' position is 4 and then SUBSTR(PHONE, 1, 4 - 1) which is SUBSTR(PHONE, 1, 3).

SUBSTR(String,'starting position','no of character') SUBSTR(PHONE, 1, 3) --gives the substring of the PHONE column from the 1st that has length of 3 chars which is 362

Ex: using this value--'362-127-4285'

SELECT '362-127-4285' Example,
SUBSTR('362-127-4285',1,INSTR('362-127-4285','-')-1) Result 
FROM DUAL

Answer 2

INSTR(PHONE, '-') gives the index of - in the PHONE column, in your case 4
and then SUBSTR(PHONE, 1, 4 - 1) or SUBSTR(PHONE, 1, 3)
gives the substring of the PHONE column from the 1st that has length of 3 chars which is 362,
if the value PHONE column is 362-127-4285.