Check out example codes for "oracle leftmost characters". It will help you in understanding the concepts better.

Code Example 1

-- For Oracle only

-- syntax 
SUBSTR(<main-string>,1,<number-of-characters>)

-- example 
SUBSTR('Useless stuff',1,10) -- OUTPUT: Useless st

-- practical example
SELECT SUBSTR('Useless stuff',1,10)
FROM DUAL;

Code Example 2

/*Using SUBSTR in Oracle (Example from hackerrank.com): */

/*Simple select query...*/ 
SELECT DISTINCT city 
FROM station 

/*Using WHERE and SUBSTR to find (distinct) cities in station table that begin 
as well as end with a vowel.*/ 
WHERE SUBSTR(city,1,1) IN ('A','E','I','O','U') 
	AND substr(city,-1) IN ('a','e','i','o','u'); 

/*Parameters for SUBSTR (Substring) in order are as follows: 
String, Start, Length.*/

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