Oracle "Starts With" text join, excluding nulls - oracle

Question

I am having some trouble. I am trying to build a SQL query that uses "starts with" logic. A little background first...
In the database that I've been tasked to write reports from, there is a "user" table and a "salesperson" table, with salespersons belonging to a user. In a not-so-brilliant move, the designer of the database decided to associate the salespersons through a substring match to their employee code. For example:
John Smith's "employee_code" would be "JS". But he has multiple "salespersons" to distinguish his different sale types. So he might have "JS1", "JS2", "JS3", etc., as his "salesperson_code".
To illustrate:
user table:
|----------|-----------|----------|---------------|
| username | firstname | lastname | employee_code |
|----------|-----------|----------|---------------|
| JSMITH | John | Smith | JS |
|----------|-----------|----------|---------------|
salesperson table:
|------------------|------------------|
| salesperson_name | salesperson_code |
|------------------|------------------|
| John Smith 1 | JS1 |
| John Smith 2 | JS2 |
| John Smith 3 | JS3 |
|------------------|------------------|
There is no foreign key on the salesperson table linking them to the user table, only the substring from the employee code.
I do not remember where I found this answer, but in my queries I've been doing this:
select user.name
from user user
inner join salesperson spn on spn.salesperson_code like user.employee_code || '%'
This logic successfully does the "starts with" match. However, there are users with blank employee codes and they, also, match this query.
What I am looking for: how do I modify this query so that if the employee_code is blank it will not match? I'm pretty newbie with Oracle queries. Other DBMS' have a starts with clause that will not match blank fields.
Thank you in advance for your help!

Answer 1

Try this
select user.name
from user user
inner join salesperson spn
on spn.salesperson_code like nvl(trim(user.employee_code),'-') || '%'

Answer 2

try
select user.name
from user user
inner join salesperson spn
on spn.salesperson_code like DECODE (user.employee_code,
NULL, NULL,
user.employee_code || '%')

Answer 3

I would suggest using a regular expression to extract the non-digit parts of the salesperson code and optionally the digits part. Create a view for the table with these added fields or use it as a table expression in the query.
SELECT regexp_substr(salesperson_code,'\D+') AS employee_code,
regexp_substr(salesperson_code,'\d+') AS employee_sales_no,
salesperson_name, salesperson_code
FROM salesperson
Note: the regular expressions match one or more non-digits and one or more digits respectively.

Answer 4

Add an IS NOT NULL condition:
select *
from user
inner join salesperson spn
on spn.salesperson_code like user.employee_code || '%'
and user.employee_code is not null;