Ordering a lookup column

[SJKonthego]

In the form below (JOB Form), the first entry looks up the customer from our table of customers. When I add a new customer, as expected, it goes to the bottom of the customer table but this means that the table order of customers is not alphabetical, but rather by entry date. How can I sort the customer list so that it appears alphabetically in the JOB form, accommodating new names as added? Note that the list of customers is actually a computer-generated field concatenating the first and last names which are entered separately if this matters. I'd like to alpha by last then first. Thanks for guidance.

SCREENSHOT OF FORM

SCREENSHOT OF CUSTOMER LIST AS IT APPEARS WHEN YOU SELECT THE DOWNWARD ARROW NEXT TO "Customer" in the JOB Form.

[Marc_Dillon]

Use ORDERBY in valid_if.

https://help.appsheet.com/en/articles/2357312-orderby

https://help.appsheet.com/en/articles/961544-dropdown-from-valid_if

[SJKonthego]

@Marc_Dillon thanks! That got me closer. But I'm failing on the syntax. 

The JOB table is linked to the CUSTOMER table by CID

In the CUSTOMER Table I have four variables that may by of interest here: 

CID (key unique ID)

CFirst (first name)

CLast (last name)

_ComputedName (system-generated concatenation of CFirst and CLast)

Currently, I get  _ComputedName but out of order and the goal is  _ComputedName in alpha order.

So, in the JOB table (not the Customer table, yes?) under Data Validty, I'm adding a Valid If statement I believe from what you are suggesting. 

I've tried different permutations, but keep getting an error code "Orderby has invalid inputs"

The three I think are closest:
ORDERBY([CID],[CID].[CLast])

ORDERBY([CID],CUSTOMER[CLast])

ORDERBY(CUSTOMER[CID],CUSTOMER[CLast])

Clearly I'm having difficulty referencing across tables. I'm so sorry, but can you guide?

[Marc_Dillon · Accepted solution]

You're so close.

https://community.appsheet.com/t/referencing-a-column-value-in-a-table-versus-referencing-all-column...

[SJKonthego]

Thank you @Marc_Dillon - really appreciate the kind guidance. I read through your tip and it helped to clarify the syntax. 

For those who might stumble upon this, the final expression that got me over the finish line was:

ORDERBY(CUSTOMER[CID],[CLast],FALSE,[CFirst])