mysql - Strange logic in SQL Query -

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i have strange case building mysql query:

here is.

  1. i need sort results based on first_name , last_name
  2. my expected result using concat(first_name, last_name) in order by

but restricted not use concat in order by. there way can build query

raw sql : 

select concat(first_name, last_name)  users  order concat(first_name, last_name)

cases: first_name can empty

example 

 first name           last name                    b                      b c                    d

now need sort order 

 ab  b cd

try this:

select full_name (     select concat(first_name, last_name) full_name users ) tmp order full_name

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