Detect double value in MySQL - mysql

Question

I have an InnoDB table with fields
firstname, lastname
While displaying names, usually only firstname is enough. Sometimes users have the same first name; so I have to get firstname and first letter of lastname:
CONCAT(firstname, ' ', SUBSTRING(lastname, 1, 1), '.')
Is there a (performant) way to only display the first letter of the last name in case of a double first name? Something like
WHEN isDouble(Firstname) THEN
CONCAT(firstname, ' ', SUBSTRING(lastname, 1, 1), '.')
ELSE firstname
/* edit */
Forgot to mention the solution I was thinking of:
Creating a column 'double_firstname', with value 1 or 0, and use a CASE statement to select. Then update the double_firstname column on user create and delete.

Answer 1

You can of course ask mysql if the number of entries for that firstname is bigger than one, so:
select IF( (select count(*) as cnt from person where firstname = p.firstname) > 1
, concat(firstname, " ", substring(lastname, 1))
, firstname)
from person
where id = 4711
;
But that is not very quick.
Better for a higher number of persons is a stable mark on person how to "call" her. That could be "firstname lastname" initially and then get more personally with reducing to firstname by a cronjob. It also could mean to call "John Doe" just John, because he entered early, and "John DaSecond" call "John D.", and "JohnDaThird" call "John DaT.".
JohnD is not unique in that scenario.
Is John Doe informed about being renamed to "John D." in your Scenario?

Answer 2

You asked for good performance as well as the ability to do it. If you have an index on names(firstname, lastname) the following should perform well:
select (case when exists (select 1
from names n2
where n2.firstname = n.firstname and n2.lastname <> n.lastname
)
then concat(firstname, ' ', left(lastname, 1))
else firstname
end)
from names n