Checking uniqueness of the systemusername with account names

[Shrinita]

I have generated systemusername using below query in global configuration.

IF(
firstname IS NOT NULL AND firstname != '' AND
lastname IS NOT NULL AND lastname != '',
UPPER(
CONCAT(
SUBSTRING(firstname, 1, 1),
IFNULL(NULLIF(SUBSTRING(middlename, 1, 1), ''), 'X'),
SUBSTRING(lastname, 1, 1),
RPAD(FLOOR(RAND() * 1000), 3, '0')
)
),
NULL
)

Now I want to check the uniqueness of the systemusername with the account names and hence I have tried the below query for the same but not able to get expected output. 
IF(IF(
firstname IS NOT NULL AND firstname != '' AND
lastname IS NOT NULL AND lastname != '',
UPPER(
CONCAT(
SUBSTRING(firstname, 1, 1),
IFNULL(NULLIF(SUBSTRING(middlename, 1, 1), ''), 'X'),
SUBSTRING(lastname, 1, 1),
RPAD(FLOOR(RAND() * 1000), 3, '0')
)
),
NULL
)) EXISTS IN ACCOUNTS.NAME

[Raghu]

@Shrinita  Here account name condition not a scope, once generate the new user accountname/system usernmae will create based on your logic,  still you want compare those you can map endpoint level account co-relation rule based on you requiremnt it work.

[Shrinita]

Could you please give me an example how it works with respect to our scenario?