Detect with Left Join duplicates entries and return the first value, also return sum of duplicates entries ¡Hi!, i have a case with two tables than can have a lot of matches entries and the time of fetching increases a lot.

This tables can be

Table A: Employees

-------------------------
| Name   | ID | Account |
-------------------------
| Nicole | 01 | 12345   |
| Alexis | 02 | 67890   |
------------------------- 


And Table B: BankAccounts

--------------------------
| Name   | ID  | Account |
--------------------------
| Nicole | 01  | 12345   |
| Nicole | 01  | 67890   | //duplicates Accounts
| Alexis | 02  | 67890   | //duplicates Accounts
--------------------------


And i want to do this with a Left Join in a Table that can have more of 450,000 different entries

Result Table C

Column_A = ¿Exists the account number in other register?
Column_B = if(NumberOfMatches &gt; 1) //this means that the account be found 
in other user AND i want to get the first value of all posibles number of matches

                         |Account exists in other user|Match in User..
-----------------------------------------------------------------------------
| Name   | ID  | Account | Column_A | NumberOfMatches | Column_B    | BadID |
--------------------------------------------------------------------|-------|
| Nicole | 01  | 12345   | No       |       1         | Nicole (OK) | null  |
| Alexis | 02  | 67890   | Yes      |       2         | Nicole (BAD)|   01  |
-----------------------------------------------------------------------------


Thanks and regards!

Note: sorry for my english, im learning :p
SELECT DISTINCT
       CASE WHEN t2.ID IS NULL 
            THEN t1.Name
            ELSE t2.Name
            END Name,
       CASE WHEN t2.ID IS NULL 
            THEN t1.ID
            ELSE t2.ID
            END ID,
       CASE WHEN t2.ID IS NULL 
            THEN t1.Account
            ELSE t2.Account
            END Account,
       CASE WHEN t2.ID IS NULL 
            THEN 'No'
            ELSE 'Yes'
            END Column_A,
       ( SELECT COUNT(t4.Account)
         FROM BankAccounts t4
         WHERE t1.Account = t4.Account ) NumberOfMatches,
       CASE WHEN t2.ID IS NULL 
            THEN CONCAT(t1.Name, ' (OK)')
            ELSE CONCAT(t1.Name, ' (BAD)')
            END Column_B,
       CASE WHEN t2.ID IS NULL 
            THEN NULL
            ELSE t1.ID
            END BadID
FROM BankAccounts t1
LEFT JOIN BankAccounts t2 ON t1.Account = t2.Account
                         AND t1.ID &lt; t2.ID
LEFT JOIN BankAccounts t3 ON t1.Account = t3.Account
                         AND t1.ID &gt; t3.ID
WHERE t3.ID IS NULL
ORDER BY Account, ID;


fiddle