Check if MySQL entry exists and if it does, how to overwrite other columns?

For this, use INSERT ON DUPLICATE KEY UPDATE command. Let us first create a table −

mysql> create table DemoTable1891
   (
   FirstName varchar(20),
   UNIQUE KEY(FirstName)
   );
Query OK, 0 rows affected (0.00 sec)

Insert some records in the table using insert command −

mysql> insert into DemoTable1891 values('Chris') on duplicate key update  FirstName='Robert';
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1891 values('David') on duplicate key update  FirstName='Robert';
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1891 values('Chris') on duplicate key update  FirstName='Robert';
Query OK, 2 rows affected (0.00 sec)

Display all records from the table using select statement −

mysql> select * from DemoTable1891;

This will produce the following output −

+-----------+
| FirstName |
+-----------+
| David     |
| Robert    |
+-----------+
2 rows in set (0.00 sec)