how to update table using trigger - mysql

Question

i am new to this and want some help.i have table with name "abc" with following entries
+------+--------+------+
| Id | Name | City |
+------+--------+------+
| 101 | john | abc |
| 102 | Miller | cbz |
+------+--------+------+
and another table "xyz"
+------+--------+------+
| Id | Name | City |
+------+--------+------+
| 102 | Miller | cbz |
+------+--------+------+
i applied trigger on table "abc" which will update the table "xyz" with recently inserted values and will delete all previous entries...
for example, when i fire insert query on "abc" table i get ,"abc" as follow
insert into abc Values(103,'Joseph','xyz');
i get output for table "abc" as,
+------+--------+------+
| Id | Name | City |
+------+--------+------+
| 101 | john | abc |
| 102 | Miller | cbz |
| 103 | Joseph | xyz |
+------+--------+------+
and table "xyz" as,
+------+--------+------+
| Id | Name | City |
+------+--------+------+
| 103 | Joseph | xyz |
+------+--------+------+
now my question is how to acheive this using only one table(i dont want to use two table as this is not my requirement).
like following ..
insert into xyz values(104,'Ridhit','pqr');
+------+--------+------+
| Id | Name | City |
+------+--------+------+
| 104 | Ridhit | pqr |
+------+--------+------+
Please help.Trigger i used is
DELIMITER !!
create trigger OnlyOne BEFORE INSERT on abc
for each row
BEGIN
DECLARE a1 INT;
Select count(1) INTO a1 from xyz;
IF a1>0 THEN
delete from xyz limit 1;
insert into xyz(Id,Name,City) values (new.Id,new.Name,new.City);
ELSE
insert into xyz(Id,Name,City) values (new.Id,new.Name,new.City);
END IF;
END;
!!
DELIMITER ;

Answer 1

Do this on application level, but you better make sure you use transactions. You probably don't want to have an empty table, when the operation crashes in the middle.
You may also consider to have following approach.
Instead of deleting/updating whenever an insert occurs, add a column like created with default value of current timestamp to your table so that it looks something like this:
CREATE TABLE abc(
id int auto_increment primary key,
name varchar(50),
city varchar(50),
created timestamp default current_timestamp
);
To get the latest entry you just do
SELECT * FROM abc ORDER BY created DESC LIMIT 1;
Or you put this in a view
CREATE VIEW just_latest_entry_from_abc AS
SELECT * FROM abc ORDER BY created DESC LIMIT 1;
Then you just do
SELECT * FROM just_latest_entry_from_abc;
When table size matters, create a cronjob or a scheduled event to delete older entries on a regular basis.