MySQL Query change row if higher? - mysql

Question

I have two Tables (account2017) and (account2018) and I want to add a 2 at the end of the names in the name row for one of the tables, but only if both tables contain the same name. Adding the 2 for that name, which has lower points value.
Overall a solution to merge two tables with unique key (name), but to decide which row gets a 2 added at his end of name, should be followed by lower points.
For example if table account2017 and account2018 have both "Alex" in the name column, add a 2 at the end of the name (=Alex2) to that table, which has lower value in points column. As Alex in accounts2017 has 20 points, and Alex in accounts2018 has only 15 points, Alex name will be changed to Alex2 for accounts2018. Accounts2017 will be untouched.
Any idea how it can work?

Answer 1

If I'm understanding correctly, sounds like you need to use 2 separate update statements, using exists to match the criteria:
update account2017
set name = concat(name, '2')
where exists (
select 1
from account2018
where account2017.name = account2018.name and account2017.score < account2018.score)
update account2018
set name = concat(name, '2')
where exists (
select 1
from account2017
where account2018.name = account2017.name and account2018.score < account2017.score)

Answer 2

You will need to do this in 2 queries. Syntax will depend on what you are using (MySQL, SQL Server, SQLite) but here is the MySQL Version:
UPDATE accounts2017 table1 SET name = concat(name, '2') WHERE exists (SELECT 1 FROM accounts2018 table2 WHERE table1.name = table2.name AND table1.score < table2.score);
Then you can just flip the query around to update the 2018 table.

Answer 3

You can do this using a multi-table update. This relies on mysql recognising no change has been made to account2017 (proved by the fact that timestamp has not changed) but beware both before and after triggers fire.
MariaDB [sandbox]> drop table if exists account2017,account2018;
Query OK, 0 rows affected (0.39 sec)
MariaDB [sandbox]> create table account2017(name varchar(10), points int,ts timestamp DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP);
Query OK, 0 rows affected (0.20 sec)
MariaDB [sandbox]> create table account2018(name varchar(10), points int,ts timestamp DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP);
Query OK, 0 rows affected (0.29 sec)
MariaDB [sandbox]>
MariaDB [sandbox]> drop trigger if exists t;
Query OK, 0 rows affected, 1 warning (0.00 sec)
MariaDB [sandbox]> delimiter $$
MariaDB [sandbox]> create trigger t before update on account2017
-> for each row
-> begin
-> insert into debug_table(msg) values (concat('before:',old.name,':',new.name));
-> end $$
Query OK, 0 rows affected (0.07 sec)
MariaDB [sandbox]> delimiter ;
MariaDB [sandbox]>
MariaDB [sandbox]> drop trigger if exists t1;
Query OK, 0 rows affected, 1 warning (0.00 sec)
MariaDB [sandbox]> delimiter $$
MariaDB [sandbox]> create trigger t1 after update on account2017
-> for each row
-> begin
-> insert into debug_table(msg) values (concat('after:',old.name,':',new.name));
-> end $$
Query OK, 0 rows affected (0.08 sec)
MariaDB [sandbox]> delimiter ;
MariaDB [sandbox]>
MariaDB [sandbox]> insert into account2017 (name,points) values('alex',20);
Query OK, 1 row affected (0.02 sec)
MariaDB [sandbox]> insert into account2018 (name,points) values('alex',15);
Query OK, 1 row affected (0.01 sec)
MariaDB [sandbox]> truncate table debug_table;
Query OK, 0 rows affected (0.17 sec)
MariaDB [sandbox]>
MariaDB [sandbox]> select * from account2017;
+------+--------+---------------------+
| name | points | ts |
+------+--------+---------------------+
| alex | 20 | 2018-12-11 16:49:25 |
+------+--------+---------------------+
1 row in set (0.00 sec)
MariaDB [sandbox]> select * from account2018;
+------+--------+---------------------+
| name | points | ts |
+------+--------+---------------------+
| alex | 15 | 2018-12-11 16:49:25 |
+------+--------+---------------------+
1 row in set (0.00 sec)
MariaDB [sandbox]>
MariaDB [sandbox]> select sleep(60);
+-----------+
| sleep(60) |
+-----------+
| 0 |
+-----------+
1 row in set (1 min 0.00 sec)
MariaDB [sandbox]> update account2017 join account2018 on account2017.name = account2018.name
-> set account2017.name = case when account2017.points < account2018.points then concat(account2017.name,'2') else account2017.name end,
-> account2018.name = case when account2018.points < account2017.points then concat(account2018.name,'2') else account2018.name end
-> where 1 = 1;
Query OK, 1 row affected (0.04 sec)
Rows matched: 2 Changed: 1 Warnings: 0
MariaDB [sandbox]>
MariaDB [sandbox]>
MariaDB [sandbox]>
MariaDB [sandbox]> select * from account2017;
+------+--------+---------------------+
| name | points | ts |
+------+--------+---------------------+
| alex | 20 | 2018-12-11 16:49:25 |
+------+--------+---------------------+
1 row in set (0.00 sec)
MariaDB [sandbox]> select * from account2018;
+-------+--------+---------------------+
| name | points | ts |
+-------+--------+---------------------+
| alex2 | 15 | 2018-12-11 16:50:26 |
+-------+--------+---------------------+
1 row in set (0.00 sec)
MariaDB [sandbox]> select * from debug_table;
+----+------------------+------+
| id | msg | MSG2 |
+----+------------------+------+
| 1 | before:alex:alex | NULL |
| 2 | after:alex:alex | NULL |
+----+------------------+------+
2 rows in set (0.00 sec)