Please explain$$how the account table, the transaction table, the savepoint command, the rollback command, and the commit command are working together:

$$

CREATE TABLE account $($

$$ account$\_$id SMALLINT UNSIGNED NOT NULL AUTO$\_$INCREMENT,

$$ avail$\_$balance DECIMAL$(10,2)$ NOT NULL,

$$ last$\_$activity$\_$date TIMESTAMP NOT NULL DEFAULT CURRENT$\_$TIMESTAMP ON UPDATE CURRENT$\_$TIMESTAMP,

$$ PRIMARY KEY$($account$\_$id$)$

$)$ ENGINE$=$InnoDB DEFAULT CHARSET$=$utf$8$mb$4$;

$$

$--$ account$\_$id avail$\_$balance last$\_$activity$\_$date

$--$ txn$\_$id txn$\_$date account$\_$id txn$\_$type$\_$cd amount

$$

CREATE TABLE transaction $($

$$ txn$\_$id SMALLINT UNSIGNED NOT NULL AUTO$\_$INCREMENT,

$$ txn$\_$date DATETIME NOT NULL,

$$ account$\_$id SMALLINT UNSIGNED,

$$ txn$\_$type$\_$cd VARCHAR$(2)$ NOT NULL,

$$ amount DECIMAL$(10,2)$ NOT NULL,

$$ PRIMARY KEY$($txn$\_$id$),$

$$ KEY idx$\_$fk$\_$account$\_$id $($account$\_$id$),$

$$ CONSTRAINT fk$\_$account$\_$txn FOREIGN KEY $($account$\_$id$)$ REFERENCES account $($account$\_$id$)$ ON DELETE SET NULL ON UPDATE CASCADE

$)$ ENGINE$=$InnoDB DEFAULT CHARSET$=$utf$8$mb$4$;

$$

INSERT INTO account

$($account$\_$id$,$ avail$\_$balance, last$\_$activity$\_$date$)$

VALUES $(789,2000,"2020-07-1020$:$53$:$27")$;

$$

INSERT INTO account

$($account$\_$id$,$ avail$\_$balance, last$\_$activity$\_$date$)$

VALUES $(123,1000,"2019-07-1020$:$53$:$27")$;

$$

SET AUTOCOMMIT$=0$;

$$

# try savepoint $*************************$

$$

START TRANSACTION;

$$

SAVEPOINT before$\_$update$\_$transactions;

$$

INSERT INTO transaction

$($txn$\_$date, account$\_$id$,$ txn$\_$type$\_$cd$,$ amount$)$

VALUES

$($now$(),123,\text{'}$D$\text{'},55)$;

INSERT INTO transaction

$($txn$\_$date, account$\_$id$,$ txn$\_$type$\_$cd$,$ amount$)$

VALUES

$($now$(),789,\text{'}$C$\text{'},55)$;

$$

SAVEPOINT before$\_$update$\_$accounts;

$$

UPDATE account

SET avail$\_$balance $=$ avail$\_$balance $-55,$

last$\_$activity$\_$date $=$ now$()$

WHERE account$\_$id $=123$;

$$

UPDATE account

SET avail$\_$balance $=$ avail$\_$balance $+55,$

last$\_$activity$\_$date $=$ now$()$

WHERE account$\_$id $=789$;

$$

SAVEPOINT after$\_$update$\_$accounts;

$$

$--$ ROLLBACK TO SAVEPOINT before$\_$update$\_$transactions;

$--$ ROLLBACK TO SAVEPOINT before$\_$update$\_$accounts;

ROLLBACK TO SAVEPOINT after$\_$update$\_$accounts;

$$

COMMIT;

$$