Read Committed Vs Repeatable Reads in MySQL?

http://ronaldbradford.com/blog/understanding-mysql-innodb-transaction-isolation-2009-09-24/

REPEATABLE-READ

session 1 :

    MariaDB [test]> DROP TABLE IF EXISTS transaction_test;
    Query OK, 0 rows affected (0.22 sec)

    MariaDB [test]> CREATE TABLE transaction_test(
        ->   id   INT UNSIGNED NOT NULL AUTO_INCREMENT,
        ->   val  VARCHAR(20) NOT NULL,
        ->   created TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
        -> PRIMARY KEY(id)
        -> ) ENGINE=InnoDB DEFAULT CHARSET latin1;
    Query OK, 0 rows affected (0.29 sec)

    MariaDB [test]>
    MariaDB [test]> INSERT INTO transaction_test(val) VALUES ('a'),('b'),('c');
    Query OK, 3 rows affected (0.08 sec)
    Records: 3  Duplicates: 0  Warnings: 0

    MariaDB [test]> START TRANSACTION;
    Query OK, 0 rows affected (0.00 sec)
MariaDB [test]> SELECT * FROM transaction_test;
+----+-----+---------------------+
| id | val | created             |
+----+-----+---------------------+
|  1 | a   | 2016-04-01 10:09:33 |
|  2 | b   | 2016-04-01 10:09:33 |
|  3 | c   | 2016-04-01 10:09:33 |
+----+-----+---------------------+
3 rows in set (0.00 sec)

MariaDB [test]> select sleep(50);

then user2 run next code :

 MariaDB [test]> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> INSERT INTO transaction_test(val) VALUES ('x'),('y'),('z');

commit;

then user 1

MariaDB [test]> SELECT * FROM transaction_test;
+----+-----+---------------------+
| id | val | created             |
+----+-----+---------------------+
|  1 | a   | 2016-04-01 10:09:33 |
|  2 | b   | 2016-04-01 10:09:33 |
|  3 | c   | 2016-04-01 10:09:33 |
+----+-----+---------------------+
3 rows in set (0.00 sec)

MariaDB [test]>

READ-COMMITTED

user1

SET SESSION tx_isolation='READ-COMMITTED';
TRUNCATE TABLE transaction_test;
INSERT INTO transaction_test(val) VALUES ('a'),('b'),('c');
MariaDB [test]> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> select sleep(60);

then user2 run next code :

MariaDB [test]> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> INSERT INTO transaction_test(val) VALUES ('x'),('y'),('zwfwfw');

Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

MariaDB [test]> commit;

then user1 finish query :

MariaDB [test]> SELECT * FROM transaction_test;
+----+--------+---------------------+
| id | val    | created             |
+----+--------+---------------------+
|  1 | a      | 2016-04-01 10:28:08 |
|  2 | b      | 2016-04-01 10:28:08 |
|  3 | c      | 2016-04-01 10:28:08 |
|  4 | x      | 2016-04-01 10:29:00 |
|  5 | 
y      | 2016-04-01 10:29:00 |
|  6 | zwfwfw | 2016-04-01 10:29:00 |
+----+--------+---------------------+
6 rows in set (0.00 sec)

As per my understanding suppose balance was 1000 before starting this transaction.

After starting transaction and updating it to "balance = balance - 200", if you run again select statement within transaction then results will be as per below-

1. If isolation level is READ COMMITTED then you will see result as 800.

2. If isolation level is REPEATABLE READ then you will see result as 1000.

In READ COMMITED, you see the information that had been committed, regardless if you are in a transaction or not, thus it´s not guaranteed integration in the information because it can changed multiple times. Instead of this is REPEATABLE READ that forbids you in a ocurrence of a transaction the modification(UPDATE) of information(integrity), but you can add information(INSERT ...)