事務是怎麼回事，之前已經聊過（參見Oracle事務的隔離 ），無關哪個資料庫，只要提到事務，都是這四種隔離級別。mysql的預設隔離級別是3——可重複讀。更重要的是，是否支援事務，跟mysql的儲存引擎相關。登陸mysql，查詢儲存引擎：

mysql> show engines;
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| Engine             | Support | Comment                                                        | Transactions
 
 | XA   | Savepoints |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| InnoDB             | DEFAULT | Supports transactions, row-level locking, and foreign keys     | YES          | YES  | YES        |
| MRG_MYISAM         | YES     | Collection of identical MyISAM tables                          | NO           | NO   | NO         |
| MEMORY             | YES     | Hash based, stored in
 
 memory, useful for temporary tables      | NO           | NO   | NO         |
| BLACKHOLE          | YES     | /dev/null storage engine (anything you write to it disappears) | NO           | NO   | NO         |
| MyISAM             | YES     | MyISAM storage engine                                          | NO           | NO   | NO         |
| CSV                | YES     | CSV storage engine                                             | NO           | NO   | NO         |
| ARCHIVE            | YES     | Archive storage engine                                         | NO           | NO   | NO         |
| PERFORMANCE_SCHEMA | YES     | Performance Schema                                             | NO           | NO   | NO         |
| FEDERATED          | NO      | Federated MySQL storage engine                                 | NULL         | NULL | NULL       |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
9
 
 rows in set (0.00 sec)

　　我們看到，這麼多儲存引擎，也就一個InnoDB支援事務。再瞧一下預設的事務隔離級別：

mysql> select @@tx_isolation;
+-----------------+
| @@tx_isolation  |
+-----------------+
| REPEATABLE-READ |
+-----------------+
1 row in set, 1 warning (0.00 sec)

mysql> select @@global.tx_isolation;
+-----------------------+
| @@global.tx_isolation |
+-----------------------+
| REPEATABLE-READ       |
+-----------------------+
1 row in set, 1 warning (0.00 sec)

　　我們看到，不管是當前會話也好，整個系統也好，預設的隔離級別都是可重複讀。

　　再看下預設的事務自動提交開關：

mysql> show variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | ON    |
+---------------+-------+
1 row in set, 1 warning (0.00 sec)

　　我們把上面的開關給關了，好測試：

mysql> set autocommit=off;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | OFF   |
+---------------+-------+
1 row in set, 1 warning (0.00 sec)

　　我們建個表測試一下：

mysql> create table test_wlf
    -> (
    -> id INT(11),
    -> name VARCHAR(25)
    -> );
Query OK, 0 rows affected (0.06 sec)

　　插入一條資料：

mysql> insert into test_wlf values(1,"wulf");
Query OK, 1 row affected (0.03 sec)

mysql> select * from test_wlf;
+------+------+
| id   | name |
+------+------+
|    1 | wulf |
+------+------+
1 row in set (0.00 sec)

　　再來一個視窗，同樣把autocommit關掉，兩事務同時搞起：

　　因為當前的事務隔離級別是可重複讀，所以右邊的事務始終可以重複讀取到wulf這條資料，即使左邊的事務新增了wumanshu這條資料，按理應該出現幻讀，可惜已經被InnoDB用mvcc（多版本併發控制）解決了。換句話說，幻讀在InnoDB已經死絕了。

　　接下來我們把兩個視窗都改成已提交讀：

mysql> set tx_isolation='READ-COMMITTED';
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| READ-COMMITTED |
+----------------+
1 row in set, 1 warning (0.00 sec)

mysql> set global tx_isolation='READ-COMMITTED';
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> select @@global.tx_isolation;
+-----------------------+
| @@global.tx_isolation |
+-----------------------+
| READ-COMMITTED        |
+-----------------------+
1 row in set, 1 warning (0.00 sec)

　　再試試修改資料：

　　以上可以看到已提交讀出現了不可重複讀。繼續，接下來我們把兩個視窗都改成未提交讀：

mysql> set tx_isolation='READ-UNCOMMITTED';
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> set global tx_isolation='READ-UNCOMMITTED';
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> select @@tx_isolation;
+------------------+
| @@tx_isolation   |
+------------------+
| READ-UNCOMMITTED |
+------------------+
1 row in set, 1 warning (0.00 sec)

mysql> select @@global.tx_isolation;
+-----------------------+
| @@global.tx_isolation |
+-----------------------+
| READ-UNCOMMITTED      |
+-----------------------+
1 row in set, 1 warning (0.00 sec)

　　再次更新id為2的name值，事務的中間結果都被查出來了，髒讀出現：