会发生不可重读、幻像读和读未提交数据。
An unrepeatable read, phantom read, and reads of uncommitted data can occur.
Readcommitted（读已提交） ——事务可以读取由另一事务提交的数据。
Read committed -- The transaction can read data committed by other transactions.
提交读，未提交读，可重复读，可串行读。
Read committed, Read uncommitted, Repeatable Read, Serializable.
提交读，未提交读，可重复读，可串行读。
Read committed, Read uncommitted, Repeatable Read, Serializable.
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