IsolationLevel

Hi,

I have a trivial question:
What does IsolationLevel mean?

e.g., in SDK:
ReadCommitted
Shared locks are held while the data is being read to avoid dirty reads, but
the data can be changed before the end of the transaction, resulting in
non-repeatable reads or phantom data.

What does this mean?

Who is reading? Who will read dirty data, who will have non-repeatable read?
The current connection/thread or other connectoins?

And locks shared by ...?

What IsolationLevel will block other thread from executing?

And by default, IsolationLevel is Unspecified when using
conn.BeginTransaction()?
Thanks a lot!
R L



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