What is the difference between phantom read and non-repeatable read?

Difference:

Non-repeatable read: Under the same conditions, the value of the read data will change when we read it again Variety.

Phantom reading: Under the same conditions, the number of records read out for the first and second times is different.

Detailed analysis:

1. Non-repeatable reading

Under the same conditions, the value of the read data will be the same when we read it again Something has changed.

Example:

In transaction 1, A reads his salary as 1000, but at this time the operation of transaction 1 has not been completed, and there is another identical read later. operate.

con1 = getConnection();
select salary from employee where employeeName ="A";

In transaction 2, the financial staff modified A’s salary to 2000 and submitted the transaction.

con2 = getConnection();    
update employee set salary = 2000 where employeeName = "A";    
con2.commit();

In transaction 1, when A reads his salary again, the salary becomes 2000.

select salary from employee where employeeName ="A";

The results of two reads in a transaction are inconsistent, resulting in non-repeatable reads.

2. Phantom reading

Under the same conditions, the number of records read out for the first and second times is different.

Example:

There are currently 5 employees with a salary of 1,000.

Transaction 1, read all employees with a salary of 1,000, and read a total of 10 records.

con1 = getConnection();    
Select * from employee where salary =1000;

At this time, another transaction inserted an employee record into the employee table, and the salary was also 1,000

con2 = getConnection();    
Insert into employee(employeeName,salary) values("B",1000);    
con2.commit();

Transaction 1 read all employees with a salary of 1,000 again, and a total of 6 records were read Record, which creates phantom reading.

//con1    
select * from employee where salary =1000;

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