Interviewer: have you made a judgment on the number of rows affected by your update?

In the business system , Use update Statement updating data is a normal scenario , We often pass through update Number of rows updated , To make some business judgments , Similar to the following pseudo code ：(mybatis + mysql scene )

if (xxxMapper.updateByPrimaryKeySelective(entity)>0){
   // The update is successful , Do other business processing
}

But there's a hole ,mysql in update Number of rows affected >0 It's conditional , If there is a table ：

There's only one record in it （ID=1）, We use it update Update one ID=1 This record of . in addition , newest MySQL The interview questions are sorted out , You can Java Interview library applet online brush questions .

This line was successfully updated , The number of affected lines is 1, Taken for granted , Then put this update Statement again ：

Now , The number of affected rows returned is 0, in other words , When the record to be updated is the same as the old value of the original record ,mysql In fact, there will be no update ..

In other words , If the data transmitted from the upstream , Equal to the old value of the database itself , When there is no change ,update The number of lines affected by the statement is 0. This is different from another scenario ：" Update a record that doesn't exist , Affect the number of rows and return 0" Indistinguishable .

Conclusion ： Do not use update The number of lines affected by the statement makes important business judgments ！