Question

What are Concurrency Conflicts with regards to SQL database?

Answer 1

Assume you have an SQL statement, e.g., UPDATE table SET a = a + 1 WHERE ..., which would correspond to the following code:

read a
a = a + 1
write a

Assume that two clients A and B execute this simultaneously. The following could happen (time flows from top to bottom):

   A           B
read a     
            read a
a = a + 1
write a
            a = a + 1
            write a

What happens? a is incremented only once, although it should have been incremented twice. This is a classical concurrency conflict. To avoid such conflicts, databases use transactions and locks.