unrepeatable read ---- I understood it means A transaction reads a row twice and reads different state each time. For example, another transaction may have written to the row, and committed, between the two reads. ---- but why it is a problem , ofcourse always the updated data is best to work with.
Second lost updates problem ---two concurrent transactions both read a row, one writes to it and commits, and then the second writes to it and commits. The changes made by the first writer are lost .... but why does it matter the changes made by first writer , anyway those will be lost after some point of time.
Phantom read-A transaction executes a query twice, and the second result set includes rows that weren't visible in the first result set ----- same question as unrepeatable read.
Please brief me with the use cases ,if any.