We often rely on our tools and just deploy new DB versions and move on.

Lets see these simple examples.
Example 1
You have Schema v1 where table's column has the name `X`. At the next Schema v2 instead of it you created column named `Y`.

v1 -> v2
X -> -
- -> Y

So the tool correctly drops the `X` and creates `Y`.
Example 2:
For downgrades it looks the similar:

v2 -> v1
Y -> -
- -> X

Simple! Right??

Example 3
Let's do it in more adv…