Explain the difference between a changelog and a changeset.

A changelog is the root file that contains or references all database changes for your project. It acts as a ledger of all modifications that should be applied to bring a database to a particular state.

A changeset is an individual unit of change within a changelog. Each changeset represents a single logical database change and has a unique identifier.

<!-- This is a changelog file -->
<databaseChangeLog>
    <!-- This is a changeset -->
    <changeSet id="1" author="john.doe">
        <createTable tableName="users">
            <column name="id" type="int" autoIncrement="true">
                <constraints primaryKey="true"/>
            </column>
            <column name="username" type="varchar(50)"/>
        </createTable>
    </changeSet>
    
    <!-- Another changeset -->
    <changeSet id="2" author="jane.smith">
        <addColumn tableName="users">
            <column name="email" type="varchar(100)"/>
        </addColumn>
    </changeSet>
</databaseChangeLog>

Key characteristics of changesets:

• Must have unique ID and author combination
• Are atomic (all-or-nothing execution)
• Can only be run once per database (unless marked as runAlways)