What are the possible values of the Hibernate hbm2ddl.auto configuration and what do they do

The configuration property is called hibernate.hbm2ddl.auto

In our development environment we set hibernate.hbm2ddl.auto=create-drop to drop and create a clean database each time we deploy, so that our database is in a known state.

In theory, you can set hibernate.hbm2ddl.auto=update to update your database with changes to your model, but I would not trust that on a production database. An earlier version of the documentation said that this was experimental, at least; I do not know the current status.

Therefore, for our production database, do not set hibernate.hbm2ddl.auto - the default is to make no database changes. Instead, we manually create an SQL DDL update script that applies changes from one version to the next.

I would use liquibase for updating your db. hibernate's schema update feature is really only o.k. for a developer while they are developing new features. In a production situation, the db upgrade needs to be handled more carefully.

validate: It validates the schema and makes no changes to the DB.
Assume you have added a new column in the mapping file and perform the insert operation, it will throw an Exception "missing the XYZ column" because the existing schema is different than the object you are going to insert. If you alter the table by adding that new column manually then perform the Insert operation then it will definitely insert all columns along with the new column to the Table. Means it doesn't make any changes/alters the existing schema/table.

update: it alters the existing table in the database when you perform operation. You can add or remove columns with this option of hbm2ddl. But if you are going to add a new column that is 'NOT NULL' then it will ignore adding that particular column to the DB. Because the Table must be empty if you want to add a 'NOT NULL' column to the existing table.

To whomever searching for default value...

It is written in the source code at version 2.0.5 of spring-boot and 1.1.0 at JpaProperties:

    /**
     * DDL mode. This is actually a shortcut for the "hibernate.hbm2ddl.auto"
     * property. Defaults to "create-drop" when using an embedded database and no
     * schema manager was detected. Otherwise, defaults to "none".
     */
    private String ddlAuto;

With all above said... Notice this property is called dll.auto and should only control dll operations(create/drop schema/table), I found surprisingly that it has to do with dml, too: only update will allow insert data, which is dml operation.

Got caught by this when trying to populate data into a in-memory database; only update works.