How to rollback a drop column in Liquibase - liquibase

Question

Which is the best way to rollback this liquibase script?
<changeSet author="me" id="drop_column_example">
<dropColumn tableName="BLABLA">
<column name="example"/>
</dropColumn>
</changeSet>
Usually when I drop a table, I restore all the data from a duplicate temporary table created before the drop, but how can I manage this?
PS: I need to restore all the old data in that column.

Answer 1

Liquibase provides commands to undo changes made to database. The intention of a rollback script is to return the database to a previous specified point in time.
There are two categories of Liquibase operations, resulting in a different generation of a rollback statement:
1. Automatic - where migration can deterministically generate steps required for rolling back
2. Manual - where we need to issue a rollback command because migration instruction cannot be used to identify the statement deterministically
For example, the rollback of a “create table” statement would be to “drop” the created table. This can be determined without a doubt, and therefore the rollback statement can be autogenerated.
On the other hand, the rollback statement for a “drop table” command is not possible to be determined. It is not possible to determine the last state of the table, and therefore the rollback statement can't be autogenerated. These types of migration statements require a manual rollback instructions.
Read more about it here and here.
In your case, rollback statement for a “drop column” command is not possible to be determined. You will manually have to write an “alter table ADD column” statement for your table.
For your example, the query will be :
ALTER TABLE BLABLA ADD COLUMN example DATATYPE;
So the complete changeset would look like this :
<changeSet author="me" id="drop_column_example">
<dropColumn tableName="BLABLA">
<column name="example"/>
</dropColumn>
<rollback>
ALTER TABLE BLABLA ADD COLUMN example COLUMN_DATATYPE;
</rollback>
</changeSet>
To restore data in the column, you will have to create a changeset to copy and insert the data from a duplicate temporary table (similar to the way you mentioned for “drop table”) to this created column. Make sure you execute your data insertion in column after the rollback query for “drop column” is executed successfully.
You could either create a separate changeset or could include the logic to copy and insert old data for restoring into new column in the above changeset as below:
<changeSet author="me" id="drop_column_example">
<dropColumn tableName="BLABLA">
<column name="example"/>
</dropColumn>
<rollback>
ALTER TABLE BLABLA ADD COLUMN example COLUMN_DATATYPE;
<!-- Logic to restore old data goes here OR include the changeset created separately for data restoration here (After creating the dropped column) -->
</rollback>
</changeSet>
Added logic to copy the old data from temporary table to new created table -
INSERT INTO `New_Table` (`example`) VALUES
(SELECT `example` FROM `Temporary_Table` WHERE
`New_Table`.`Some_Common_Column` = `Temporary_Table`.`Some_Common_Column`);
You need to maintain the old data into a table in such a way that you can insert it at the time of restoration with proper mapping to the primary key column or some column which is common between the newly created table and backup table. Without having such structure you won't be able to map the required column data for data restoration in new table column.