Delete (cruD) using Perl and DBD::ORACLE

In this post, we’re going to take a look at the D in CRUD: Delete.

We use the DBD::Oracle driver to delete some data in the database tables, using the connection object created in the Initial Setup section of the first post in this series.

PLEASE REVIEW ALL EXAMPLE CODE AND ONLY RUN IT IF YOU ARE SURE IT WILL NOT CAUSE ANY PROBLEMS WITH YOUR SYSTEM.

Helper Function

My helper function get_all_rows encapsulates a select statement used to verify that the deletes worked. The select functionality is covered in the R part of this series, so I won’t go into the details here.

Add this function to the top of your file.

Resetting the data

To keep the examples clean and precise, I will reset the data at times.

Create a new file called reset_data.perl with the following code and run it whenever you would like to reset the data. (Notice this version adds people and pet data not included in other sections.)

Boilerplate template

The template we will be using is:

For each exercise, replace the “# Your code here” line with your code.

Reset the data

First, let’s run reset_data.perl to set up our data.

Simple delete

We will perform a simple delete that removes a single record from the lcs_people table.  These are the steps performed in the code snippet below.

  • Prepare a SQL DELETE statement, deleting the record with an id of 1.
  • Bind the id value. (See the R part of this series for an explanation of bind variables.)
  • Execute the statement.
When I run this code in my Perl session, I see:

Extra Fun 1

Delete all the birds .

Your results should be:

Answer

Reset the data

Now is a good time to run reset_data.perl.

Boilerplate change

Change the boilerplate get_all_rows statements to get people and pet data.

Deleting records referenced by Foreign Keys

If you are using integrity constraints in your database (of course you are, because then you let the database do some heavy lifting for you), you will sometimes need to change the way you process your changes.

In our design, we have a Foreign Key constraint in lcs_pets that ensures if a pet has an owner, that owner exists.

This is the statement that creates the constraint in the Creating the Database Objects section of the Initial Setup post.

If we attempt to delete a record in lcs_people that is referenced in lcs_pets (Person has a pet,) we get an error.

When I run this code in my Perl session, I see:

Before deleting the person you have to handle the pet (watch out for claws and teeth).

There are a few options here, depending on your database design:

  • If: pets are not required to have an owner and you only want to delete the person, not the pets.  Then: you can update the pets and set their owner to null.
  • If: pets are required to have an owner.  Then: you can delete the pets for the owner.

In either of the above scenarios, you can update the pets and set their owner to another person.

Bob is moving out of our area and his new apartment doesn’t allow pets, so he’s giving them to Kim.  Let’s use that last option here.

  • Prepare a SQL UPDATE statement, changing owner to 2 (Kim) for the records with an owner of 1 (Bob).  Updating is covered in the U part of this series.
  • Bind the new and old owner values.
  • Execute the statement.
  • Prepare a SQL DELETE statement, deleting records with an id of 1 (Bob).
  • Bind the id value.
  • Execute the statement.
When I run this code in my Perl session, I see:

When you change data it’s a good idea to verify the number of affected rows.  This is covered in the R part of this series.

Extra Fun 2

Due to a zoning change, snakes are no longer allowed in our area.  Stacey has decided to move and take Sneaky with her.

Let’s fix our data.

Your results should be:

Answer

Some other things you could try
  • Change the database constraints to delete or Null the child record on delete (a cascading delete).  Delete a person and let the database handle the children.
  • Remove the people who don’t have any pets.

Series sections

Initial Setup
Create records
Retrieve records
Update records
Delete records

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