Lesson 4.3: Deleting data

To delete data from the database, we use a Delete query. Delete queries always consist of a match clause followed by a delete clause. The way we write Delete queries depends on if we want to delete an entity, relation, or attribute. To begin with, we’ll look at an example of an attribute.

It turns out that Fundamentals of Data Engineering has been mistakenly assigned the genre "children’s fiction" (in addition to "nonfiction" and "technology"). We should probably change that before there are too many upset children, which we’ll do with the following Delete query. To run a Delete query, use a data session and write transaction, run the query, and commit the transaction.

Run and commit

To begin with, we’ve matched the book entity by its ISBN in the match clause. Then, we’ve matched the genre attribute by its value. Finally, we’ve deleted the entity’s ownership of the attribute. In TypeQL, Delete queries delete data per statement rather than per variable, so the has statement in the delete clause only deletes the attribute ownership, not the entity itself.

Next, we’ll see how to delete relations. The courier that is supposed to be delivering order "o0033" has notified us that they will be unable to fulfill the order and are cancelling the pickup. Until we are able to arrange an alternative courier, we should record this fact in the database by deleting the delivery relation that the order plays a role in, which we do with the following query.

Run and commit

Similar to the previous example, we’ve begun by matching the order by its order ID. Next, we’ve matched the delivery relation in which it plays a role. We’ve omitted the role and let TypeDB infer it. Role inference means we’ll match any delivery in which $order-33 plays any role, but in this case only one such relation exists, in which $order-33 plays delivery:delivered. We’ve also omitted the roleplayers we aren’t interested in from the tuple: the courier and the address. We don’t need to know them to identify or delete the relation!

Finally, the delete clause deletes the relation. In Insert queries, we use isa statements to create entities and relations, and has statements to create their attributes. Likewise in Delete queries, we use isa statements to delete entities and relations, and has statements to delete their attributes.

Last, we’ll see how to delete entities. A user has decided to delete a review they made of the book. In the following queries, we perform their request. First of all, we need to retrieve the ID of the review to be deleted.

Now that we know the review’s ID, we can go ahead and delete it. We start by deleting any relations that the review plays a role in.

Run

Here, we delete the relations by using the same query structure as we used to delete the delivery relation earlier, but we’re using a parametric statement to delete all relations rather than just deliveries. Once that’s done, we can delete the review itself.

Run and commit

When deleting an entity, it’s important to also delete any relations it plays a role in. If we don’t, then those relations would be left behind without the entity as a roleplayer. A relation is only deleted automatically if all of its roleplayers are deleted.