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let's also delete their affiliation with their artwork all in one go.
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This converts our 2-step process into a 1-step process.
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So let's visualize this.
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Let's say we have now applied this constraint ON DELETE CASCADE.
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So if I delete the artist, I'll also delete their affiliation
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with their work.
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We'll have the same two tables--
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artists, collections, and created.
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Now again, artist_id references this primary key in artists.
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Now, I'm going to try.
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I want to delete Unidentified artist here.
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Well, I could do that.
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I could just delete their row.
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And now, instead of a FOREIGN KEY constraint error, what I get
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is the following.
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[INAUDIBLE] [? created ?] that this row is also gone.
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The row that had the artist_id of 3 gets removed.
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We've cascaded the deletion onward to the created table.
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So let's try this now with a new database schema.
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I'll go back to my computer here.
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So I'm back in my [? TERMINAL ?] here.
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And I can type sqlite3 mfa.db to reopen this database.
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And notice how if I type .schema, I've updated this schema to now have ON
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DELETE CASCADE.
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Let me show you over here on the screen.
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Here on the created table, I now have the very same kind of table schema,
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but now my only difference is I've applied this ON DELETE action
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to my [? foreign ?] [? key ?] constraints.
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In particular, I'm going to [? cascade ?] the delete
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from the artist's table to the created table.
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So let's try that out in SQLite.
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I'll come back over here, and I will now try just
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DELETE FROM the "artists" table WHERE the "name" equals "Unidentified artist.
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semicolon.
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Now, I'll hit Enter, and I don't get a FOREIGN KEY constraint anymore.
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But if I say SELECT [? star ?] FROM "created" semicolon,
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notice how I've also deleted the artist's affiliation with their work.
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So that is wherever in the artist_id column,
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I saw the id for the artist I deleted, I would to delete that row,
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so I have no references to that primary key, which is now gone from my table.
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So let me ask here.
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We've seen how to delete single rows, how to delete multiple rows,
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and now how to delete data among some constraints,
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like our [? foreign ?] [? key ?] constraints.
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What questions do we have on those techniques?
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Let's go to [? Han. ?]
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SPEAKER 13: I have a question regarding deleting.
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The [? id ?] numbers have been removed while we were removing the record.
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I was wondering if do we have to clean it up somehow
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later or will they be populated with the new data as it comes along?
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SPEAKER 1: Yeah, great question.
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So the question is, what happens to our primary keys when we delete our data.
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So for this one, let me show you the visual
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again that we had before of our tables nicely printed on the side.
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So I'll come back here, and I will go back to our idea of these joint tables
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where we had an artists table, a collections table, and a created table.
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And we saw before that we're going to delete
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this artist called Unidentified artist.
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So I'll delete them, and to your point, [? well ?] the idea of 3
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no longer exists in this case.
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Now by default, at least in SQLite, if I insert a new row, what I'll do
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is take the highest id value and I will then
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make that the new primary key for the new row that I insert.
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That is the default situation.
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I could if I wanted to get more specific,
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and if I had done in my id integer column here in my [? create ?]
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[? table, ?] if I had also said this [? keyword ?] [? call ?]
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[? autoincrement, ?] all one word, what would happen instead is I would
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actually reinsert an ID that is not used.
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So in this case 3 is not used, I could insert that one here.
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So up to you what you want to do.
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In general, SQlite by default will take the highest ID, add one from there.
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If you specify this constraint [? called ?] [? autoincrement, ?] you
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will instead take whatever id is not being used and use that in your insert
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afterwards.
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OK, so now we've seen how to insert, and how to delete data.
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But of course, we make mistakes when we add data or even when we delete data.
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So we'll see in just a minute how to update our values
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as well to correct typos and even to update associations
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between artists and artwork.
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And we're back.
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So we've so far seen how to insert some data into our tables
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and how to delete it.
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But sometimes, we don't want to fully delete something,
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we just want to change its value to correct some typo
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or correct some association.
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So let's think to our MFA example, [? where ?] the Museum of Fine Arts
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has some tables that look like this.
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They have artists in their collection, and they also
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have art work in their collection.
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They also have a created table to associate artists with their artwork.
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Now in this case, we know that I have this Unidentified artist.
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And we can see that they authored this item in the collections,
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"Farmers working at dawn."
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But let's say that later on, we find out it wasn't an Unidentified artist.
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It was instead Li Yin, who created "Farmers working at dawn."
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How could we update our tables to make sure it's Li Yin, who we have
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creating "Farmers working at dawn."
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SPEAKER 14: So what we could do is--
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we know that in created, we have our artist_id and [? collection id, ?]
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