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I now see I'm down to two paintings, two pieces of artwork that
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were acquired only after 1909.
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OK, so given this part on DELETE, we're able to delete one row or even multiple
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rows at a time.
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But there are still instances where you might
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want to think about, should we delete something or can we delete something.
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We'll talk about those in just a minute in terms of constraints.
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We back in a few.
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And we're back.
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So we just saw how to delete one row, even multiple rows.
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We haven't yet talked about whether we should delete some data
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or whether we should delete some data and particularly
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in the context of these constraints, where
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you might have maybe a piece of data you actually
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shouldn't delete to maintain the integrity of your table.
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Now, one example of this is a [? Foreign ?] [? Key ?]
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[? Constraint, ?] which means that you have some table with a primary key that
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is referenced by some other table.
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And we'll get concrete about this in a minute.
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But it might mean that if you were to delete that primary key,
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that other table would have nothing to reference.
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So let's update our schema here for the MFA, the Museum of Fine Arts.
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And let's say that they have not just a collection now, but also artists
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involved.
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And there exists a [? many ?] [? to ?] [? many ?] relationship among artists
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and items in the collection.
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We could say that an artist might make more than one piece of art
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in the collection.
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And a piece of art might be created by more than one artist.
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Now concretely in our database, this might look as follows.
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I could have a table for artists and a table for collections.
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And notice how each has their own primary key.
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Artists has an id column, called our primary key column.
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Collections too has an id called our primary key column here.
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Now, in the middle is this table created that symbolizes,
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that represents the relationship among artists and items in the collection.
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Here on the first row, we see the artist id of 1 created the piece of art
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with the collection id of 2.
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So who did what here?
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Well, we can see that the artist with the ID of 1 is Li Yin.
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Li Yin created this piece of art, the id of 2, which is Imaginative landscape.
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So we can kind of relate in this case artists with collections
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as we saw just a few weeks ago.
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But let's say we decide to delete a particular artist,
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maybe we delete Unidentified artists down here.
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Well, we could just delete from the artist table.
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Maybe we find a condition to select this row and we delete from artists.
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We'll do that here remove that row.
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But what have we done wrong?
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If we look at our created table in particular,
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what have we now done wrong?
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What kind of problem might we [? run ?] [? into? ?] Let's go to [? Karim. ?]
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SPEAKER 11: [? Yeah ?] [INAUDIBLE] [? was ?] [? only ?] [? deleted ?]
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[? from ?] [? the ?] [? artist ?] [INAUDIBLE] without the other
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[INAUDIBLE].
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SPEAKER 1: Yeah, good point.
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We only deleted the artist with the id of 3 from the artists table.
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Now if I look at the created table and I look for the artist with id of 3,
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do you see them?
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They don't exist anymore.
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So I can't understand this relationship.
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I don't know the artist with the id of 3.
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So let's try this now with a new kind of schema now in our database.
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I'll go back to my computer.
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And let me open up our new version of our database.
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I'll type sqlite3, in this case, mfa.db, same name, but now a different schema.
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If I type .schema, notice a few things here.
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I have those same tables we saw visually.
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I have collections, artists, and a created table.
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And [? let me ?] focus in particular on this created table.
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If I say .schema, in this case, created, Enter,
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I'll see I have a FOREIGN KEY constraint on my "artist_id"
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column and my "collection_id" column.
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So if I tried to delete an artist that is referenced in my created table,
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I would probably raise a constraint error--
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a FOREIGN KEY [? constraint ?] error.
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So let's just try this and see what happens.
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Let me try DELETE FROM the [? artist's ?] [? table, ?] DELETE FROM
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"artists" WHERE, let's say the "name" of the artist is literally "Unidentified
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artist."
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This is the name they have in the Museum of Fine Arts collections--
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Unidentified artist.
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So now I want to delete them from that table.
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Well, if I hit Enter, I do get that FOREIGN KEY constraint error.
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So does this mean I just can't delete this artist or is there a workaround?
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Let's look at it visually again.
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I'll go back to what we had before as our table,
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and let me ask the audience here.
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What's the solution?
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If I can't delete Unidentified artist, because they
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have an id referenced by this table, what should I maybe do first?
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What should I do instead?
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SPEAKER 12: Yeah, I want to say that first we
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can delete the id that is being referenced by this [? id ?]
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that we want to delete, then we can delete it.
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In fact, if we want to delete it from the artists,
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first we have to delete from the created table [INAUDIBLE] table, which
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it is referencing, then we can delete from the artist [INAUDIBLE]
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SPEAKER 1: Yeah, I like your thinking here.
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So if we look at the created table, you notice
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