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Bus Troubles
2:17Which teachers don't have a class during 1st period?
Wow, that must keep her pretty busy.
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The last thing I'd like you to figure out,
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is which teachers don't
have a first period class?
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They're having trouble unloading
the buses on time, and
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are wondering if they can get
some of the teachers to help out.
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To figure out which teachers don't
have a class during first period,
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we can actually do
something pretty clever.
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Instead of trying to solve
this question directly,
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we can figure out which teachers do
have a class during first period, and
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just subtract those teachers away
from all the other teachers.
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So let's start by figuring out,
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which teachers do have
a class during first period.
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So let's SELECT* FROM TEACHERS, And
then to get the period information,
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we'll need to join to the classes table.
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So JOIN CLASSES ON TEACHERS.ID
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= CLASSES.TEACHER_ID.
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And let's add a WHERE clause.
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And if we look at the CLASSES table,
we've got a PERIOD_ID column.
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And if you don't have a CLASSES table
over here, you can always just select
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star from classes or
whatever table you need to look into.
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So WHERE PERIOD.ID = 1.
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All right, here's the teachers
that have a class first period.
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Now, all we need to do, is take
a selection of all the teachers, and
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subtract away these teachers.
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So above this,
let's SELECT* FROM TEACHERS.
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And then, between these two queries,
all we need to write is EXCEPT,
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which in some languages
will be the minus keyword.
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Now, if we run the query, we get an error.
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When you're using except, you need to
make sure that you have exactly the same
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columns in your first query and
your second query.
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So instead of selecting
everything from both of these,
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since the second one
includes the classes table,
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let's go ahead and
just select TEACHERS.* and TEACHERS.*.
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And now if we run it, it looks like
we've only got one teacher back,
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Cassandra McGuire.
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Great work figuring out
all of these queries.
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I'm not sure where they'd be without you.
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On that note,
if you've got any questions or
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comments about what you've seen here,
be sure to post about it in the community.
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Until next time.
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