0:00
The last operator that I'm
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going to cover in this video is the theta join operator.
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Like natural join, theta join is
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actually an abbreviation that doesn't add expressive power to the language.
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Let me just write it.
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The theta join operator takes
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two expressions and combines them
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with the bow tie looking
0:16
operator, but with a subscript theta.
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That theta is a condition.
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It's a condition in the style
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of the condition in the selection operator.
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And what this actually says
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- it's pretty simple -
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is it's equivalent to applying
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the theta condition to the
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cross-product of the two expressions.
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So you might wonder why
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I even mention the theta
0:37
join operator, and the reason I
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mention it is that most
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database management systems implement the
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theta join as their basic
0:44
operation for combining relations.
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So the basic operation is
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take two relations, combine all tuples,
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but then only keep the combinations
0:51
that pass the theta condition.
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Often when you talk to
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people who build database systems or
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use databases, when they
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use the word join, they really mean the theta join.
1:02
So, in conclusion, relational algebra is a formal language.
1:06
It operates on sets of
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relations and produces relations as a result.
1:10
The simplest query is just
1:12
the name of a relation and
1:13
then operators are used
1:14
to filter relations, slice them, and combine them.
1:17
So far, we've learned the
1:19
select operator for selecting rows;
1:21
the project operator for selecting
1:22
columns; the cross-product
1:24
operator for combining every possible
1:27
pair of tuples from two
1:28
relations; and then two
1:29
abbreviations, the natural join,
1:31
which a very useful way to combine
1:32
relations by enforcing a equality
1:35
on certain columns; and the theta join operator.
1:38
In the next video, we'll learn
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some additional operators of relational
1:41
algebra and also some alternative
1:42
notations for relational algebra expressions.