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110 lines
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<TITLE>The POSTGRES95 User Manual - EXTENDING SQL: AGGREGATES</TITLE>
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<H1>10. EXTENDING SQL: AGGREGATES</H1>
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<HR>
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Aggregates in POSTGRES are expressed in terms of state
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transition functions. That is, an aggregate can be
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defined in terms of state that is modified whenever an
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instance is processed. Some state functions look at a
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particular value in the instance when computing the new
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state (<B>sfunc1</B> in the create aggregate syntax) while
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others only keep track of their own internal state
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(<B>sfunc2</B>).
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If we define an aggregate that uses only <B>sfunc1</B>, we
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define an aggregate that computes a running function of
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the attribute values from each instance. "Sum" is an
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example of this kind of aggregate. "Sum" starts at
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zero and always adds the current instance's value to
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its running total. We will use the <B>int4pl</B> that is
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built into POSTGRES to perform this addition.
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<pre> CREATE AGGREGATE complex_sum (
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sfunc1 = complex_add,
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basetype = complex,
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stype1 = complex,
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initcond1 = '(0,0)'
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);
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SELECT complex_sum(a) FROM test_complex;
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+------------+
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|complex_sum |
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+------------+
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|(34,53.9) |
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+------------+
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</pre>
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If we define only <B>sfunc2</B>, we are specifying an aggregate
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that computes a running function that is independent of
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the attribute values from each instance.
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"Count" is the most common example of this kind of
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aggregate. "Count" starts at zero and adds one to its
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running total for each instance, ignoring the instance
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value. Here, we use the built-in <B>int4inc</B> routine to do
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the work for us. This routine increments (adds one to)
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its argument.
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<pre> CREATE AGGREGATE my_count (sfunc2 = int4inc, -- add one
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basetype = int4, stype2 = int4,
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initcond2 = '0')
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SELECT my_count(*) as emp_count from EMP;
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+----------+
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|emp_count |
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+----------+
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|5 |
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+----------+
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</pre>
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"Average" is an example of an aggregate that requires
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both a function to compute the running sum and a function
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to compute the running count. When all of the
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instances have been processed, the final answer for the
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aggregate is the running sum divided by the running
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count. We use the <B>int4pl</B> and <B>int4inc</B> routines we used
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before as well as the POSTGRES integer division
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routine, <B>int4div</B>, to compute the division of the sum by
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the count.
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<pre> CREATE AGGREGATE my_average (sfunc1 = int4pl, -- sum
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basetype = int4,
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stype1 = int4,
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sfunc2 = int4inc, -- count
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stype2 = int4,
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finalfunc = int4div, -- division
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initcond1 = '0',
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initcond2 = '0')
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SELECT my_average(salary) as emp_average FROM EMP;
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+------------+
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|emp_average |
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+------------+
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|1640 |
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+------------+
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</pre>
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