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Intro to Set Functions in SQL

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• Set Functions
  • Set Functions compute an aggregate value from a set of values

  • The set functions available in SQL are:

    SUM( ): sums a set of values AVG( ): takes the average of a set of values MAX( ): finds the maximum value of a set of values MIN( ): finds the minimum value of a set of values COUNT( ): returns the number of elements in a set (duplicates are counted multiple times !!!) COUNT(DISTINCT ...): returns the number of distinct elements in a set ( duplicates are counted ONCE )

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• Applying set function on a set of values
  • Fact:

    SELECT returns a set of tuples

    Therefore:

    We can therefore apply one or more set functions on the result (set) of the SELECT command.

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  • Syntax:

    SELECT SetFunction( attribute ) , ... FROM .... WHERE ....

    Meaning:

    Apply the set function SetFunction( ) on the attribute values in the selected set of tuples returned by the SELECT command

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  • Example:

    SELECT salary FROM employee Result (set): SALARY ---------- 30000 40000 25000 43000 38000 25000 25000 55000 SELECT SUM(salary) FROM employee Result: SUM(SALARY) ----------- 281000

  • The query returns the total salary, the average salary, the max salary, the min salary and the number of salary values of all employees.

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  • Note:

    COUNT( ) will count duplicate values as multiple times To count duplicate value once, use COUNT(DISTINCT ...)

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• The set of tuples used by Set Functions
  • Important note:

    The output of a set function depends on the set of selected tuples output by the WHERE clase of the SELECT command

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  • Example:

    select salary from employee where dno=4 salary --------- 25000.00 43000.00 <--- Max 25000.00 select max(salary) from employee where dno=4 MAX(SALARY) ----------- 43000.00

  • On the other hand:

    select salary from employee where dno=5 salary --------- 31004.00 50000.00 <--- Max 38000.00 25000.00 select max(salary) from employee where dno=5 MAX(SALARY) ----------- 50000.00

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  • Therefore:

    How well you can use set functions will depend very much on how well you can formulate the where clause that select the correct set of tuples !!!!

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• Renaming the selected set function attribute
  • Names given to set functions attributes:

    select max(salary), avg(salary), sum(salary) from employee max(salary) avg(salary) sum(salary) ----------- ------------- ------------------------------- 55000.00 33875.000000 271000.00

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  • You can rename the selected attributes with as alias clause inside the SELECT clause:

    select max(salary) Max, avg(salary) Average, sum(salary) from employee Max Average sum(salary) --------- ------------- ------------------------------- 55000.00 33875.000000 271000.00