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uni/year2/semester1/logseq-stuff/pages/Aggregate Clauses, Group By, & Having Clauses.md

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  • #CT230 - Database Systems I
  • Previous Topic: SQL SELECT: Working with Strings & Subqueries
  • Next Topic: Entity Relationship Models
  • Relevant Slides: Topic 6 SQL_DML_aggregateFns and Group By Having.pdf

  • Aggregate Functions #card

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    • Aggregate Functions are only supported in SELECT clauses & HAVING clauses.
    • Keywords SUM, AVG, MIN, MAX work as expected and can only be applied to numeric data.
    • Keyword COUNT can be used to count the number of tuples / values / rows specified in a query.
    • We can also use mathematical operations as part of an aggregate function on numeric data, e.g., +, -, *, /.

  • GROUP BY #card

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    • GROUP BY <group attributes>
    • The GROUP BY clause allows the grouping of rows of a table together so that all occurrences within a specified group are collected together.
    • Aggregate clauses can then be applied to groups.

    • Using Aggregate Functions with GROUP BY #card

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      • The GROUP BY clause specifies the group and the aggregate function is applied to the group.
      • COUNT(*) can be used to count the number of rows (tuples) in the specified groups.
      • SUM, AVG, MIN, MAX can be used to find the sum, average, min, & max of a numerical value in a specified group.
    • ^^Important: You must GROUP BY all attributes in the SELECT clause unless they are involved in an aggregation.^^
      • This ^^wouldn't work^^ as we do not GROUP BY all the attributes in the SELECT clause - salary remains ungrouped. 
        • SELECT dno, salary
FROM employee
GROUP BY dno -- THIS IS WRONG

  • HAVING #card

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    • HAVING <group condition>
      • The HAVING clause is used in conjunction with GROUP BY and allows the specification of conditions on groups.
      • The column names in the HAVING clause must also appear in the GROUP BY list or be contained within an aggregate function, i.e., you cannot apply a HAVING condition to something that has not been calculated already.