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Sql group functions



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Sql group functions Sql group functions Presentation Transcript

  • SQL Group Functions
  • Chapter Objectives
    • Differentiate between single-row and multiple-row functions
    • Use the SUM and AVG functions for numeric calculations
    • Use the COUNT function to return the number of records containing non-NULL values
    • Use COUNT(*) to include records containing NULL values
  • Chapter Objectives
    • Use the MIN and MAX functions with non-numeric fields
    • Determine when to use the GROUP BY clause to group data
    • Identify when the HAVING clause should be used
    • List the order of precedence for evaluating WHERE, GROUP BY, and HAVING clauses
  • Chapter Objectives
    • State the maximum depth for nesting group functions
    • Nest a group function inside a single-row function
    • Calculate the standard deviation and variance of a set of data, using the STDDEV and VARIANCE functions
  • Group Functions
    • Return one result per group of rows processed
    • Also called multiple-row and aggregate functions
    • All group functions ignore NULL values except COUNT(*)
    • Use DISTINCT to suppress duplicate values
  • SUM Function
    • Calculates total amount stored in a numeric column for a group of rows
  • AVG Function
    • Calculates average of numeric values in a specified column
  • COUNT Function
    • Two purposes:
      • Count non-NULL values
      • Count total records, including those with NULL values
  • COUNT Function – Non-NULL Values
    • Include column name in argument to count number of occurrences
  • COUNT Function – NULL Values
    • Include asterisk in argument to count number of rows
  • MAX Function
    • Returns largest value
  • MIN Function
    • Returns smallest value
  • GROUP BY Clause
    • Used to group data
    • Must be used for individual column in the SELECT clause with a group function
    • Cannot reference column alias
  • GROUP BY Example
  • HAVING Clause//
    • Serves as the WHERE clause for grouped data
  • Order of Clause Evaluation
    • When included in the same SELECT statement, evaluated in order of:
      • WHERE
      • GROUP BY
      • HAVING
  • Nesting Functions
    • Inner function resolved first
    • Maximum nesting depth: 2
  • Statistical Group Functions
    • Based on normal distribution
    • Includes:
      • STDDEV
      • VARIANCE
  • STDDEV Function
    • Calculates standard deviation for grouped data
  • VARIANCE Function
    • Determines data dispersion within a group