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Sql group functions(2)


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Sql group functions(2)

  1. 1. Introduction to 1 SQL Group Functions
  2. 2. Introduction to 2 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
  3. 3. Introduction to 3 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
  4. 4. Introduction to 4 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
  5. 5. Introduction to 5 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
  6. 6. Introduction to 6 SUM Function Calculates total amount stored in a numeric column for a group of rows
  7. 7. Introduction to 7 AVG Function Calculates average of numeric values in a specified column
  8. 8. Introduction to 8 COUNT Function Two purposes: – Count non-NULL values – Count total records, including those with NULL values
  9. 9. Introduction to 9 COUNT Function – Non-NULL Values Include column name in argument to count number of occurrences
  10. 10. Introduction to 10 COUNT Function – NULL Values Include asterisk in argument to count number of rows
  11. 11. Introduction to 11 MAX Function Returns largest value
  12. 12. Introduction to 12 MIN Function Returns smallest value
  13. 13. Introduction to 13 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
  14. 14. Introduction to 14 GROUP BY Example
  15. 15. Introduction to 15 HAVING Clause// Serves as the WHERE clause for grouped data
  16. 16. Introduction to 16 Order of Clause Evaluation When included in the same SELECT statement, evaluated in order of: – WHERE – GROUP BY – HAVING
  17. 17. Introduction to 17 Nesting Functions • Inner function resolved first • Maximum nesting depth: 2
  18. 18. Introduction to 18 Statistical Group Functions • Based on normal distribution • Includes: – STDDEV – VARIANCE
  19. 19. Introduction to 19 STDDEV Function Calculates standard deviation for grouped data
  20. 20. Introduction to 20 VARIANCE Function Determines data dispersion within a group