MANAGING DATA(BASES) USING SQL (NON-PROCEDURAL SQL, X401.9)

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1 Technology & Information Management Instructor: Michael Kremer, Ph.D. Class 6 Professional Program: Data Administration and Management MANAGING DATA(BASES) USING SQL (NON-PROCEDURAL SQL, X401.9)

2 AGENDA 9. Aggregate Queries 9.1 Aggregate Functions 9.2 Grouping Data 9.3 Filtering Grouped/Aggregated Data

3 Aggregate Queries 9.

9.1 AGGREGATE FUNCTIONS 169 Aggregate function: Values of multiple rows are grouped together as input on certain criteria to form a single value of more significant meaning or measurement

4 9.1 AGGREGATE FUNCTIONS 169 Aggregate function: Values of multiple rows are grouped together as input on certain criteria to form a single value of more significant meaning or measurement Aggregate queries are still part of SELECT queries (DQL) Following functions are part of SQL standard: Additional aggregate functions include StDev, Var, First, Last (db platform dependent)

5 9.1 AGGREGATE FUNCTIONS 170 Create expressions combining multiple aggregate functions together Rules about aggregate function expressions:

6 9.1 AGGREGATE FUNCTIONS 171 Use of SQL predicates (such as DISTINCT) is allowed: Use WHERE clause to apply aggregation over subset of data:

7 9.1 AGGREGATE FUNCTIONS 172 SELECT query using dynamic WHERE clause criteria based on aggregate query Hard-coded WHERE clause, average salary of ST_CLERK (based on previous example) Use of subquery in WHERE clause to dynamically calculate average salary of ST_CLERK

8 9.1 AGGREGATE FUNCTIONS 173 Subqueries will be covered in next chapter in more detail Basic rules of subqueries:

9.2 GROUPING DATA 174 So far, aggregation took place over entire table or subset using WHERE clause Grouping of data Partition table data into groups and aggregate data within each group New Keyword:

9 9.2 GROUPING DATA 174 So far, aggregation took place over entire table or subset using WHERE clause Grouping of data Partition table data into groups and aggregate data within each group New Keyword: GROUP BY Partition of a set divides the set into subsets such that the union of the subsets returns the original set (pizza, pie) GROUP BY clause takes the result of the FROM and WHERE clauses and arranges the rows into groups defined as having the same values for the columns listed in the Group By clause Each group is reduced to a single row in the resulting set

10 9.2 GROUPING DATA 175

11 9.2 GROUPING DATA 176 If the grouped fields contain null values, all null values are arranged into one group as if they were all equal SELECT syntax including GROUP BY clause Rules for aggregate functions:

12 9.2 GROUPING DATA 177 NULL behavior of aggregate functions (Aggregate functions do not consider NULL values) Rules for GROUP BY:

13 9.2 GROUPING DATA 178 Error message when not using GROUP BY in SELECT query Same query using GROUP BY

14 9.2 GROUPING DATA 179 Use multiple columns in GROUP BY clause Order in GROUP BY clause has no bearing on results To achieve desired order, use ORDER BY clause

9.2 GROUPING DATA 180 Use multiple aggregate functions Group on expressions, this allows you to customize your query in a very specific way based on certain business rules You have to list

15 9.2 GROUPING DATA 180 Use multiple aggregate functions Group on expressions, this allows you to customize your query in a very specific way based on certain business rules You have to list expression twice, in SELECT clause as well as in GROUP BY clause Due to the fact that GROUP BY clause is processed before SELECT clause Better solution is to use subquery (demonstrated later in this course)

16 9.2 GROUPING DATA 181 Group on expression Aggregation based on query using multiple tables

17 9.2 GROUPING DATA 182 When using multiple tables, make sure to aggregate data only on lowest child table When using multiple tables, you denormalize data and display parent records multiple times aggregation takes place on redundant parent data!

18 9.2 GROUPING DATA 183 Remove COUNT function to demonstrate number of records To get the correct count, use the DISTINCT predicate

19 9.2 GROUPING DATA 184 Display distinct departments Only shows count of departments that have employees associated with them (INNER JOIN) To count all departments, use OUTER JOIN!

20 9.2 GROUPING DATA 185 Nest aggregate functions up to a level of two in Oracle (this is not according to the SQL standard) Must use GROUP BY for inner aggregate function returns multiple values Error message when omitting GROUP BY: Add GROUP BY clause to be used for inner aggregate function:

21 9.2 GROUPING DATA 186 Count employees hired in the same year

22 9.2 GROUPING DATA 187 Transpose data results in complex expression

9.3 FILTERING GROUPED/AGGREGATED DATA 188 3 different kind of columns in aggregate SELECT statement: Group By column Aggregate column

23 9.3 FILTERING GROUPED/AGGREGATED DATA different kind of columns in aggregate SELECT statement: Group By column Aggregate column Non-Select column (only used for additional filtering) Hence there 3 filtering situations: Filtering on Non-Select column WHERE clause:

24 9.3 FILTERING GROUPED/AGGREGATED DATA 189 Same query as before but without WHERE clause Resulting data is different!

9.3 FILTERING GROUPED/AGGREGATED DATA 190 Filtering on Group By column WHERE clause Previous two examples: WHERE clause filters data before aggregation takes place

25 9.3 FILTERING GROUPED/AGGREGATED DATA 190 Filtering on Group By column WHERE clause Previous two examples: WHERE clause filters data before aggregation takes place Next example shows filtering on aggregate column HAVING clause HAVING clause is used to restrict the number of rows after the aggregation of data used for aggregate columns

26 9.3 FILTERING GROUPED/AGGREGATED DATA 191 HAVING clause comes after GROUP BY HAVING clause is processed after aggregation takes place WHERE clause, on the other hand, is processed before! You can place HAVING clause before GROUP BY This is just semantic, it does not represent processing order!

9.3 FILTERING GROUPED/AGGREGATED DATA 192 Filtering on aggregate column HAVING clause In order to filter average salary greater than 10000, we must know the average salary

27 9.3 FILTERING GROUPED/AGGREGATED DATA 192 Filtering on aggregate column HAVING clause In order to filter average salary greater than 10000, we must know the average salary first! Aggregation must take place first not very efficient as the database must aggregate all rows first before applying the HAVING clause to filter out rows SQL processing order:

28 9.3 FILTERING GROUPED/AGGREGATED DATA 193 Based on processing order, we understand now that we cannot reference an expression from the SELECT clause in the GROUP BY clause Same is true for WHERE clause But we can reference expressions in the ORDER BY clause by using column alias

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