大型数据库管理系统 oracle.ppt

大型数据库管理系统 Aggregating Data Using Group Functions Objectives After completing this lesson, you should be able to do the following: Identify the available group functions Describe the use of group functions Group data using the GROUP BY clause Include or exclude grouped rows by using the HAVING clause What Are Group Functions? Group functions operate on sets of rows to give one result per group. Types of Group Functions AVG COUNT MAX MIN STDDEV SUM VARIANCE Using Group Functions Using AVG and SUM Functions You can use AVG and SUM for numeric data. Using MIN and MAX Functions You can use MIN and MAX for any datatype. Using the COUNT Function COUNT(*) returns the number of rows in a table. Using the COUNT Function COUNT(expr) returns the number of nonnull rows. Group Functions and Null Values Group functions ignore null values in the column. Using the NVL Function with Group Functions The NVL function forces group functions to include null values. Creating Groups of Data Creating Groups of Data: GROUP BY Clause Divide rows in a table into smaller groups by using the GROUP BY clause. Using the GROUP BY Clause All columns in the SELECT list that are not in group functions must be in the GROUP BY clause. Using the GROUP BY Clause The GROUP BY column does not have to be in the SELECT list. Grouping by More Than One Column Using the GROUP BY Clause on Multiple Columns Illegal Queries Using Group Functions Any column or expression in the SELECT list that is not an aggregate function must be in the GROUP BY clause. Illegal Queries Using Group Functions You cannot use the WHERE clause to restrict groups. You use the HAVING clause to restrict groups. Excluding Group Results Excluding Group Results: HAVING Clause Use the HAVING clause to restrict groups Rows are grouped. The group function is applied. Groups matching the HAVING clause are displayed. Using the HAVING Clause Using the HAVING Clause Nesting Group Functions Display the maximum average salary. Summar