PostgreSQL Aggregate Query

PostgreSQL is an open source relational Database management system that provides rich aggregate query functions. Before introducing different aggregation queries, let's assume the following table structure and sample data: Table name: students |Id | name | age | grade| |----|--------|-----|-------| |1 | Alice | 18 | 12| |2 | Bob | 17 | 11| |3 | Charlie | 19 | 12| |4 | David | 16 | 10| |5 | Ethan | 18 | 11| Next, we will introduce some common aggregation queries and their implementation methods: 1. COUNT function: used to calculate the number of rows or the number of non NULL values. -Calculate the total number of students: 'SELECT COUNT (*) from students` -Number of students aged 18 and above: 'SELECT COUNT (*) From students where age>=18` 2. SUM function: used to calculate the sum of a specified column. -Calculate the total age of all students: 'SELECT SUM (age) from students` -Calculate the total age of 12th grade students: 'SELECT SUM (age) from students where grade=12` 3. AVG function: used to calculate the average value of a specified column. -Calculate the average age of all students: 'SELECT AVG (age) from students` -Calculate the average age of 10th grade students: 'SELECT AVG (age) From students where grade=10` 4. MAX function: used to find the maximum value of the specified column. -Find the oldest student: 'SELECT MAX (age) From students` -Find the oldest student among 12th grade students: 'SELECT MAX (age) From students where grade=12` 5. MIN function: used to find the minimum value of a specified column. -Find the youngest student: 'SELECT MIN (age) From students` -Find the youngest student among 10th grade students: 'SELECT MIN (age) From students where grade=10` 6. GROUP BY clause: used to group results by specified columns and perform aggregation calculations on each group. -Calculate the number of students in each grade by grade: 'SELECT grade, COUNT (*) From students GROUP BY grade` -Calculate the average age of each grade by grade: 'SELECT grade, AVG (age) From students GROUP BY grade` These are just some basic examples of aggregate queries, and PostgreSQL also supports more complex aggregate query operations such as HAVING clauses, DISTINCT keywords, and so on. In practical applications, suitable aggregation functions and combination conditions can be used according to specific needs to achieve more complex query operations.