SQL aggregate function (AVG, Count, first, last, Max, Min, sum)

The operation of the Aggregate function targets a series of values and returns a single value.

Note: If a SELECT statement is used in many other expressions in the list of items in the SELECT statement, the select must use the GROUP by statement!

First, all SQL instances are used to the following table

o_id	OrderDate	Orderprice	Customer
1	2008/12/29	1000	Bush
2	2008/11/23	1600	Carter
3	2008/10/05	700	Bush
4	2008/09/28	300	Bush
5	2008/08/06	2000	Adams
6	2008/07/21	100	Carter

SQL AVG

The AVG function returns the average of a numeric column. NULL values are not included in the calculation.

　　Grammar

SELECT AVG (column_name) from table_name

　　Example 1, calculating the average of the "Orderprice" field

SELECT AVG (Orderprice) as Orderaverage from Orders

The results are as follows

Orderaverage
950

Example 2, find customers with Orderprice values above the Orderprice average

Select Customerfrom orderswhere orderprice> (select AVG (orderprice) from Orders)

The results are as follows

Customer
Bush
Carter
Adams

SQL COUNT

The count () function returns the number of rows that match the specified condition (Null does not count in)

SELECT COUNT (column_name) from table_name

SQL COUNT (*) returns the number of records in the table

SELECT COUNT (*) from table_name

SQL COUNT (DISTINCT column_name) returns the number of different values for the specified column

SELECT COUNT (DISTINCT column_name) from table_name

Note: COUNT (DISTINCT) applies to Oracle and SQL SERVER, but is not available for access

Example 1, calculating the number of orders for the customer "Carter"

SELECT COUNT (Customer) as Customernilsen from Orderswhere customer= ' Carter '

The result of the above SQL statement is 2 because there are 2 orders for customer Carter:

Customernilsen
2

Example 2, using count (*) to get the total number of rows in a table

SELECT COUNT (*) as Numberofordersfrom Orders

The result set looks like this:

Numberoforders
6

Example 3, using count (DISTINCT column_name) to calculate the number of different customers in the Orders table

SELECT COUNT (DISTINCT Customer) as Numberofcustomersfrom Orders

The result set looks like this:

Numberofcustomers
3

This is the number of different customers (Bush, Carter, and Adams) in the Orders table.

SQL First

The first () function returns the value of record one in the specified field

Records can be sorted by using the ORDER BY statement

Grammar

SELECT First (column_name) from table_name

Instance to find the first value of the "Orderprice" column

SELECT First (Orderprice) as Firstorderprice from Orders

The result set looks like this:

Firstorderprice
1000

SQL Last

The last () function returns the value of the final record in the specified field

　　Records can be sorted by using the ORDER BY statement

Grammar

SELECT last (column_name) from table_name

Instance to find the last value in the "Orderprice" column

SELECT last (Orderprice) as Lastorderprice from Orders

The result set looks like this:

Lastorderprice
100

SQL MAX

The max () function returns the maximum value in a column. Null values are not included in the calculation

Grammar

SELECT MAX (column_name) from table_name

MIN and MAX can also be used for text columns to get the highest or lowest values in alphabetical order

instance, finding the maximum value for the "Orderprice" column

SELECT MAX (Orderprice) as Largestorderprice from Orders

The result set looks like this:

Largestorderprice
2000

SQL MIN

The min () function returns the minimum value in a column. Null values are not included in the calculation

Grammar

SELECT MIN (column_name) from table_name

MIN and MAX can also be used in text columns to get the highest or lowest values in alphabetical order.

instance to find the minimum value in the "Orderprice" column

SELECT MIN (Orderprice) as Smallestorderprice from Orders

The result set looks like this:

Smallestorderprice
100

SQL SUM

The sum () function returns the total number of numeric columns

Grammar

SELECT SUM (column_name) from table_name

Instance to find out the total number of "Orderprice" fields

SELECT SUM (Orderprice) as OrderTotal from Orders

The result set looks like this:

OrderTotal
5700

5700= 1000+1600+700+300+2000+100

SQL aggregate function (AVG, Count, first, last, Max, Min, sum)