SQL tutorial: introduction to SQL aggregate functions

What are aggregate functions

They are functions that operate on a set of rows to calculate and return a single value , We see the aggregate function here ：

• AVG()： Returns the average value of the column .
• COUNT()： Returns the number of rows in the column .
• MAX()： Returns the maximum value of the column .
• MIN()： Returns the minimum value of the column .
• SUM()： Returns the sum of column values .

Using aggregate functions

SQL Provides special functions for summarizing data without retrieving the entire data set . This is a common requirement . Use these functions ,SQL Queries are often used to retrieve data for reporting and analysis .

Be careful ： All aggregate functions can be used to perform calculations on multiple columns using standard mathematical operators , for example ：AVG(prod_price*quantity).

function AVG()

AVG() This function returns the average value of the column by calculating the sum of the number of rows and their values .AVG() It can be used to return the average value of a specific column or row .

Be careful NULL： The function ignores columns and rows that contain values AVG().

function COUNT()

COUNT() Essentially , It is very important . You can use COUNT() Function to determine the number of rows in the table or the number of rows that meet specific conditions .

COUNT() Can pass 2 How to use ：

be used for COUNT() Count the number of rows in the table , Whether the column contains values or NULL value .
be used for COUNT(column) Calculate the number of rows with values in a particular column , Ignore NULL value .
Be careful NULL： The function ignores columns and rows that contain values COUNT(), But if you use an asterisk (<