Data query of MySQL (aggregate function)

        SELECT The expression of clause can contain the so-called aggregate function （Aggregation Function）. Aggregate functions are often used to calculate a set of values , And then return a single value . except COUNT() Out of function , Aggregate functions ignore null values . Aggregate functions are usually associated with GROUP BY Clauses are used together . if SELECT There is a GROUP BY Clause , Then the aggregate function works on all columns . If there is no , be SELECT Statement produces only one line as the result .

Common aggregate functions ：

COUNT： Find the number of items in the group , return int Type integer

MAX： For maximum

MIN： For the minimum

SUM： Returns the sum of all values in an expression

AVG： Find the average of the group median

 1、COUNT() function

The most commonly used in aggregate functions is COUNT() function , It is used to count the number of rows or total rows that meet the conditions in the table , return SELECT Statement retrieves rows that are not NULL The number of values , If no match is found , Then return to 0.

The grammar format is ：COUNT（ { [ALL | DISTINCT] expression } | *）

  example ： Ask the total number of members

SELECT COUNT(*)AS ' Number of members '
FROM Members;

  example ： Count the number of closed orders

SELECT COUNT( Whether to settle )AS' Number of orders closed '
FROM Sell;

 2、MAX() Functions and MIN() function

MAX and MIN Functions are used to find the maximum and minimum values of all value items in the expression

Grammar format ：MAX / MIN （[ALL | DISTINCT] expression ）

example ： Please order the book number IS-001 The maximum number of orders and the minimum number of orders  

SELECT MAX( Number of copies ordered ),MIN( Number of copies ordered )
FROM Sell
WHERE  Book number ='IS-001';

 3、SUM() Functions and AVG() function

SUM and AVG Respectively used to find the sum and average of all value items in the expression

Grammar format ：SUM / AVG ( [ ALL | DISTINCT ] expression )

  example ： Please order the book number IS-01 The total number of books ordered

SELECT SUM( Number of copies ordered )AS' Total number of copies ordered '
FROM Sell
WHERE  Book number ='IS-01';

  example ： Please order books with the number IS-02 The average number of books ordered

SELECT AVG( Number of copies ordered )AS' The average number of copies per order '
FROM Sell
WHERE  Book number ='IS-02';