Detailed explanation of MySQL statistical function count

MySql Statistical function COUNT Detailed explanation

• 1. COUNT() Function Overview
• 2. COUNT() Parameter description
• 3. COUNT() To judge the existence of
• 4. COUNT() Ali Development Specification

1. COUNT() Function Overview

COUNT() It's an aggregate function , Returns the number of rows matching the specified criteria . In development, it is often used to count the data in the table , All data , Not for NULL data , Or de duplication data .

2. COUNT() Parameter description

COUNT(1)： Statistics are not NULL The record of .
COUNT(*)： Count all the records （ Include NULL）.

COUNT( Field )： Statistics should " Field " Not for NULL The record of .
1. If this field is defined as not null Words , Read the field line by line from the record , Judgment cannot be null, Add by line .
2. If the field definition allows null Words , To judge that it might be null, Also take out the value and judge , No null Just add up .

COUNT(DISTINCT Field )： Statistics should " Field " Weight removal without NULL The record of .

-- MySql Statistical function count test 
--  Create a user table , New test data 
CREATE TABLE `user` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT COMMENT 'ID Primary key ',
  `name` varchar(64) DEFAULT NULL COMMENT ' full name ',
  `sex` varchar(8) DEFAULT NULL COMMENT ' Gender ',
  `age` int(4) DEFAULT NULL COMMENT ' Age ',
  `born` date DEFAULT NULL COMMENT ' Date of birth ',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 ROW_FORMAT=DYNAMIC COMMENT=' User table ';

INSERT INTO `category`.`user`(`id`, `name`, `sex`, `age`, `born`) VALUES (1, '% Zhang San %', ' male ', 22, '2022-04-22');
INSERT INTO `category`.`user`(`id`, `name`, `sex`, `age`, `born`) VALUES (2, ' Li Si ', ' Woman ', 12, '2022-04-01');
INSERT INTO `category`.`user`(`id`, `name`, `sex`, `age`, `born`) VALUES (3, ' Wang Xiaoer ', ' Woman ', 12, '2022-04-28');
INSERT INTO `category`.`user`(`id`, `name`, `sex`, `age`, `born`) VALUES (4, ' Zhao si ', ' male ', 23, '2022-04-28');
INSERT INTO `category`.`user`(`id`, `name`, `sex`, `age`, `born`) VALUES (5, '', ' Woman ', 23, '2022-04-28');
INSERT INTO `category`.`user`(`id`, `name`, `sex`, `age`, `born`) VALUES (6, NULL, ' Woman ', 60, '2022-04-28');
INSERT INTO `category`.`user`(`id`, `name`, `sex`, `age`, `born`) VALUES (7, NULL, ' Woman ', 61, '2022-04-28');

select * from user;

--  statistics ：7 Data , Count all the records （ Include NULL）.
select count(*) from user;

--  statistics ：7 Data , Statistics are not NULL  The record of .
select count(1) from user;

--  statistics ：5 Data ,COUNT( Field )： Statistics should " Field " Not for NULL  The record of , Note that null Not empty '' character string 
select count(name) from user;

--  statistics ：5 Data ,COUNT(DISTINCT  Field )： Statistics should " Field " Weight removal without NULL  The record of .
select count(distinct name) from user;

3. COUNT() To judge the existence of

SQL No longer use count, Instead, use LIMIT 1, Let the database query to return when encountering an item , Don't keep looking for how many more , In the business code, you can directly judge whether it is not empty .
select 1 from emp LIMIT 1; Efficiency is the highest , Especially the need for limit Limit the number of rows , It's easy to ignore .

-- SQL Find out if " There is "
--  The employee table , Delete if it exists 
drop table if EXISTS emp;
create table emp(
    id int unsigned primary key auto_increment,
    empno mediumint unsigned not null default 0,
    empname varchar(20) not null default "",
    job varchar(9) not null default "",
    mgr mediumint unsigned not null default 0,
    hiredate datetime not null,
    sal decimal(7,2) not null,
    comn decimal(7,2) not null,
    depno mediumint unsigned not null default 0
);

--  newly added cehsi data 
 Test data ：https://blog.csdn.net/m0_37583655/article/details/124385347

-- cahxun 
select * from emp ;

--  Time ：1.082s, data ：5000000
explain select count(*) from emp;

id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	  SIMPLE																																						Select tables optimized away

--  Time ：1.129s, data ：5000000
explain select count(1) from emp;
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	  SIMPLE																																						Select tables optimized away

--  Time ：1.695s, data ：5000000
explain select 1 from emp;
id	select_type	table	partitions	type	possible_keys	key	          key_len	ref	rows	  filtered	Extra
1	  SIMPLE			emp																		idx_emp_depno		3					4981060		100.00	Using index

-- SQL No longer use count, Instead, use LIMIT 1, Let the database query to return when encountering an item , Don't keep looking for how many more , In the business code, you can directly judge whether it is not empty 
--  Time ：0.001s, data ：5000000
explain select 1 from emp LIMIT 1;
id	select_type	table	partitions	type	possible_keys	key	          key_len	ref	rows	  filtered	Extra
1	  SIMPLE			emp																		idx_emp_depno		3					4981060		100.00	Using index

4. COUNT() Ali Development Specification

1.【 mandatory 】 Do not use count( Name ) or count( Constant ) To replace count(),count() yes SQL92 The standard of definition Syntax of quasi statistical line number , It's not about the database , Follow NULL He Fei NULL irrelevant . explain ：count(*) The statistical value is NULL The line of , and count( Name ) This column is not counted NULL Row of values .

2.【 mandatory 】count(distinct col) Calculate the column Division NULL Number of non repeating lines outside , Be careful count(distinct col1, col2) If one of the columns is all NULL, So even if the other column has different values , Also returned as 0.