表件数count使用总结

1，常用connt形式的区别

count(1)
count(*)
count(列名）	列值是NULL的时候不统计

2，多个表的件数统计

select * from
(select count(*) as employees_cnt from employees) employees,
(select count(*) as departments_cnt from departments) departments,
(select count(*) as salaries_cnt from salaries) salaries

3, 根据列来取得件数

select hire_date,count(*)  from employees  group by hire_date

4，重复去除的件数取得

SELECT COUNT(DISTINCT department_id) FROM user;

5，最快速表件数的取得

5.1 mysql

select 
    table_rows 
from 
    information_schema.tables 
where 
    table_name = 'employees'