mysql的常用函数

可爱的小小小狼2024-07-12 9:09

String

select concat(c1,' ',c2);

SELECT CONCAT(first_name, ' ', last_name) AS full_name FROM users;

select substring(c1,1,3);

length()

SELECT username FROM users WHERE LENGTH(username) < 5;

round(num,3)四舍五入

select a1.machine_id ,round(avg(a2.timestamp - a1.timestamp),3) processing_time

from Activity a1

left join Activity a2 on

a1.machine_id = a2.machine_id and

a1.process_id = a2.process_id and

a1.activity_type = 'start' and

a2.activity_type = 'end'

group by machine_id

datediff（c1,c2）；

SELECT DATEDIFF(NOW(), '2024-01-01') AS days_since_new_year;

SELECT order_id, order_date, delivery_date

FROM orders

WHERE DATEDIFF(delivery_date, order_date) > 30;

DATEDIFF() 函数返回 date1 和 date2 之间的天数差。请注意，date1 和 date2 的顺序会影响结果的正负值。

count（*）返回行数，如果（c1）则c1为null不算

sum（c）返回一列的和

avg（c）返回列的平均值

max（），min（）