Functions in SQL (strings, values, dates, process functions)

The employee table is as follows (will be used in the following demonstration operations):

1 String functions

(1) Function implementation

function	Function
concat(s1,s2,…,sn)	String concatenation, concatenate s1, s2...sn into a string
lower（str）	Convert all string str to lowercase
Upper(str)	Convert all string str to uppercase
lpad（str,n,pad）	Left padding, use string pad to pad the left side of str to reach n string lengths
rpad(str,n,pad)	Right padding, fill the right side of str with string pad to reach n string length
trim(str)	Remove leading and trailing spaces from string
substring(str,start,len)	Returns a string of len lengths from the start position of the string str.

-- --------------------------- 函数演示 --------------------------------

-- ----------------------- 1.字符串函数 ----------------------------
-- concat：字符串拼接
select concat('Hello', ' MySQL!');-- Hello MySQL!

-- lower：全部转小写
select lower('Hello');-- hello

-- Upper：全部转大写
select upper('Hello');-- HELLO

-- lpad：左填充
select lpad('01',6,'X');-- XXXX01

-- rpad：右填充
select rpad('01',6,'X');-- 01XXXX

-- trim：去除首尾空格
select trim(' Hello MySQL! ');-- Hello MySQL!

-- substring：截取子字符串
select substring(' Hello MySQL! ',1,13);-- Hello MySQL!

(2) Realization of business needs

-- 业务需求员工号统一为6位数 不足则在前面补0
update employee set workno = lpad(workno,6,'0');

2 Numerical functions

Function implementation

function	Function
ceil(x)	Rounded up
floor(x)	Round down
mod(x,y)	Returns the modulus of x/y
rand()	Returns a random number between 0 and 1
round(x,y)	Find the rounded value of parameter x, retaining y decimal places

-- ----------------------- 2.数值函数 ----------------------------
-- ceil(x)：向上取整
select ceil(1.1);-- 2

-- floor(x)：向下取整
select floor(1.1);-- 1

-- mod(x,y)：取模
select mod(6,5);-- 1

-- rand：获取0~1随机数
select rand();

-- round(x,y)：四舍五入 保留y位小数
select round(6.6);-- 7
select round(6.3);-- 6
select round(6.365,2);-- 6.37


-- 案例：通过数据库的函数 随机生成六位验证码
select lpad(round(rand()*1000000),6,'0');

3 date functions

(1) Function implementation

function	Function
curdate()	Return current date
curtime()	Return current time
now()	Returns the current date and time
year()	Get the year of the specified date
month()	Get the month of the specified date
day()	Get the date of the specified date
date_add(date,interval exprtype)	Returns a date/time value plus a time value expr
datediff(date1,date2)	Returns the number of days between the start time date1 and the end time date2

-- ----------------------- 3.日期函数 ----------------------------
-- curdate：当前日期
select curdate();

-- curtime：当前时间
select curtime();

-- now：当前日期和时间
select now();

-- year month day：当前年 月 日
select year(now());
select month(now());
select day(now());

-- date_add：增加指定的时间间隔
select date_add(now(),interval 70 year);

-- datediff：获取两个日期相差的天数
select datediff('2022-12-01','2021-12-11');
select datediff('2023-3-26','2022-10-16');

(2) Realization of business needs

-- 案例：查询所有员工入职天数 并根据入职天数倒序排序
-- 思路：入职天数-->当前日期-入职日期 调用datediff函数完成
select name,datediff(curdate(),entrydate) as 'entrydays' from employee order by entrydays desc;

4 process function

(1) Function implementation

function	Function
if(value,t,f)	If value is true, return t, otherwise return f
ifnull(value1,value2)	If value1 is not empty, return value1, otherwise return value2
case when [val1] then [res1]…else [fedault] end	If val1 is true, return res1,...otherwise return default value
case [expr] when [val1] then [res1] … else [default] end	If the value of expr is equal to val1, return res1,...otherwise return the default value

-- ----------------------- 4.流程控制函数 ----------------------------
-- if(value,t,f) 如果value为true 返回t 否则返回f
select if(false,'OK','ERROR');

-- ifnull(value1,value2) 如果value1不为空 返回value1 否则返回value2
select ifnull('OK','Default');
select ifnull('','Default');
select ifnull(null,'Default');

-- case when then else end
-- 需求：查询employee表的员工姓名和年龄（age<=19-->少年, 其他-->老年）
select
    name,
    (case when age<=19 then '少年' else '老年' end) as '类分'
from employee;

(2) Realization of business needs

-- 案例：统计班级各个学员的成绩 展示的规则如下：
-- >=90 优秀
-- >=60 及格
-- 否则 不及格
create table score(
  id int comment 'ID',
  name varchar(20) comment '姓名',
  math int comment '数学',
  english int comment '英语',
  chinese int comment '语文'
) comment '学员成绩表';
insert into score values(1,'Tom',67,88,95),(2,'Rose',23,66,90),(3,'Jack',56,98,76);

select
    id,
    name,
    (case when math>=90 then '优秀' when math>=60 then '及格' else '不及格' end) '数学',
    (case when english>=90 then '优秀' when english>=60 then '及格' else '不及格' end) '英语',
    (case when chinese>=90 then '优秀' when chinese>=60 then '及格' else '不及格' end) '语文'
from score;