select name ,LENGTH(name) from student;
select LENGTH('12中国3ab');
select CHAR_LENGTH('12zg是的');

select REPLACE('124456739',3,'qwe');
select INSERT('123456789',1,4,'qwe');

SELECT LEFT('123456',4);
SELECT RIGHT('123456',3);
SELECT SUBSTRING('123456' FROM 5 FOR 3)

SELECT CONCAT('abc','sfds','fwa221')

SELECT REVERSE('123445632')

SELECT UPPER('afas')

SELECT LOWER('FSFSF')

SELECT TRIM('     asfe  ')
SELECT LTRIM('     asfe  ')
SELECT RTRIM('     asfe  ')

SELECT NOW()

SELECT  UNIX_TIMESTAMP('2021-09-02 15:13:28')
SELECT FROM_UNIXTIME(1630566808)

SELECT *,(money>=0,'正常','异常')资金状态 FROM user；
SELECT *,
(
 CASE
  WHEN money>=0 THEN
       '正常'
  ELSE
       '异常'
  END
)资金状态
 from user;

SELECT *,
(
CASE STATUS
WHEN 1 THEN
		'在线'
	WHEN 2 THEN
	'离线'
	WHEN 3 THEN
	'限制登入'
	WHEN 4 THEN
	'被删除'
	ELSE
		'未知状态'
END
)用户状态
 from user;

SELECT *,IFNULL(IDCODE,0)IDCODE FROM USER;