SQL - common function usage

select lower(ename) from emp
# Convert uppercase to lowercase 

select upper(ename) from emp
# Convert lowercase to uppercase 

select concat('www','baidu','.com') as url 
# Link together 

select char_length('aaa')
# Know the length of the string 

select substring('goodgoodstudy',0,5)
# Intercepting string 

select ltrim/rtrim/trim(' aa bb ') 
# Remove the empty string on the left , Remove the empty string on the right , Remove the empty string on both sides 

select ceil(3.14)

select obs(-3.14)
# Take the absolute value 

select floor(-3.14)
# Cut off everything after the decimal point 

select round(3.14232,3)
# Keep three decimal places , And round it up 

select truncate(3.14232,3)
# Keep three decimal places , Directly intercept later 

select curdate()
# Take the current date 
#

select curtime()
# Take the current time 
#

select now()
# Take the current date plus time 
#

select month(curdate())
# Take the current month 
#

select month/year(hiredate) from emp
# Take the data in the table , Which month did you start 
#