select ascii('abc') ;
select char_length('aaa哈哈');
select length('aaa哈哈');
select concat_ws('-','hello','word','beijing');
select insert('hello',2,2,'mmm')
from dual;
select replace('helloll','ll','mm')
from dual;
select upper('abC'),lower('abC'),ucase('abC'),lcase('abC');
select left(123456,4),right(123456,4);
select ltrim(' aaa'),rtrim(' a '),trim(' av ')
from dual;
select trim('oo' from 'ooooollasdaoo');
select repeat('aaa',4);
select substr('hello world',1,10);
select substr(now(),1,16);
select locate('lo','hello');
select reverse('hello');
select nullif(null,null);
select *,(select department_name from departments where department_id = e.department_id) as department_name
from employees e;
select *
from departments;
select now(),curdate(),curtime();
select year('2021-12-23'),month('2021-12-23'),day('2021-12-23'),
hour(now()),minute(now()),second(now());
select unix_timestamp(),unix_timestamp('2021-02-05');
select from_unixtime(1633104000,'%Y年%m月%d日');
select date_add(now(),interval -1 year),date_add(now(),interval 2 year ),
adddate(now(),interval -7 minute );
select adddate(now(),interval '1_1' year_month ),adddate(now(),interval '-1_-1' year_month );
select datediff(now(),adddate(now(),interval -1 year ));
select last_day(now());
select date_format(now(),'%Y-%m-%d %H:%i:%s');
select str_to_date('2021年-10月-05 17:-05-35','%Y年-%m月-%d %H:-%i-%s');
select date_format(now(),get_format(DATETIME ,'ISO'));
select last_name,salary,if(salary > 6000,'高工资','低工资') as 'grades_desc'
from employees;
select last_name,salary,case when salary > 1000 and salary < 3000 then '下等工资'
when salary >= 3000 and salary < 8000 then '中等工资'
when salary >= 8000 and salary < 10000 then '高等工资'
else '特高工资' end
from employees;
select last_name,case department_id when 10 then salary*1.1
when 20 then salary*1.2
when 30 then salary*1.3
else salary * 1.4
end as salary_bei,salary,department_id
from employees;
select last_name,salary, if(salary >= 6000,'6000以上','6000以下') as salary_grade
from employees;