[DB] SQL 헷갈리는 함수 모음

#함수
-- 1.집계함수(그룹함수): count(), sum(), avg(), min(), max()
select count(*), avg(salary), max(salary)
from emp;

-- 2.숫자 관련 함수
select ceil(8.1); #9
select floor(8.9); #8
select round(8.9); #9
select greatest(100, 34, 888) max, least(100, 34, 888) min;

-- 3. 문자열 관련 함수
select empid, concat(fname, ' ', lname) name
from emp;
select fname, replace(fname, 'e', 'x') name
from emp;
select fname, substring(fname, 1, 2) name
from emp;

-- 4. 날짜/시간 관련 함수
select now(), sysdate(),curdate(), curtime();
select now(), date_add(now(), interval 1 day);
select year(now()), month(now()), monthname(now()), dayofweek(now());

-- 5.json
select json_object('num',empid,'name', fname) obj
from emp;

-- like
select fname
from emp
where fname like 'A\%%'; -- 'A%'로 시작

select fname
from emp
where fname like 'A__'; -- '_3번

select fname
from emp
where fname like 'A%'; -- A로 시작하는