--去重，截取，计算，替换，替换，非，范围，排序
select distinct(productid),substr(productname,1,3),productprice*1.25 productprice,orgion，
replace(productname,'器','abc')，replace(productname,'器','abc') from productinfo
where productprice <> 100 and productprice between 1000 and 3000 order by orgion nulls first

--模糊查询
select * from productinfo where productname like '%_器%' or productname like '%美'

--分组，平均，统计数量，累加
select category,avg(productprice),count(1),sum(productprice) from productinfo group by category having avg(productprice) >2000

--日期及计算
select to_char(sysdate,'YYYY-MM-DD hh;mm:ss') from dual
select add_months(to_date('2019-07-18','YYYY-MM-DD'),1) FROM DUAL

--截取   1，从字符的下标开始截取到最后，2，从字符的下标开始截取，第三个为要截取的字符长度
select substr('field',-2),substr('field',1,2) from dual;
--拼接
select '11'|| '--' || '22',concat('11','22','33') from dual;
--选择
select case '1' when '1' then '001' else  '222' end as dealNum from dual;
select decode('bb','aa','11','bb','22','33') from dual;

--空值替换
select nvl('',0) from dual;
select coalesce(null,1,null,3) from dual;

--最大最小
select greatest(1,3,5) from dual;
select least(3,2,5) from dual;

--特殊的null
    null不支持加减乘除，且null是个特殊的存在，扰乱计算规则和函数的使用
    field is null

--行和下标
select rownum , a.* from table_a a where rownum = 1 / rownum <= 2

--随机抽取
select * from (
    select productid,productname,dbms_random.value from productinfo order by dbms_random.value
) t where rownum <=3

--排序
select a,b,c,d,e,f,g from table_a order by g nulls last/first;
select a,b,c,d,e,f,g from table_a order by case b when '2' then '1' else '2' end,3
select 1 from dual where rownum ='1'
select rownum,t.* from dual