04 映射

想要获取的列。

select * from info;
select id, name                from info;
select id, name as NM         from info;
select id, name as NM, 123  from info;
select id, name as NM, 123 as age  from info;

select 
    id,
    name,
    666 as num,
    ( select max(id) from depart ) as mid, -- max/min/sum
    ( select min(id) from depart) as nid, -- max/min/sum
    age
from info;
# 注意：效率很低

select 
    id,
    name,
    case depart_id when 1 then "第1部门" end v1
from info;
select 
    id,
    name,
    case depart_id when 1 then "第1部门" else "其他" end v2
from info;
select 
    id,
    name,
    case depart_id when 1 then "第1部门" end v1,
    case depart_id when 1 then "第1部门" else "其他" end v2,
    case depart_id when 1 then "第1部门" when 2 then "第2部门" else "其他" end v3,
    case when age<18 then "少年" end v4,
    case when age<18 then "少年" else "油腻男" end v5,
    case when age<18 then "少年" when age<30 then "青年" else "油腻男" end v6
from info;