1.1 區域人口總數統計

轉換成

-- 註意group by後跟的條件
select case pref_name 
    when ‘北京‘ then ‘北方‘
    when ‘哈爾濱‘ then ‘北方‘
    when ‘廣州‘ then ‘南方‘
    when ‘深圳‘ then ‘南方‘
else ‘其他‘ end as district,
sum(population) as total
from poptbl
group by case pref_name 
    when ‘北京‘ then ‘北方‘
    when ‘哈爾濱
 
‘ then ‘北方‘
    when ‘廣州‘ then ‘南方‘
    when ‘深圳‘ then ‘南方‘
    else ‘其他‘ end

1.2 行統計轉為列統計

轉換為

select pref_name, 
    sum(case when sex=1 then population else 0 end) as male,  -- 註意使用了sum函數
    sum(case when sex=2 then population else 0 end) as female
from poptbl2
group by pref_name

1.3 在update語句裏面進行條件分支

??變為 ??

update salaries 
set salary = case 
    when salary >= 300000 then salary*0.9
    when salary >= 250000 and salary < 280000 then salary*1.2
    else
 
 salary end;  -- 註意一定要寫else，否則不在上述條件裏面的工資會被置為NULL

1.4 生成課程表

????==>??

select course_name,
　　case when exists (
　　　　select * from opencourses OC where CM.course_id = OC.course_id and OC.month = ‘200706‘
　　) then ‘o‘ else ‘x‘ end as ‘6月‘,
　　case when exists (
　　　　select * from opencourses OC where CM.course_id = OC.course_id and OC.month = ‘200707‘
　　) then ‘o‘ else ‘x‘ end as ‘7月‘,
　　case when exists (
　　　　select * from opencourses OC where CM.course_id = OC.course_id and OC.month = ‘200708‘
　　) then ‘o‘ else ‘x‘ end as ‘8月‘
from coursemaster CM

1.5 case表達式使用聚合函數

??

-- 先查出只加入了一個社團的學生的社團ID
select std_id, max(club_id) from studentclub
group by std_id
having count(*) = 1
-- 再查出有主社團標誌的社團ID
select std_id, club_id
from studentclub
where main_club_flg = ‘Y‘

解法2：使用case表達式，一個sql即可完成

-- select後跟的字段，要麽是分組字段，要麽被包含到聚合函數中
-- 聚合函數計算某個字段時自動忽略null行
select std_id,
case when count(*)=1
then max(club_id)
else max(case when main_club_flg=‘Y‘ then club_id else null end)
end as main_club
from studentclub
group by std_id

1.6 求行的極值

??==》??

select `key`, greatest(x,y,z) as max
from greatests

解法2：用case表達式

select `key`,
case when x < y then (case when y < z then z else y end)
else (case when x < z then z else x end)
end as max
from greatests

解法3：行列轉換後使用max函數

SELECT `key`, MAX(col) AS greatest
FROM (
SELECT `key`, x AS col FROM Greatests
UNION ALL
SELECT `key`, y AS col FROM Greatests
UNION ALL
SELECT `key`, z AS col FROM Greatests
)TMP
GROUP BY `key`;

1.7 自定義order by

??

select `key` from greatests
order by `key`

得到結果如下

??，修改sql語句使其變成這樣：??

select `key` from greatests
order by case
when `key`=‘B‘ then 1
when `key`=‘A‘ then 2
when `key`=‘D‘ then 3
when `key`=‘C‘ then 4
else null end;

很神奇的腦洞。

《SQL進階教程》抄書筆記01-case表達式