隨著越來越偏向資料開發，發現 只會 select * from 已經不足以滿足業務需求了，系統學習學習內建函式和自定義函式，自我感覺比昨天的那個他高大上一些！

一些過於low的就不再廢話了~~~

##count函式
select 
    deptno,
    count(1) 總人數,
    count(case when job ='SALESMAN' then '1' end) 銷售人數,
    count(case when job ='MANAGER' then '1' end) 主管人數
from emp
group by deptno;--如果不group，會認為所有資料是一組，返回一個數據
 

##left join先形成一箇中間表，中間表中通過分組標識name進行count條數
SELECT 
    Websites.name,
    COUNT(access_log.aid) AS nums 
FROM    
    access_log 
LEFT JOIN 
    Websites 
ON access_log.site_id=Websites.id 
GROUP BY Websites.name;

##找出總訪問量大於200的網站，括號中select可以省略直接 表名 inner join 表名
SELECT Websites.name, Websites.url, SUM
 
(access_log.count) AS nums FROM (access_log
INNER JOIN Websites
ON access_log.site_id=Websites.id)
GROUP BY Websites.name
HAVING SUM(access_log.count) > 200;           

##加一個where條件
SELECT Websites.name, SUM(access_log.count) AS nums FROM Websites
INNER JOIN access_log
ON Websites.id=access_log.site_id
 
WHERE Websites.alexa < 200 
GROUP BY Websites.name
HAVING SUM(access_log.count) > 200;

##小括號有值就展示相關的name，url
SELECT Websites.name, Websites.url 
FROM Websites 
WHERE EXISTS (SELECT count FROM access_log WHERE Websites.id = access_log.site_id AND count > 200);

工作中更多的是自定義函式，隨後用到的話再次進行總結！