像Excel一樣使用SQL進行數(shù)據(jù)分析
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SQL不僅可以從數(shù)據(jù)庫中讀取數(shù)據(jù)，還能通過不同的SQL函數(shù)語句直接返回所需要的結(jié)果，從而大大提高了自己在客戶端應(yīng)用程序中計算的效率。
1 ?重復(fù)數(shù)據(jù)處理
查找重復(fù)記錄
SELECT?* FROM?user?
Where?(nick_name,password) in
(
SELECT?nick_name,password?
FROM?user?
group?by?nick_name,password?
having?count(nick_name)>1
);
查找去重記錄
查找id最大的記錄
SELECT?* FROM?user?
WHERE?id?in
(SELECT?max(id) FROM?user
group?by?nick_name,password?
having?count(nick_name)>1
);
刪除重復(fù)記錄
只保留id值最小的記錄
DELETE??c1
FROM??customer c1,customer c2
WHERE?c1.cust_email=c2.cust_email
AND?c1.id>c2.id;DELETE?FROM?user?Where?(nick_name,password) in
(SELECT?nick_name,password?FROM
????(SELECT?nick_name,password?FROM?user?
????group?by?nick_name,password?
????having?count(nick_name)>1) as?tmp1
)
and?id?not?in
(SELECT?id?FROM
????(SELECT?min(id) id?FROM?user?
?????group?by?nick_name,password?
?????having?count(nick_name)>1) as?tmp2
);
2??缺失值處理
查找缺失值記錄
SELECT?* FROM?customer
WHERE?cust_email IS?NULL;UPDATE?sale set?city = "未知"?
WHERE?city IS?NULL;
UPDATE?orderitems set?
price_new=IFNULL(price_new,5.74);SELECT?AVG(price_new) FROM?orderitems;
SELECT?IFNULL(price_new,5.74) AS?bus_ifnull
FROM?orderitems;
3 ?計算列
更新表添加計算列
ALTER?TABLE?orderitems ADD?price_new DECIMAL(8,2) NOT?NULL;
UPDATE?orderitems set?price_new= item_price*count;
查詢計算列
SELECT?item_price*count?as?sales FROM?orderitems;4 ?排序
多列排序
SELECT?* FROM?orderitems
ORDER?BY?price_new DESC,quantity;查詢排名前幾的記錄
SELECT ?* FROM?orderitems
ORDER?BY?price_new DESC?Limit 5;查詢第10大的值
SELECT?DISTINCT?price_new
FROM?orderitems
ORDER?BY?price_new DESC?LIMIT?9,1;排名
數(shù)值相同的排名相同且排名連續(xù)
SELECT?prod_price,
(SELECT?COUNT(DISTINCT?prod_price)
FROM?products
WHERE?prod_price>=a.prod_price
) AS?rank
FROM?products AS?a
ORDER?BY?rank?;
5 字符串處理
字符串替換
UPDATE?data1 SET?city=REPLACE(city,'SH','shanghai');
SELECT?city FROM?data1;按位置字符串截取
字符串截取可用于數(shù)據(jù)分列
MySQL 字符串截取函數(shù)：left(), right(), substring(), substring_index()
SELECT?left('example.com', 3);SELECT?substring('example.com', 4);從字符串的第 4 個字符位置開始取，只取 2 個字符
SELECT?substring('example.com', 4, 2);按關(guān)鍵字截取字符串
取第一個分隔符之前的所有字符，結(jié)果是www
SELECT?substring_index('www.google.com','.',1);取倒數(shù)第二個分隔符之后的所有字符，結(jié)果是google.com;
SELECT?substring_index('www.google.com','.',-2);6 篩選
通過操作符實現(xiàn)高級篩選
使用 AND OR IN NOT 等操作符實現(xiàn)高級篩選過濾
SELECT?prod_name，prod_price FROM?Products
WHERE?vend_id IN('DLL01','BRS01');
SELECT?prod_name FROM?Products WHERE?NOT?vend_id='DLL01';通配符篩選
常用通配符有% _ [] ^
SELECT?* from?customers WHERE?country LIKE?"CH%";7 表聯(lián)結(jié)
SQL表連接可以實現(xiàn)類似于Excel中的Vlookup函數(shù)的功能
SELECT?vend_id,prod_name,prod_price
FROM?Vendors INNER?JOIN?Products
ON?Vendors.vend_id=Products.vend_id;
SELECT?prod_name,vend_name,prod_price,quantity
FROM?OderItems,Products,Vendors
WHERE?Products.vend_id=Vendors.vend_id
AND?OrderItems.prod_id=Products.prod_id
AND?order_num=20007;
自聯(lián)結(jié) 在一條SELECT語句中多次使用相同的表
SELECT?c1.cust_od,c1.cust_name,c1.cust_contact
FROM?Customers as?c1,Customers as?c2
WHERE?c1.cust_name=c2.cust_name
AND?c2.cust_contact='Jim Jones';
8 數(shù)據(jù)透視
數(shù)據(jù)分組可以實現(xiàn)Excel中數(shù)據(jù)透視表的功能
數(shù)據(jù)分組
group by 用于數(shù)據(jù)分組 having 用于分組后數(shù)據(jù)的過濾
SELECT?order_num,COUNT(*) as?items
FROM?OrderItems
GROUP?BY?order_num HAVING?COUNT(*)>=3;
交叉表
SELECT?data1.city,
CASE?WHEN?colour = "A"?THEN?price END?AS?A,
CASE?WHEN?colour = "B"?THEN?price END?AS?B,
CASE?WHEN?colour = "C"?THEN?price END?AS?C,
CASE?WHEN?colour = "F"?THEN?price END?AS?F
FROM?data1——End——
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