1 重複數據處理
查找重複記錄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
);
刪除重複記錄
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;
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
數值相同的排名相同且排名連續
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
字符串截取可用於數據分列
MySQL 字符串截取函數：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);
按關鍵字截取字符串
取第一個分隔符之前的所有字符，結果是www
SELECT substring_index('www.google.com','.',1);
取倒數第二個分隔符之後的所有字符，結果是google.com;
SELECT substring_index('www.google.com','.',-2);
6 篩選
通過操作符實現高級篩選使用 AND OR IN NOT 等操作符實現高級篩選過濾
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 表聯結
SQL表連接可以實現類似於Excel中的Vlookup函數的功能
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;
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 數據透視
數據分組可以實現Excel中數據透視表的功能
數據分組group by 用於數據分組 having 用於分組後數據的過濾
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
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