SQL不仅可以从数据库中读取数据，还能通过不同的SQL函数语句直接返回所需要的结果，从而大大提高了自己在客户端应用程序中计算的效率。
1 重复数据处理
查找重复记录
SELECT * FROM user Where (nick_name,password) in ( SELECT nick_name,password FROM user group by nick_name,password having count(nick_name)>1 );
查找去重记录
查找id最大的记录
SELECT * FROM user WHERE id in (SELECT max(id) FROM user group by nick_name,password having count(nick_name)>1 );
删除重复记录
只保留id值最小的记录
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 缺失值处理
查找缺失值记录
SELECT * FROM customer WHERE cust_email IS NULL;
更新列填充空值
UPDATE sale set city = "未知" WHERE city IS NULL; UPDATE orderitems set price_new=IFNULL(price_new,5.74);
查询并填充空值列
SELECT AVG(price_new) FROM orderitems; SELECT IFNULL(price_new,5.74) AS bus_ifnull FROM orderitems;
3 计算列
更新表添加计算列
ALTER TABLE orderitems ADD price_new DECIMAL(8,2) NOT NULL; UPDATE orderitems set price_new= item_price*count;
查询计算列
SELECT item_price*count as sales FROM orderitems;
4 排序
多列排序
SELECT * FROM orderitems ORDER BY price_new DESC,quantity;
查询排名前几的记录
SELECT * FROM orderitems ORDER BY price_new DESC Limit 5;
查询第10大的值
SELECT DISTINCT price_new FROM orderitems ORDER BY price_new DESC LIMIT 9,1;
