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sql 去重求和_SQL知识大全(二):SQL的基础知识你都掌握了吗？

news 来源：原创 2024/5/20 20:56:31

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今天是SQL系列的第二讲，会着重从SQL的基础语法出发，讲解去重，聚合函数，常用的数值处理函数，条件筛选和排序

数据去重

Distinct数据去重

从Products中筛选出无重复的vend_id 。

#去除重复值SELECT DISTINCT vend_id FROM Products;

聚合函数

AVG()返回平均值

SELECT AVG(prod_price) AS avg_price FROM Products;

SUM() 汇总求和

SELECT SUM(quantity) AS items_ordered FROM OrderItemsWHERE order_num = 20005;

COUNT()返回数量

#count(*)返回所有值，包括nullSELECT COUNT(*) AS num_custFROM Customers;#count(字段名)只返回该字段不为null的值SELECT COUNT(cust_email) AS num_custFROM Customers;

MAX()/MIN()求最大最小值

SELECT MAX(prod_price) AS max_price FROM Products;SELECT dt,       COUNT(*) AS num_items,       MIN(prod_price) AS price_min,       MAX(prod_price) AS price_max,       AVG(prod_price) AS price_avgFROM ProductsGROUP BY dt;

数值处理函数

数值处理函数主要是将数值型的数据转换为其余数，绝对值，余弦值，正切值等，具体用如下：

SELECT dt,       COUNT(*) AS num_items,       MOD(prod_price) AS price_mod,       ROUND(prod_price) AS price_round,       ABS(prod_price) AS price_abs,       SQRT(prod_price) AS price_sqrtFROM ProductsGROUP BY dt;

条件筛选

WHERE的用法

SELECT prod_name, prod_price FROM Products WHERE prod_price = 3.49; #ANDSELECT prod_id, prod_price, prod_name FROM Products WHERE vend_id = 'DLL01' AND prod_price <= 4; #ORSELECT prod_name, prod_price FROM Products WHERE vend_id = 'DLL01' OR vend_id = ‘BRS01’;

IN的用法

SELECT prod_name, prod_priceFROM ProductsWHERE vend_id IN ( 'DLL01', 'BRS01' )ORDER BY prod_name;

LIKE相关通配符

1.%通配符

#查找有bean bag的字段SELECT prod_id, prod_nameFROM ProductsWHERE prod_name LIKE '%bean bag%';

2._下划线通配符

SELECT prod_id, prod_nameFROM ProductsWHERE prod_name LIKE '__ inch teddy bear';

3.[]通配符

#寻找J或M开头的姓名SELECT cust_contact FROM CustomersWHERE cust_contact LIKE '[JM]%'ORDER BY cust_contact;#此通配符之前加一个括号可以表示否定，即寻找不是J或M开头的人SELECT cust_contactFROM CustomersWHERE cust_contact LIKE '[^JM]%' ORDER BY cust_contact;

HAVING分组过滤

#对价格为4元以上的商品统计其被购买次数，并筛选出被购买两次以上的商品SELECT vend_id, COUNT(*) AS num_prods FROM Products WHERE prod_price >= 4 GROUP BY vend_id HAVING COUNT(*) >= 2;

数据排序

ORDER BY 数据排序

SELECT prod_id, prod_price, prod_nameFROM ProductsORDER BY prod_price DESC, prod_name;#分组查询过滤排序SELECT order_num, COUNT(*) AS items FROM OrderItemsGROUP BY order_numHAVING COUNT(*) >= 3ORDER BY items, order_num;

参考书籍：《SQL必知必会》