SQL知识大全(二):SQL的基础知识你都掌握了吗？

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 OrderItems
WHERE order_num = 20005;

COUNT()返回数量

#count(*)返回所有值，包括null
SELECT COUNT(*) AS num_cust
FROM Customers;


#count(字段名)只返回该字段不为null的值
SELECT COUNT(cust_email) AS num_cust
FROM 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_avg
FROM Products
GROUP 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_sqrt
FROM Products
GROUP BY dt;

条件筛选

WHERE的用法

SELECT prod_name, prod_price FROM Products
 WHERE prod_price = 3.49; 
#AND
SELECT prod_id, prod_price, prod_name 
FROM Products 
WHERE vend_id = 'DLL01' AND prod_price <= 4; 


#OR
SELECT prod_name, prod_price
 FROM Products
 WHERE vend_id = 'DLL01' OR vend_id = ‘BRS01’;

IN的用法

SELECT prod_name, prod_price
FROM Products
WHERE 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_name
FROM Products
WHERE prod_name LIKE '%bean bag%';

2._下划线通配符

SELECT prod_id, prod_name
FROM Products
WHERE prod_name LIKE '__ inch teddy bear';

3.[]通配符

#寻找J或M开头的姓名
SELECT cust_contact 
FROM Customers
WHERE 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_contact
FROM Customers
WHERE 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_name
FROM Products
ORDER BY prod_price DESC, prod_name;


#分组查询过滤排序
SELECT order_num, COUNT(*) AS items FROM OrderItems
GROUP BY order_num
HAVING COUNT(*) >= 3
ORDER BY items, order_num;