目录
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;