学习笔记-数据查询语言DQL

1.基本语法

select [all|distinct]  字段1 [别名1],字段2 [别名2]...
from 表名 
where 条件 
group by 分组字段                                   
having 分组之后的条件 [asc|desc]
order by 排序
limit 数字或列表

2.数据准备

CREATE TABLE product
(
    pid         INT PRIMARY KEY,
    pname       VARCHAR(20),
    price       DOUBLE,
    category_id VARCHAR(32)
);
# 插入数据
INSERT INTO product(pid,pname,price,category_id) VALUES(1,'联想',5000,'c001');
INSERT INTO product(pid,pname,price,category_id) VALUES(2,'海尔',3000,'c001');
INSERT INTO product(pid,pname,price,category_id) VALUES(3,'雷神',5000,'c001');
INSERT INTO product(pid,pname,price,category_id) VALUES(4,'杰克琼斯',800,'c002');
INSERT INTO product(pid,pname,price,category_id) VALUES(5,'真维斯',200,'c002');
INSERT INTO product(pid,pname,price,category_id) VALUES(6,'花花公子',440,'c002');
INSERT INTO product(pid,pname,price,category_id) VALUES(7,'劲霸',2000,'c002');
INSERT INTO product(pid,pname,price,category_id) VALUES(8,'香奈儿',800,'c003');
INSERT INTO product(pid,pname,price,category_id) VALUES(9,'相宜本草',200,'c003');
INSERT INTO product(pid,pname,price,category_id) VALUES(10,'面霸',5,'c003');
INSERT INTO product(pid,pname,price,category_id) VALUES(11,'好想你枣',56,'c004');
INSERT INTO product(pid,pname,price,category_id) VALUES(12,'香飘飘奶茶',1,'c005');
INSERT INTO product(pid,pname,price,category_id) VALUES(13,'海澜之家',1,'c002');
INSERT INTO product(pid,pname,price,category_id) VALUES(14,'小米',1999,'');
INSERT INTO product(pid,pname,price,category_id) VALUES(15,'华为',6999,'null');
INSERT INTO product(pid,pname,price,category_id) VALUES(16,'蜜雪冰城',1,null);

3.简单查询

# 1.查询所有的商品.  
select *  from product;
# 2.查询商品名和商品价格. 
select pname,price from product;
# 3.查询结果是表达式（运算查询）：将所有商品的价格+10元进行显示.
select pname,price+10 from product;

4.条件查询

(1)比较查询

# 查询商品名称为“花花公子”的商品所有信息：
SELECT * FROM product WHERE pname = '花花公子';
# 查询价格为800商品
SELECT * FROM product WHERE price = 800;
# 查询价格不是800的所有商品
SELECT * FROM product WHERE price != 800;
SELECT * FROM product WHERE price <> 800;
SELECT * FROM product WHERE not (price = 800);
# 查询商品价格大于60元的所有商品信息
SELECT * FROM product WHERE price > 60;
# 查询商品价格小于等于800元的所有商品信息
SELECT * FROM product WHERE price <= 800;

(2).范围查询

# 查询商品价格在200到1000之间所有商品
SELECT * FROM product WHERE price BETWEEN 200 AND 1000;
# 查询商品价格是200或800的所有商品
SELECT * FROM product WHERE price IN (200,800);
# 查询商品价格不是200或800的所有商品
SELECT * FROM product WHERE price NOT IN (200,800);

(3).逻辑查询

# 查询商品价格在200到1000之间所有商品
SELECT * FROM product WHERE price >= 200 AND price <=1000;
# 查询商品价格是200或800的所有商品
SELECT * FROM product WHERE price = 200 OR price = 800;
# 查询价格不是800的所有商品
SELECT * FROM product WHERE NOT(price = 800);

(4).模糊查询

# 模糊 查询: 关键字:like   %:0个或者多个字符   _:一个字符
# 需求1: 查询商品名称以'香'开头的所有商品信息
select * from product where pname like '香%';
# 需求2: 查询商品名称包含'想'字的所有商品信息
select * from product where pname like '%想%';
# 需求3: 查询商品名称以'斯'字结尾的所有商品信息
select * from product where pname like '%斯';
# 需求4: 查询商品名称第三个字是'斯'的商品信息
select * from product where pname like '__斯%';
# 需求5: 查询商品名称以'香'开头并且是三个字的所有商品信息
select * from product where pname like '香__';

(5).非空查询

# 查询没有分类的商品
SELECT * FROM product WHERE category_id IS NULL;
# 查询有分类的商品
SELECT * FROM product WHERE category_id IS NOT NULL;

5.排序查询

• ASC 升序 (默认)
• DESC 降序

# 1.使用价格排序(降序)
SELECT * FROM product ORDER BY price DESC;
# 2.在价格排序(降序)的基础上，以分类排序(降序)
SELECT * FROM product ORDER BY price DESC,category_id DESC;

6.聚合查询

# 1、查询商品的总条数
SELECT COUNT(*) FROM product;
# 2、查询价格大于200商品的总条数
SELECT COUNT(*) FROM product WHERE price > 200;
# 3、查询分类为'c001'的所有商品的总和
SELECT SUM(price) FROM product WHERE category_id = 'c001‘;
# 4、查询分类为'c002'所有商品的平均价格
SELECT AVG(price) FROM product WHERE categ ory_id = 'c002‘;
# 5、查询商品的最大价格和最小价格
SELECT MAX(price),MIN(price) FROM product;

7.分组查询

#1 统计各个分类商品的个数
SELECT category_id ,COUNT(*) FROM product GROUP BY category_id ;
#2 统计各个分类商品的个数,且只显示个数大于1的信息
SELECT category_id ,COUNT(*) FROM product GROUP BY category_id HAVING COUNT(*) > 1;

having与where的区别:
1).having是在分组后对数据进行过滤,where是在分组前对数据进行过滤
2).having后面可以使用分组函数(统计函数),where后面不可以使用分组函数。

8.分页查询

# 1、查询商品的第1-5条
SELECT * FROM product limit 0,5