sql的查询
排序
- 什么是排序?
order by
- 如何进行排序?
SELECT * FROM 表名 ORDER BY 排序字段 ASC(升序)|DESC(降序)
- 特点
指定列
指定升序或者降序
order by 排序只对数字和英文符串有效,对汉字无效
可以指定一个或者多个列作为排序条件
- 字典顺序 对中文无效
admin delete
初始化数据
CREATE DATABASE day13;
category
CREATE TABLE products(
pid INT PRIMARY KEY ,
pname VARCHAR(50),
price INT,
flag VARCHAR(2),
category_id INT
);
ALTER TABLE products ADD FOREIGN KEY(category_id) REFERENCES category(cid);
INSERT INTO category(cid,cname) VALUES(1,'家电');
INSERT INTO category(cid,cname) VALUES(2,'服饰');
INSERT INTO category(cid,cname) VALUES(3,'化妆品');
INSERT INTO products(pid, pname,price,flag,category_id) VALUES('1','联想',5000,'1',1);
INSERT INTO products(pid, pname,price,flag,category_id) VALUES('2','海尔',3000,'1',1);
INSERT INTO products(pid, pname,price,flag,category_id) VALUES('3','雷神',5000,'1',1);
INSERT INTO products (pid, pname,price,flag,category_id) VALUES('4','杰克琼斯',800,'1',2);
INSERT INTO products (pid, pname,price,flag,category_id) VALUES('5','真维斯',200,'1',2);
INSERT INTO products (pid, pname,price,flag,category_id) VALUES('6','花花公子',440,'1',2);
INSERT INTO products (pid, pname,price,flag,category_id) VALUES('7','劲霸',2000,'1',2);
INSERT INTO products (pid, pname,price,flag,category_id) VALUES('8','香奈儿',800,'1',2);
INSERT INTO products (pid, pname,price,flag,category_id) VALUES('9','相宜本草',200,'1',2);
SELECT * FROM products ORDER BY price DESC;
- 2.在价格排序(降序)的基础上,以分类排序(降序)-- 可以指定多个列
SELECT * FROM products ORDER BY price DESC,category_id DESC;
- 3.显示商品的价格(去重复),并排序(降序) – distinct去掉重复
SELECT DISTINCT price FROM products ORDER BY price DESC;
聚合函数
- 聚合函数的含义
聚合函数(类似与Java中的方法:函数名())
- 常用的聚合函数
count:统计指定列不为NULL的记录函数
sum:计算指定列的数值和,如果指定列不是数值类型,则计算结果为0
max:计算指定列的最大值,如果指定列是字符串类型,则使用字符串排序运算
min:计算指定列的最小值,如果指定列是字符串类型,则使用字符串排序运算
avg:计算指定列的平均值,如果指定列不是数值类型,则计算结果为0
- 语句的执行顺序
from ->where ->count( *)
- 4.查询商品的总条数
SELECT COUNT(*)FROM products;
SELECT COUNT(*) FROM products WHERE price > 200;
SELECT SUM(price) FROM products WHERE category_id = 1;
SELECT AVG(price) FROM products WHERE category_id = 2;
分组查询
- 什么是分组
将数据按照每个规则或者某个特征分为N组
一般不对id进行分组
如果是对分组之后的结果进行过滤,不能使用where,必须使用having
- 如何进行分组
select 字段1,字段2…from 表名 group by 分组字段 having 分组条件
- 8.统计各个分类商品的个数
SELECT category_id, COUNT(*) FROM products GROUP BY category_id;
- 9.统计各个分类商品的个数,且只显示个数大于3的信息
SELECT category_id,COUNT(*) c FROM products GROUP BY category_id HAVING c > 3 ;
分页
- 什么是分页
数据库分页也是在数据库里写查询语句,不同的是查询的都是指定条数到指定条数的数据,不是一次性把数据全部查询出来
- 如何分页
SELECT 字段1,字段2… FROM 表名 LIMIT M,N
(M:表示从哪一行的索引(从0开始)开始显示,N表示要显示几行)
第1页: N*(1-1) 第2页: N*(2-1) 第page页: N*(page-1)
- 显示prouct表的前5行
sql的练习
CREATE TABLE emp(
empno INT,
ename VARCHAR(50),
job VARCHAR(50),
mgr INT,
hiredate DATE,
sal INT,
comm INT,
deptno INT
) ;
INSERT INTO emp VALUES(7369,'SMITH','CLERK',7902,'1980-12-17',800,NULL,20);
INSERT INTO emp VALUES(7499,'ALLEN','SALESMAN',7698,'1981-02-20',1600,300,30);
INSERT INTO emp VALUES(7521,'WARD','SALESMAN',7698,'1981-02-22',1250,500,30);
INSERT INTO emp VALUES(7566,'JONES','MANAGER',7839,'1981-04-02',2975,NULL,20);
INSERT INTO emp VALUES(7654,'MARTIN','SALESMAN',7698,'1981-09-28',1250,1400,30);
INSERT INTO emp VALUES(7698,'BLAKE','MANAGER',7839,'1981-05-01',2850,NULL,30);
INSERT INTO emp VALUES(7782,'CLARK','MANAGER',7839,'1981-06-09',2450,NULL,10);
INSERT INTO emp VALUES(7788,'SCOTT','ANALYST',7566,'1987-04-19',3000,NULL,20);
INSERT INTO emp VALUES(7839,'KING','PRESIDENT',NULL,'1981-11-17',5000,NULL,10);
INSERT INTO emp VALUES(7844,'TURNER','SALESMAN',7698,'1981-09-08',1500,0,30);
INSERT INTO emp VALUES(7876,'ADAMS','CLERK',7788,'1987-05-23',1100,NULL,20);
INSERT INTO emp VALUES(7900,'JAMES','CLERK',7698,'1981-12-03',950,NULL,30);
INSERT INTO emp VALUES(7902,'FORD','ANALYST',7566,'1981-12-03',3000,NULL,20);
INSERT INTO emp VALUES(7934,'MILLER','CLERK',7782,'1982-01-23',1300,NULL,10);
- 1.按员工编号升序排列不在10号部门工作的员工信息
SELECT * FROM emp;
SELECT * FROM emp WHERE deptno != 10 ORDER BY empno ASC;
- 2.查询姓名第二个字母不是”A”且薪水大于800元的员工信息,按年薪降序排列 not()取反
SELECT * ,(sal*12+IFNULL(comm,0)) nx FROM emp WHERE NOT(ename = '_A%') AND sal>800 ORDER BY nx;
SELECT deptno, AVG(sal) FROM emp GROUP BY deptno;
SELECT deptno,MAX(sal) FROM emp GROUP BY deptno;
SELECT job,deptno,MAX(sal) FROM emp GROUP BY deptno,job;