mysql中对查询到的姓名字段进行星号名称脱敏

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flybirdfly 2024-09-22 22:59:21

在练习SQL查询语句时我们往往会复杂问题无从下手，其实再复杂的问题也是由简单的问题构成

想要成为随心所欲的查询和输出自己想要的数据其实可以分三步去练习即可

1.首先要了解SQL一般关键字执行顺序

<1> from 首先指明从哪个表中取数据
<2> on, join 表与表之间的连接(内连接，外连接，左连接，右连接等)
<3> where 进行初步过滤
<4> group by 按指定字段进行分组
<5> 聚合函数对分组后数据使用聚集函数，分别作用于每组中，而不是整体数据
<6> Having 对分组后数据进行后序过滤(group by一起，在group by后)
<7> select 投影需要的数据
<8> distinct 对投影得到的数据进行去重
<9> order by 对投影去重后的数据进行排序
<10> limit 取最终预处理后数据前n行

2. 带着这个顺序去看SQL必知必会（第四版）

可以先自建一个数据库，在创建如下表

-- ----------------------
-- Create Customers table
-- ----------------------
CREATE TABLE Customers
(
  cust_id      char(10)  NOT NULL ,
  cust_name    char(50)  NOT NULL ,
  cust_address char(50)  NULL ,
  cust_city    char(50)  NULL ,
  cust_state   char(5)   NULL ,
  cust_zip     char(10)  NULL ,
  cust_country char(50)  NULL ,
  cust_contact char(50)  NULL ,
  cust_email   char(255) NULL ,
    PRIMARY KEY(cust_id)
);

-- -----------------------
-- Create OrderItems table
-- -----------------------
CREATE TABLE OrderItems
(
  order_num  int          NOT NULL ,
  order_item int          NOT NULL ,
  prod_id    char(10)     NOT NULL ,
  quantity   int          NOT NULL ,
  item_price decimal(8,2) NOT NULL 
);


-- -------------------
-- Create Orders table
-- -------------------
CREATE TABLE Orders
(
  order_num  int      NOT NULL ,
  order_date datetime NOT NULL ,
  cust_id    char(10) NOT NULL 
);

-- ---------------------
-- Create Products table
-- ---------------------
CREATE TABLE Products
(
  prod_id    char(10)      NOT NULL ,
  vend_id    char(10)      NOT NULL ,
  prod_name  char(255)     NOT NULL ,
  prod_price decimal(8,2)  NOT NULL ,
  prod_desc  text          NULL 
);

-- --------------------
-- Create Vendors table
-- --------------------
CREATE TABLE Vendors
(
  vend_id      char(10) NOT NULL ,
  vend_name    char(50) NOT NULL ,
  vend_address char(50) NULL ,
  vend_city    char(50) NULL ,
  vend_state   char(5)  NULL ,
  vend_zip     char(10) NULL ,
  vend_country char(50) NULL 
);


-- -------------------
-- Define primary keys
-- -------------------
ALTER TABLE Customers ADD PRIMARY KEY (cust_id);
ALTER TABLE OrderItems ADD PRIMARY KEY (order_num, order_item);
ALTER TABLE Orders ADD PRIMARY KEY (order_num);
ALTER TABLE Products ADD PRIMARY KEY (prod_id);
ALTER TABLE Vendors ADD PRIMARY KEY (vend_id);


-- -------------------
-- Define foreign keys
-- -------------------
ALTER TABLE OrderItems ADD CONSTRAINT FK_OrderItems_Orders FOREIGN KEY (order_num) REFERENCES Orders (order_num);
ALTER TABLE OrderItems ADD CONSTRAINT FK_OrderItems_Products FOREIGN KEY (prod_id) REFERENCES Products (prod_id);
ALTER TABLE Orders ADD CONSTRAINT FK_Orders_Customers FOREIGN KEY (cust_id) REFERENCES Customers (cust_id);
ALTER TABLE Products ADD CONSTRAINT FK_Products_Vendors FOREIGN KEY (vend_id) REFERENCES Vendors (vend_id);

接着初始化各表数据，代码如下

-- ------------------------
-- Populate Customers table
-- ------------------------
INSERT INTO Customers(cust_id, cust_name, cust_address, cust_city, cust_state, cust_zip, cust_country, cust_contact, cust_email)
VALUES('1000000001', 'Village Toys', '200 Maple Lane', 'Detroit', 'MI', '44444', 'USA', 'John Smith', 'sales@villagetoys.com');
INSERT INTO Customers(cust_id, cust_name, cust_address, cust_city, cust_state, cust_zip, cust_country, cust_contact)
VALUES('1000000002', 'Kids Place', '333 South Lake Drive', 'Columbus', 'OH', '43333', 'USA', 'Michelle Green');
INSERT INTO Customers(cust_id, cust_name, cust_address, cust_city, cust_state, cust_zip, cust_country, cust_contact, cust_email)
VALUES('1000000003', 'Fun4All', '1 Sunny Place', 'Muncie', 'IN', '42222', 'USA', 'Jim Jones', 'jjones@fun4all.com');
INSERT INTO Customers(cust_id, cust_name, cust_address, cust_city, cust_state, cust_zip, cust_country, cust_contact, cust_email)
VALUES('1000000004', 'Fun4All', '829 Riverside Drive', 'Phoenix', 'AZ', '88888', 'USA', 'Denise L. Stephens', 'dstephens@fun4all.com');
INSERT INTO Customers(cust_id, cust_name, cust_address, cust_city, cust_state, cust_zip, cust_country, cust_contact)
VALUES('1000000005', 'The Toy Store', '4545 53rd Street', 'Chicago', 'IL', '54545', 'USA', 'Kim Howard');

-- ----------------------
-- Populate Vendors table
-- ----------------------
INSERT INTO Vendors(vend_id, vend_name, vend_address, vend_city, vend_state, vend_zip, vend_country)
VALUES('BRS01','Bears R Us','123 Main Street','Bear Town','MI','44444', 'USA');
INSERT INTO Vendors(vend_id, vend_name, vend_address, vend_city, vend_state, vend_zip, vend_country)
VALUES('BRE02','Bear Emporium','500 Park Street','Anytown','OH','44333', 'USA');
INSERT INTO Vendors(vend_id, vend_name, vend_address, vend_city, vend_state, vend_zip, vend_country)
VALUES('DLL01','Doll House Inc.','555 High Street','Dollsville','CA','99999', 'USA');
INSERT INTO Vendors(vend_id, vend_name, vend_address, vend_city, vend_state, vend_zip, vend_country)
VALUES('FRB01','Furball Inc.','1000 5th Avenue','New York','NY','11111', 'USA');
INSERT INTO Vendors(vend_id, vend_name, vend_address, vend_city, vend_state, vend_zip, vend_country)
VALUES('FNG01','Fun and Games','42 Galaxy Road','London', NULL,'N16 6PS', 'England');
INSERT INTO Vendors(vend_id, vend_name, vend_address, vend_city, vend_state, vend_zip, vend_country)
VALUES('JTS01','Jouets et ours','1 Rue Amusement','Paris', NULL,'45678', 'France');

-- -----------------------
-- Populate Products table
-- -----------------------
INSERT INTO Products(prod_id, vend_id, prod_name, prod_price, prod_desc)
VALUES('BR01', 'BRS01', '8 inch teddy bear', 5.99, '8 inch teddy bear, comes with cap and jacket');
INSERT INTO Products(prod_id, vend_id, prod_name, prod_price, prod_desc)
VALUES('BR02', 'BRS01', '12 inch teddy bear', 8.99, '12 inch teddy bear, comes with cap and jacket');
INSERT INTO Products(prod_id, vend_id, prod_name, prod_price, prod_desc)
VALUES('BR03', 'BRS01', '18 inch teddy bear', 11.99, '18 inch teddy bear, comes with cap and jacket');
INSERT INTO Products(prod_id, vend_id, prod_name, prod_price, prod_desc)
VALUES('BNBG01', 'DLL01', 'Fish bean bag toy', 3.49, 'Fish bean bag toy, complete with bean bag worms with which to feed it');
INSERT INTO Products(prod_id, vend_id, prod_name, prod_price, prod_desc)
VALUES('BNBG02', 'DLL01', 'Bird bean bag toy', 3.49, 'Bird bean bag toy, eggs are not included');
INSERT INTO Products(prod_id, vend_id, prod_name, prod_price, prod_desc)
VALUES('BNBG03', 'DLL01', 'Rabbit bean bag toy', 3.49, 'Rabbit bean bag toy, comes with bean bag carrots');
INSERT INTO Products(prod_id, vend_id, prod_name, prod_price, prod_desc)
VALUES('RGAN01', 'DLL01', 'Raggedy Ann', 4.99, '18 inch Raggedy Ann doll');
INSERT INTO Products(prod_id, vend_id, prod_name, prod_price, prod_desc)
VALUES('RYL01', 'FNG01', 'King doll', 9.49, '12 inch king doll with royal garments and crown');
INSERT INTO Products(prod_id, vend_id, prod_name, prod_price, prod_desc)
VALUES('RYL02', 'FNG01', 'Queen doll', 9.49, '12 inch queen doll with royal garments and crown');

-- ---------------------
-- Populate Orders table
-- ---------------------
INSERT INTO Orders(order_num, order_date, cust_id)
VALUES(20005, '2012-05-01', '1000000001');
INSERT INTO Orders(order_num, order_date, cust_id)
VALUES(20006, '2012-01-12', '1000000003');
INSERT INTO Orders(order_num, order_date, cust_id)
VALUES(20007, '2012-01-30', '1000000004');
INSERT INTO Orders(order_num, order_date, cust_id)
VALUES(20008, '2012-02-03', '1000000005');
INSERT INTO Orders(order_num, order_date, cust_id)
VALUES(20009, '2012-02-08', '1000000001');

-- -------------------------
-- Populate OrderItems table
-- -------------------------
INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20005, 1, 'BR01', 100, 5.49);
INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20005, 2, 'BR03', 100, 10.99);
INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20006, 1, 'BR01', 20, 5.99);
INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20006, 2, 'BR02', 10, 8.99);
INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20006, 3, 'BR03', 10, 11.99);
INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20007, 1, 'BR03', 50, 11.49);
INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20007, 2, 'BNBG01', 100, 2.99);
INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20007, 3, 'BNBG02', 100, 2.99);
INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20007, 4, 'BNBG03', 100, 2.99);
INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20007, 5, 'RGAN01', 50, 4.49);
INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20008, 1, 'RGAN01', 5, 4.99);
INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20008, 2, 'BR03', 5, 11.99);
INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20008, 3, 'BNBG01', 10, 3.49);
INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20008, 4, 'BNBG02', 10, 3.49);
INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20008, 5, 'BNBG03', 10, 3.49);
INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20009, 1, 'BNBG01', 250, 2.49);
INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20009, 2, 'BNBG02', 250, 2.49);
INSERT INTO OrderItems(order_num, order_item, prod_id, quantity, item_price)
VALUES(20009, 3, 'BNBG03', 250, 2.49);

最后跟着这本书上的语句慢慢练习就行

3. 通过前两步，你基本对于简单的查询没有问题了，接下来可以通过经典复杂查询来进行锻炼

其中学校类的数据库查询最为常见

可以建立一个school数据库

然后创建经典的四个表：学生表，课程表，成绩表，教师表

其代码如下所示

--学生表
DROP TABLE IF EXISTS `Student`;
CREATE TABLE `Student`(
`s_id` VARCHAR(20),
`s_name` VARCHAR(20) NOT NULL DEFAULT '',
`s_birth` VARCHAR(20) NOT NULL DEFAULT '',
`s_sex` VARCHAR(10) NOT NULL DEFAULT '',
PRIMARY KEY(`s_id`)
);

--课程表
DROP TABLE IF EXISTS `Course`;
CREATE TABLE `Course`(
`c_id` VARCHAR(20),
`c_name` VARCHAR(20) NOT NULL DEFAULT '',
`t_id` VARCHAR(20) NOT NULL,
PRIMARY KEY(`c_id`)
);

--教师表
DROP TABLE IF EXISTS `Teacher`;
CREATE TABLE `Teacher`(
`t_id` VARCHAR(20),
`t_name` VARCHAR(20) NOT NULL DEFAULT '',
PRIMARY KEY(`t_id`)
);

--成绩表
DROP TABLE IF EXISTS `Score`;
CREATE TABLE `Score`(
`s_id` VARCHAR(20),
`c_id` VARCHAR(20),
`s_score` INT(3),
PRIMARY KEY(`s_id`,`c_id`)
);

初始化各表数据，如下所示

--插入学生表测试数据
insert into Student values('01' , '赵雷' , '1990-01-01' , '男');
insert into Student values('02' , '钱电' , '1990-12-21' , '男');
insert into Student values('03' , '孙风' , '1990-05-20' , '男');
insert into Student values('04' , '李云' , '1990-08-06' , '男');
insert into Student values('05' , '周梅' , '1991-12-01' , '女');
insert into Student values('06' , '吴兰' , '1992-03-01' , '女');
insert into Student values('07' , '郑竹' , '1989-07-01' , '女');
insert into Student values('08' , '王菊' , '1990-01-20' , '女');

--课程表测试数据
insert into Course values('01' , '语文' , '02');
insert into Course values('02' , '数学' , '01');
insert into Course values('03' , '英语' , '03');

--教师表测试数据
insert into Teacher values('01' , '张三');
insert into Teacher values('02' , '李四');
insert into Teacher values('03' , '王五');

--成绩表测试数据
insert into Score values('01' , '01' , 80);
insert into Score values('01' , '02' , 90);
insert into Score values('01' , '03' , 99);
insert into Score values('02' , '01' , 70);
insert into Score values('02' , '02' , 60);
insert into Score values('02' , '03' , 80);
insert into Score values('03' , '01' , 80);
insert into Score values('03' , '02' , 80);
insert into Score values('03' , '03' , 80);
insert into Score values('04' , '01' , 50);
insert into Score values('04' , '02' , 30);
insert into Score values('04' , '03' , 20);
insert into Score values('05' , '01' , 76);
insert into Score values('05' , '02' , 87);
insert into Score values('06' , '01' , 31);
insert into Score values('06' , '03' , 34);
insert into Score values('07' , '02' , 89);
insert into Score values('07' , '03' , 98);

最后通过以下50道练习题进行训练，当你自己很轻松写出时，你SQL查询能力已经得到了很大的提升

这50道练习题参考于

-- 1、查询"01"课程比"02"课程成绩高的学生的信息及课程分数 
select st.*,sc.s_score as '语文' ,sc2.s_score '数学' 
from student st
left join score sc on sc.s_id=st.s_id and sc.c_id='01' 
left join score sc2 on sc2.s_id=st.s_id and sc2.c_id='02'  
where sc.s_score>sc2.s_score

-- 2、查询"01"课程比"02"课程成绩低的学生的信息及课程分数
select st.*,sc.s_score '语文',sc2.s_score '数学' from student st
left join score sc on sc.s_id=st.s_id and sc.c_id='01'
left join score sc2 on sc2.s_id=st.s_id and sc2.c_id='02'
where sc.s_score<sc2.s_score

-- 3、查询平均成绩大于等于60分的同学的学生编号和学生姓名和平均成绩
select st.s_id,st.s_name,ROUND(AVG(sc.s_score),2) "平均成绩" from student st
left join score sc on sc.s_id=st.s_id
group by st.s_id having AVG(sc.s_score)>=60

-- 4、查询平均成绩小于60分的同学的学生编号和学生姓名和平均成绩
        -- (包括有成绩的和无成绩的)
select st.s_id,st.s_name,(case when ROUND(AVG(sc.s_score),2) is null then 0 else ROUND(AVG(sc.s_score),2) end ) "平均成绩" from student st
left join score sc on sc.s_id=st.s_id
group by st.s_id having AVG(sc.s_score)<60 or AVG(sc.s_score) is NULL

-- 5、查询所有同学的学生编号、学生姓名、选课总数、所有课程的总成绩
select st.s_id,st.s_name,count(sc.c_id) "选课总数",sum(case when sc.s_score is null then 0 else sc.s_score end) "总成绩" 
from student st 
left join score sc on st.s_id = sc.s_id 
group by st.s_id
-- 6、查询"李"姓老师的数量 
select t.t_name,count(t.t_id) from teacher t
group by t.t_id having t.t_name like "李%"; 

-- 7、查询学过"张三"老师授课的同学的信息 
select st.* from student st 
left join score sc on sc.s_id=st.s_id
left join course c on c.c_id=sc.c_id
left join teacher t on t.t_id=c.t_id
 where t.t_name="张三"

-- 8、查询没学过"张三"老师授课的同学的信息 
 -- 张三老师教的课
 select c.* from course c left join teacher t on t.t_id=c.t_id where  t.t_name="张三"
 -- 有张三老师课成绩的st.s_id
 select sc.s_id from score sc where sc.c_id in (select c.c_id from course c left join teacher t on t.t_id=c.t_id where  t.t_name="张三")
 -- 不在上面查到的st.s_id的学生信息,即没学过张三老师授课的同学信息
 select st.* from student st where st.s_id not in(
  select sc.s_id from score sc where sc.c_id in (select c.c_id from course c left join teacher t on t.t_id=c.t_id where  t.t_name="张三")
  )

-- 9、查询学过编号为"01"并且也学过编号为"02"的课程的同学的信息
select st.* from student st 
inner join score sc on sc.s_id = st.s_id
inner join course c on c.c_id=sc.c_id and c.c_id="01"
where st.s_id in (
select st2.s_id from student st2 
inner join score sc2 on sc2.s_id = st2.s_id
inner join course c2 on c2.c_id=sc2.c_id and c2.c_id="02"
)

复制代码
select a.* 
from
    student a,
    score b,
    score c
where
    a.s_id = b.s_id
    and a.s_id = c.s_id
    and b.c_id = '01'
    and c.c_id = '02';
复制代码

-- 10、查询学过编号为"01"但是没有学过编号为"02"的课程的同学的信息
select st.* from student st 
inner join score sc on sc.s_id = st.s_id
inner join course c on c.c_id=sc.c_id and c.c_id="01"
where st.s_id not in (
select st2.s_id from student st2 
inner join score sc2 on sc2.s_id = st2.s_id
inner join course c2 on c2.c_id=sc2.c_id and c2.c_id="02"
)

-- 11、查询没有学全所有课程的同学的信息
select * from student where s_id not in (
select st.s_id from student st 
inner join score sc on sc.s_id = st.s_id and sc.c_id="01"
where st.s_id  in (
select st1.s_id from student st1 
inner join score sc2 on sc2.s_id = st1.s_id and sc2.c_id="02"
) and st.s_id in (
select st2.s_id from student st2 
inner join score sc2 on sc2.s_id = st2.s_id and sc2.c_id="03"
))

复制代码
select a.*
from student a
left join score b on a.s_id = b.s_id
group by
a.s_id
having
count(b.c_id) != '3';
复制代码
-- 12、查询至少有一门课与学号为"01"的同学所学相同的同学的信息
select distinct st.* from student st 
left join score sc on sc.s_id=st.s_id
where sc.c_id in (
select sc2.c_id from student st2
left join score sc2 on sc2.s_id=st2.s_id
where st2.s_id ='01'
)

-- 13、查询和"01"号的同学学习的课程完全相同的其他同学的信息
select  st.* from student st 
left join score sc on sc.s_id=st.s_id
group by st.s_id
having group_concat(sc.c_id) = 
(
select  group_concat(sc2.c_id) from student st2
left join score sc2 on sc2.s_id=st2.s_id
where st2.s_id ='01'
)

-- 14、查询没学过"张三"老师讲授的任一门课程的学生姓名
select st.s_name from student st 
where st.s_id not in (
select sc.s_id from score sc 
inner join course c on c.c_id=sc.c_id
inner join teacher t on t.t_id=c.t_id and t.t_name="张三"
)

-- 15、查询两门及其以上不及格课程的同学的学号，姓名及其平均成绩
select st.s_id,st.s_name,avg(sc.s_score) from student st
left join score sc on sc.s_id=st.s_id
where sc.s_id in (
select sc.s_id from score sc 
where sc.s_score<60 or sc.s_score is NULL
group by sc.s_id having COUNT(1)>=2
)
group by st.s_id

-- 16、检索"01"课程分数小于60，按分数降序排列的学生信息
select st.*,sc.s_score from student st 
inner join score sc on sc.s_id=st.s_id and sc.c_id="01" and sc.s_score<60
order by sc.s_score desc

select st.*,sc.s_score from student st 
left join score sc on sc.s_id=st.s_id 
where sc.c_id="01" and sc.s_score<60
order by sc.s_score desc

-- 17、按平均成绩从高到低显示所有学生的所有课程的成绩以及平均成绩
select st.s_id,st.s_name,avg(sc4.s_score) "平均分",sc.s_score "语文",sc2.s_score "数学",sc3.s_score "英语" from student st
left join score sc  on sc.s_id=st.s_id  and sc.c_id="01"
left join score sc2 on sc2.s_id=st.s_id and sc2.c_id="02"
left join score sc3 on sc3.s_id=st.s_id and sc3.c_id="03"
left join score sc4 on sc4.s_id=st.s_id
group by st.s_id 
order by avg(sc4.s_score) desc
复制代码
select st.s_id,st.s_name,
(case when avg(sc4.s_score) is null then 0 else avg(sc4.s_score) end) "平均分",
(case when sc.s_score is null then 0 else sc.s_score end) "语文",
(case when sc2.s_score is null then 0 else sc2.s_score end) "数学",
(case when sc3.s_score is null then 0 else sc3.s_score end) "英语" 
from student st
left join score sc  on sc.s_id=st.s_id  and sc.c_id="01"
left join score sc2 on sc2.s_id=st.s_id and sc2.c_id="02"
left join score sc3 on sc3.s_id=st.s_id and sc3.c_id="03"
left join score sc4 on sc4.s_id=st.s_id
group by st.s_id 
order by avg(sc4.s_score) desc
复制代码


-- 18.查询各科成绩最高分、最低分和平均分：以如下形式显示：课程ID，课程name，最高分，最低分，平均分，及格率，中等率，优良率，优秀率
-- 及格为>=60，中等为：70-80，优良为：80-90，优秀为：>=90
select c.c_id,c.c_name,max(sc.s_score) "最高分",MIN(sc2.s_score) "最低分",avg(sc3.s_score) "平均分" 
,((select count(s_id) from score where s_score>=60 and c_id=c.c_id )/(select count(s_id) from score where c_id=c.c_id)) "及格率"
,((select count(s_id) from score where s_score>=70 and s_score<80 and c_id=c.c_id )/(select count(s_id) from score where c_id=c.c_id)) "中等率"
,((select count(s_id) from score where s_score>=80 and s_score<90 and c_id=c.c_id )/(select count(s_id) from score where c_id=c.c_id)) "优良率"
,((select count(s_id) from score where s_score>=90 and c_id=c.c_id )/(select count(s_id) from score where c_id=c.c_id)) "优秀率"
from course c
left join score sc on sc.c_id=c.c_id 
left join score sc2 on sc2.c_id=c.c_id 
left join score sc3 on sc3.c_id=c.c_id 
group by c.c_id

-- 19、按各科成绩进行排序，并显示排名(实现不完全)
-- mysql没有rank函数
-- 加@score是为了防止用union all 后打乱了顺序
select c1.s_id,c1.c_id,c1.c_name,@score:=c1.s_score,@i:=@i+1 from (select c.c_name,sc.* from course c 
left join score sc on sc.c_id=c.c_id
where c.c_id="01" order by sc.s_score desc) c1 ,
(select @i:=0) a
union all 
select c2.s_id,c2.c_id,c2.c_name,c2.s_score,@ii:=@ii+1 from (select c.c_name,sc.* from course c 
left join score sc on sc.c_id=c.c_id
where c.c_id="02" order by sc.s_score desc) c2 ,
(select @ii:=0) aa 
union all
select c3.s_id,c3.c_id,c3.c_name,c3.s_score,@iii:=@iii+1 from (select c.c_name,sc.* from course c 
left join score sc on sc.c_id=c.c_id
where c.c_id="03" order by sc.s_score desc) c3;
set @iii=0;


-- 20、查询学生的总成绩并进行排名
select st.s_id,st.s_name
,(case when sum(sc.s_score) is null then 0 else sum(sc.s_score) end)
 from student st
left join score sc on sc.s_id=st.s_id
group by st.s_id order by sum(sc.s_score) desc

-- 21、查询不同老师所教不同课程平均分从高到低显示 
select t.t_id,t.t_name,c.c_name,avg(sc.s_score) from teacher t 
left join course c on c.t_id=t.t_id 
left join score sc on sc.c_id =c.c_id
group by t.t_id
order by avg(sc.s_score) desc

-- 22、查询所有课程的成绩第2名到第3名的学生信息及该课程成绩
select a.* from (
select st.*,c.c_id,c.c_name,sc.s_score from student st
left join score sc on sc.s_id=st.s_id
inner join course c on c.c_id =sc.c_id and c.c_id="01"
order by sc.s_score desc LIMIT 1,2 ) a
union all
select b.* from (
select st.*,c.c_id,c.c_name,sc.s_score from student st
left join score sc on sc.s_id=st.s_id
inner join course c on c.c_id =sc.c_id and c.c_id="02"
order by sc.s_score desc LIMIT 1,2) b
union all
select c.* from (
select st.*,c.c_id,c.c_name,sc.s_score from student st
left join score sc on sc.s_id=st.s_id
inner join course c on c.c_id =sc.c_id and c.c_id="03"
order by sc.s_score desc LIMIT 1,2) c

-- 23、统计各科成绩各分数段人数：课程编号,课程名称,[100-85],[85-70],[70-60],[0-60]及所占百分比
select c.c_id,c.c_name 
,((select count(1) from score sc where sc.c_id=c.c_id and sc.s_score<=100 and sc.s_score>80)/(select count(1) from score sc where sc.c_id=c.c_id )) "100-85"
,((select count(1) from score sc where sc.c_id=c.c_id and sc.s_score<=85 and sc.s_score>70)/(select count(1) from score sc where sc.c_id=c.c_id )) "85-70"
,((select count(1) from score sc where sc.c_id=c.c_id and sc.s_score<=70 and sc.s_score>60)/(select count(1) from score sc where sc.c_id=c.c_id )) "70-60"
,((select count(1) from score sc where sc.c_id=c.c_id and sc.s_score<=60 and sc.s_score>=0)/(select count(1) from score sc where sc.c_id=c.c_id )) "60-0"
from course c order by c.c_id

-- 24、查询学生平均成绩及其名次 
set @i=0;
select a.*,@i:=@i+1 from (
select st.s_id,st.s_name,round((case when avg(sc.s_score) is null then 0 else avg(sc.s_score) end),2) "平均分" from student st
left join score sc on sc.s_id=st.s_id
group by st.s_id order by sc.s_score desc) a

-- 25、查询各科成绩前三名的记录
select a.* from (
 select st.s_id,st.s_name,c.c_id,c.c_name,sc.s_score from student st
 left join score sc on sc.s_id=st.s_id
 inner join course c on c.c_id=sc.c_id and c.c_id='01'
 order by sc.s_score desc LIMIT 0,3) a
union all 
select b.* from (
 select st.s_id,st.s_name,c.c_id,c.c_name,sc.s_score from student st
 left join score sc on sc.s_id=st.s_id
 inner join course c on c.c_id=sc.c_id and c.c_id='02'
 order by sc.s_score desc LIMIT 0,3) b
union all
select c.* from (
 select st.s_id,st.s_name,c.c_id,c.c_name,sc.s_score from student st
 left join score sc on sc.s_id=st.s_id
 inner join course c on c.c_id=sc.c_id and c.c_id='03'
 order by sc.s_score desc LIMIT 0,3) c

-- 26、查询每门课程被选修的学生数 
select c.c_id,c.c_name,count(1) from course c 
left join score sc on sc.c_id=c.c_id
inner join student st on st.s_id=c.c_id
group by st.s_id

-- 27、查询出只有两门课程的全部学生的学号和姓名
select st.s_id,st.s_name from student st 
left join score sc on sc.s_id=st.s_id
inner join course c on c.c_id=sc.c_id 
group by st.s_id having count(1)=2

-- 28、查询男生、女生人数
select st.s_sex,count(1) from student st group by st.s_sex

-- 29、查询名字中含有"风"字的学生信息
select st.* from student st where st.s_name like "%风%";

-- 30、查询同名同性学生名单，并统计同名人数 
select st.*,count(1) from student st group by st.s_name,st.s_sex having count(1)>1

-- 31、查询1990年出生的学生名单
select st.* from student st where st.s_birth like "1990%";

-- 32、查询每门课程的平均成绩，结果按平均成绩降序排列，平均成绩相同时，按课程编号升序排列 
select c.c_id,c.c_name,avg(sc.s_score) from course c
inner join score sc on sc.c_id=c.c_id  
group by c.c_id order by avg(sc.s_score) desc,c.c_id asc

-- 33、查询平均成绩大于等于85的所有学生的学号、姓名和平均成绩
select st.s_id,st.s_name,avg(sc.s_score) from student st
left join score sc on sc.s_id=st.s_id
group by st.s_id having avg(sc.s_score)>=85

-- 34、查询课程名称为"数学"，且分数低于60的学生姓名和分数 
select st.s_id,st.s_name,sc.s_score from student st
inner join score sc on sc.s_id=st.s_id and sc.s_score<60
inner join course c on c.c_id=sc.c_id and c.c_name ="数学" 

-- 35、查询所有学生的课程及分数情况；
select st.s_id,st.s_name,c.c_name,sc.s_score from student st
left join score sc on sc.s_id=st.s_id
left join course c on c.c_id =sc.c_id
order by st.s_id,c.c_name

-- 36、查询任何一门课程成绩在70分以上的姓名、课程名称和分数
select st2.s_id,st2.s_name,c2.c_name,sc2.s_score from student st2
left join score sc2 on sc2.s_id=st2.s_id
left join course c2 on c2.c_id=sc2.c_id 
where st2.s_id in(
select st.s_id from student st 
left join score sc on sc.s_id=st.s_id 
group by st.s_id having min(sc.s_score)>=70)
order by s_id

-- 37、查询不及格的课程
select st.s_id,c.c_name,st.s_name,sc.s_score from student st
inner join score sc on sc.s_id=st.s_id and  sc.s_score<60
inner join course c on c.c_id=sc.c_id 

-- 38、查询课程编号为01且课程成绩在80分以上的学生的学号和姓名
select st.s_id,st.s_name,sc.s_score from student st
inner join score sc on sc.s_id=st.s_id and sc.c_id="01" and sc.s_score>=80

-- 39、求每门课程的学生人数
select c.c_id,c.c_name,count(1) from course c
inner join score sc on sc.c_id=c.c_id
group by c.c_id

-- 40、查询选修"张三"老师所授课程的学生中，成绩最高的学生信息及其成绩 
select st.*,c.c_name,sc.s_score,t.t_name from student st
inner join score sc on sc.s_id=st.s_id
inner join course c on c.c_id=sc.c_id 
inner join teacher t on t.t_id=c.t_id and  t.t_name="张三"
order by sc.s_score desc
limit 0,1

-- 41、查询不同课程成绩相同的学生的学生编号、课程编号、学生成绩 
select st.s_id,st.s_name,sc.c_id,sc.s_score from student st 
left join score sc on sc.s_id=st.s_id
left join course c on c.c_id=sc.c_id
where (
select count(1) from student st2 
left join score sc2 on sc2.s_id=st2.s_id
left join course c2 on c2.c_id=sc2.c_id
where sc.s_score=sc2.s_score and c.c_id!=c2.c_id 
)>1

-- 42、查询每门功成绩最好的前两名 
select a.* from (select st.s_id,st.s_name,c.c_name,sc.s_score from student st
left join score sc on sc.s_id=st.s_id
inner join course c on c.c_id=sc.c_id and c.c_id="01"
order by sc.s_score desc limit 0,2) a
union all
select b.* from (select st.s_id,st.s_name,c.c_name,sc.s_score from student st
left join score sc on sc.s_id=st.s_id
inner join course c on c.c_id=sc.c_id and c.c_id="02"
order by sc.s_score desc limit 0,2) b
union all
select c.* from (select st.s_id,st.s_name,c.c_name,sc.s_score from student st
left join score sc on sc.s_id=st.s_id
inner join course c on c.c_id=sc.c_id and c.c_id="03"
order by sc.s_score desc limit 0,2) c
 
-- 借鉴(更准确,漂亮):
 select a.s_id,a.c_id,a.s_score from score a
 where (select COUNT(1) from score b where b.c_id=a.c_id and b.s_score>=a.s_score)<=2 order by a.c_id

-- 43、统计每门课程的学生选修人数（超过5人的课程才统计）。要求输出课程号和选修人数，查询结果按人数降序排列，
--     若人数相同，按课程号升序排列  
select sc.c_id,count(1) from score sc
left join course c on c.c_id=sc.c_id
group by c.c_id having count(1)>5
order by count(1) desc,sc.c_id asc

-- 44、检索至少选修两门课程的学生学号 
select st.s_id from student st 
left join score sc on sc.s_id=st.s_id
group by st.s_id having count(1)>=2

-- 45、查询选修了全部课程的学生信息
select st.* from student st 
left join score sc on sc.s_id=st.s_id
group by st.s_id having count(1)=(select count(1) from course)

-- 46、查询各学生的年龄
 select st.*,timestampdiff(year,st.s_birth,now()) from student st

-- 47、查询本周过生日的学生
  -- 此处可能有问题,week函数取的为当前年的第几周,2017-12-12是第50周而2018-12-12是第49周,可以取月份,day,星期几(%w),
  -- 再判断本周是否会持续到下一个月进行判断,太麻烦,不会写
select st.* from student st 
where week(now())=week(date_format(st.s_birth,'%Y%m%d'))

-- 48、查询下周过生日的学生
select st.* from student st 
where week(now())+1=week(date_format(st.s_birth,'%Y%m%d'))

-- 49、查询本月过生日的学生
select st.* from student st 
where month(now())=month(date_format(st.s_birth,'%Y%m%d'))

-- 50、查询下月过生日的学生
 -- 注意:当 当前月为12时,用month(now())+1为13而不是1,可用timestampadd()函数或mod取模
select st.* from student st 
where month(timestampadd(month,1,now()))=month(date_format(st.s_birth,'%Y%m%d'))
-- 或
select st.* from student st where (month(now()) + 1) mod 12 = month(date_format(st.s_birth,'%Y%m%d'))

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