MySQL表操作+表联结

task3:超过5名学生的课

创建如下所示的courses 表 ，有: student (学生) 和 class (课程)。
编写一个 SQL 查询，列出所有超过或等于5名学生的课。
应该输出:
±--------+ | class | ±--------+ | Math | ±--------+
Note:
学生在每个课中不应被重复计算。

mysql> use yiibaidb;
mysql> CREATE TABLE IF NOT EXISTS courses (
    -> student VARCHAR(50) NOT NULL,
    -> class VARCHAR(50) NOT NULL
    -> );
mysql> INSERT INTO courses VALUES('A','Math');
mysql> INSERT INTO courses VALUES('B','English');
mysql> INSERT INTO courses VALUES('C','Math');
mysql> INSERT INTO courses VALUES('D','Biology');
mysql> INSERT INTO courses VALUES('E','Math');
mysql> INSERT INTO courses VALUES('F','Computer');
mysql> INSERT INTO courses VALUES('G','Math');
mysql> INSERT INTO courses VALUES('H','Math');
mysql> INSERT INTO courses VALUES('I','Math');
mysql> INSERT INTO courses VALUES('A','Math');

构建courses表，含有student和class两项。

mysql> select * from courses
    -> ;
+---------+----------+
| student | class    |
+---------+----------+
| A       | Math     |
| B       | English  |
| C       | Math     |
| D       | Biology  |
| E       | Math     |
| F       | Computer |
| G       | Math     |
| H       | Math     |
| I       | Math     |
| A       | Math     |
+---------+----------+

剔除表格的重复项并group之后用having筛选：

mysql> ALTER IGNORE TABLE courses ADD PRIMARY KEY (student,class);
mysql> select * from courses;
+---------+----------+
| student | class    |
+---------+----------+
| A       | Math     |
| B       | English  |
| C       | Math     |
| D       | Biology  |
| E       | Math     |
| F       | Computer |
| G       | Math     |
| H       | Math     |
| I       | Math     |
+---------+----------+
mysql> select class from courses group by class having count(*)>=5;
+-------+
| class |
+-------+
| Math  |
+-------+

task4:交换工资

性别交换。有m=男性 和 f=女性的值 。交换所有的 f 和 m 值(例如，将所有 f 值更改为 m，反之亦然)。要求使用一个更新查询，并且没有中间临时表。

构建salary表。

mysql> create table if not exists salary (
    -> id int NOT NULL PRIMARY KEY,
    -> name VARCHAR(50) NOT NULL,
    -> sex VARCHAR(50) NOT NULL,
    -> salary INT NOT NULL
    -> );
mysql> INSERT INTO salary VALUES(1,'A','m',2500);
mysql> INSERT INTO salary VALUES(2,'B','f',1500);
mysql> INSERT INTO salary VALUES(3,'C','m',5500);
mysql> INSERT INTO salary VALUES(4,'D','f',500);
mysql> select * from salary;
+----+------+-----+--------+
| id | name | sex | salary |
+----+------+-----+--------+
|  1 | A    | m   |   2500 |
|  2 | B    | f   |   1500 |
|  3 | C    | m   |   5500 |
|  4 | D    | f   |    500 |
+----+------+-----+--------+

用update和replace进行替换

mysql> UPDATE `salary` SET `sex`=replace(`sex`,'f','M');
mysql> UPDATE `salary` SET `sex`=replace(`sex`,'m','f');
mysql> UPDATE `salary` SET `sex`=replace(`sex`,'M','m');
mysql> select * from salary;
+----+------+-----+--------+
| id | name | sex | salary |
+----+------+-----+--------+
|  1 | A    | f   |   2500 |
|  2 | B    | m   |   1500 |
|  3 | C    | f   |   5500 |
|  4 | D    | m   |    500 |
+----+------+-----+--------+

task5：组合两张表

在数据库中创建表1和表2，并各插入三行数据（自己造）
构建表格Person和Address.

mysql> create table if not exists Person (
    -> PersonId int not null primary key,
    -> FirstName VARCHAR(50) NOT NULL,
    -> LastName VARCHAR(50) NOT NULL
    -> );
mysql> INSERT INTO Person VALUES('1','ming','li');
mysql> INSERT INTO Person VALUES('2','ya','wang');
mysql> INSERT INTO Person VALUES('3','feili','sun');
mysql> select * from Person;
+----------+-----------+----------+
| PersonId | FirstName | LastName |
+----------+-----------+----------+
|        1 | ming      | li       |
|        2 | ya        | wang     |
|        3 | feili     | sun      |
+----------+-----------+----------+
mysql> create table if not exists Address (
    -> AddressId int not null primary key,
    -> PersonId int not null,
    -> City VARCHAR(50),
    -> State VARCHAR(50)
    -> );
mysql> INSERT INTO Address VALUES(0001,2,'','guangdong');
mysql> INSERT INTO Address VALUES(0002,3,'chengdu','sichuan');
mysql> INSERT INTO Address VALUES(0003,1,'','');
mysql> select * from Address;
+-----------+----------+---------+-----------+
| AddressId | PersonId | City    | State     |
+-----------+----------+---------+-----------+
|         1 |        2 |         | guangdong |
|         2 |        3 | chengdu | sichuan   |
|         3 |        1 |         |           |
+-----------+----------+---------+-----------+

mysql> select P.FirstName,P.LastName as LastName, A.City,A.State as State from Person P,Address A where P.PersonId=A.PersonId;
+-----------+----------+---------+-----------+
| FirstName | LastName | City    | State     |
+-----------+----------+---------+-----------+
| ming      | li       |         |           |
| ya        | wang     |         | guangdong |
| feili     | sun      | chengdu | sichuan   |
+-----------+----------+---------+-----------+

task6：删除重复的邮箱

编写一个 SQL 查询，来删除 email 表中所有重复的电子邮箱，重复的邮箱里只保留 **Id ***最小 *的那个。

mysql> select ID,Email from email group by Email having ID=min(ID);
+----+---------+
| ID | Email   |
+----+---------+
|  1 | a@b.com |
|  2 | c@d.com |
+----+---------+