数据导入导出
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导入table
http://www.runoob.com/mysql/mysql-database-import.html -
导出table
http://www.runoob.com/mysql/mysql-database-export.html
SELECT * FROM runoob_tblINTO OUTFILE '/tmp/runoob.txt'设置分隔符
SELECT a,b,a+b INTO OUTFILE '/tmp/result.text'FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"'LINES TERMINATED BY '\n'FROM test_table;作业
- 1.项目七: 各部门工资最高的员工（难度：中等）
创建Employee 表，包含所有员工信息，每个员工有其对应的 Id, salary 和 department Id。
+----+-------+--------+--------------+| Id | Name | Salary | DepartmentId |+----+-------+--------+--------------+| 1 | Joe | 70000 | 1 || 2 | Henry | 80000 | 2 || 3 | Sam | 60000 | 2 || 4 | Max | 90000 | 1 |+----+-------+--------+--------------+创建Department 表，包含公司所有部门的信息。
+----+----------+| Id | Name |+----+----------+| 1 | IT || 2 | Sales |+----+----------+编写一个 SQL 查询，找出每个部门工资最高的员工。例如，根据上述给定的表格，Max 在 IT 部门有最高工资，Henry 在 Sales 部门有最高工资。
+------------+----------+--------+| Department | Employee | Salary |+------------+----------+--------+| IT | Max | 90000 || Sales | Henry | 80000 |+------------+----------+--------+解答：
CREATE TABLE employee(Id INT NOT NULL PRIMARY KEY,Name VARCHAR(20) NOT NULL,Salary INT,DepartmentId INT);INSERT INTO employee(Id,Name,Salary,DepartmentId)VALUES(1,'Joe',70000,1),(2,'Henry',80000,2),(3,'Sam',60000,2),(4,'Max',90000,2);CREATE TABLE Department(Id INT NOT NULL,Name VARCHAR(20));INSERT INTO Department(Id,Name)VALUES(1,'IT'),(2,'Sales');SELECT Department.Name AS Department, employee.Name As Employee, employee.Salary AS SalaryFROM DepartmentJOIN employeeWHERE Department.Id = employee.DepartmentIdGROUP BY employee.DepartmentIdHAVING MAX(Salary)结果：
- 2.项目八: 换座位（难度：中等）
小美是一所中学的信息科技老师，她有一张 seat 座位表，平时用来储存学生名字和与他们相对应的座位 id。
其中纵列的 id 是连续递增的
小美想改变相邻俩学生的座位。
你能不能帮她写一个 SQL query 来输出小美想要的结果呢？
请创建如下所示seat表：
示例：
+---------+---------+| id | student |+---------+---------+| 1 | Abbot || 2 | Doris || 3 | Emerson || 4 | Green || 5 | Jeames |+---------+---------+假如数据输入的是上表，则输出结果如下：
+---------+---------+| id | student |+---------+---------+| 1 | Doris || 2 | Abbot || 3 | Green || 4 | Emerson || 5 | Jeames |+---------+---------+注意：
如果学生人数是奇数，则不需要改变最后一个同学的座位。
解答：
CREATE TABLE seat(id INT NOT NULL PRIMARY KEY,student VARCHAR(20));INSERT INTO seatVALUES(1,'Abbot'),(2,'Doris'),(3,'Emerson'),(4,'Green'),(5,'Jeames');SELECT s.id,s.student from ( SELECT id-1 as id, student FROM seat WHERE id%2 = 0 UNION SELECT id+1 as id, student FROM seat WHERE id%2 =1 AND id != (SELECT MAX(id) FROM seat) UNION SELECT id,student FROM seat WHERE id%2=1 AND id = (SELECT MAX(id) FROM seat);)sORDER BY s.id;结果：
- 3.项目九: 分数排名（难度：中等）
编写一个 SQL 查询来实现分数排名。如果两个分数相同，则两个分数排名（Rank）相同。请注意，平分后的下一个名次应该是下一个连续的整数值。换句话说，名次之间不应该有“间隔”。
创建以下score表：
+----+-------+| Id | Score |+----+-------+| 1 | 3.50 || 2 | 3.65 || 3 | 4.00 || 4 | 3.85 || 5 | 4.00 || 6 | 3.65 |+----+-------+例如，根据上述给定的 Scores 表，你的查询应该返回（按分数从高到低排列）：
+-------+------+| Score | Rank |+-------+------+| 4.00 | 1 || 4.00 | 1 || 3.85 | 2 || 3.65 | 3 || 3.65 | 3 || 3.50 | 4 |+-------+------+CREATE TABLE score(id int,score DECIMAL(3,2));INSERT INTO scoreVALUES(1,3.50),(2,3.65),(3,4.00),(4,3.85),(5,4.00),(6,3.65);SELECT id, score,CASEWHEN @pre_score = score THEN @rank_tmpWHEN @pre_score:=score THEN @rank_tmp:=@rank_tmp+1END AS 'rank'FROM score,(SELECT @rank_tmp:=0,@pre_score:=NULL) tmpORDER BYscore.score DESC;结果：
这里用了变量和 CASE WHEN 语句，十分巧妙。
在MySQL 8.x 版本引入了窗口函数，包括 rank()、dense_rank()；所以可以这样写
SELECT id, score, dense_rank ( ) over ( ORDER BY score.score DESC ) AS 'rank'FROM scoreORDER BY score DESC;轻松优雅美滋滋。
参考
- 排序部分参考了 https://www.jianshu.com/p/bb1b72a1623e
- 换座位参考了 https://blog.csdn.net/wal1314520/article/details/80117658
- 窗口函数参考了 https://blog.51cto.com/11103985/2341971
