#建立一个数据库 create database why; #新建一个用户表 create table why(employee_id bigint,name char(16),gerder enum(
#建立一个数据库
create database why;
#新建一个用户表
create table why(employee_id bigint,name char(16),gerder enum('M','F'),dept_id enum('caiwu','renshi','shichang','jishu'),join_time date,salary double,phone char(11),address char(40),description text);
#建立用户记录
insert into why values(20121200*(#序号),'name','F/M','caiwu/renshi/shichang/jishu','2012-10-07,*(#工资),'*'(#电话),'address','description')
#实现sql查询
按名字查找
select * from why where; #查找某个人的个人信息
按照薪水>查找
select * from why where salary>=3000 and salary
查找每个部门的人数
select dent_id,count(*) as dentsum from why group by dept_id;
每个部门的平均薪资
select dent_id,avg(salary) as avgsalary from why group by dept_id;
描述字段为空的员工
select * from why where description is null; #所有信息均为空(包括用户姓名)
select * from why where description='';#只有描述信息为空
求出每个部门女员工工资最高的职工的薪水
select dept_id max(salary) from why where gerder='F' group by dept_id;
名字以l开头的员工个数
select * from why where name like "l%"

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MySQLisbetterforspeedandsimplicity,suitableforwebapplications;PostgreSQLexcelsincomplexdatascenarioswithrobustfeatures.MySQLisidealforquickprojectsandread-heavytasks,whilePostgreSQLispreferredforapplicationsrequiringstrictdataintegrityandadvancedSQLf

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The EXPLAIN statement can be used to analyze and improve SQL query performance. 1. Execute the EXPLAIN statement to view the query plan. 2. Analyze the output results, pay attention to access type, index usage and JOIN order. 3. Create or adjust indexes based on the analysis results, optimize JOIN operations, and avoid full table scanning to improve query efficiency.

Using mysqldump for logical backup and MySQLEnterpriseBackup for hot backup are effective ways to back up MySQL databases. 1. Use mysqldump to back up the database: mysqldump-uroot-pmydatabase>mydatabase_backup.sql. 2. Use MySQLEnterpriseBackup for hot backup: mysqlbackup--user=root-password=password--backup-dir=/path/to/backupbackup. When recovering, use the corresponding life


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