学习笔记7——数据库基础知识以及mysql的查询语句

学习笔记系列开头惯例发布一些寻亲消息

链接：https://baobeihuijia.com/bbhj/contents/3/199913.html

数据库

• 三个概念区分

  • DB：数据库，存储数据的仓库，有组织的数据容器
  • DBMS:数据库管理系统
  • SQL：几乎所有的DBMS都支持的语言

• 对数据库的大致了解过程

  MySQL详细学习教程（建议收藏）-CSDN博客

  聚集索引（聚类索引）与非聚集索引（非聚类索引） - 知乎 (zhihu.com)

  主键约束（PRIMARY KEY）和唯一约束（UNIQUE）的区别_主键约束和唯一约束有什么区别-CSDN博客

  事务ACID理解-CSDN博客

  数据库三级模式：外模式、模式和内模式_数据库模式内模式外模式-CSDN博客

• 基础知识

  • mysql的索引从1开始，但是分页查询是从0开始起始索引
  • 起别名,as或者空格，需要注意**【由于mysql先执行from，所以起了别名后，其他地方就不能再使用原名】**
  • mysql中+只有运算符含义，如果遇到字符串就会将字符转化为数据：转化成功继续计算，转化失败则将字符型转化为0，如果其中一个为null那么拼接结果为null
  • 字符串拼接：SELECT CONCAT(first_name,last_name) AS “name” FROM employees; 如果其中一个为null那么拼接结果为null
  • DESC departments;
  • 字段用着重符，如果是加入的字符串要用单引
  • IFNULL(字段，0) 字段为null则改为0，不为null则仍为原数据
  • 在GBK中一个字符占2个字节，utf8中一个字符占3个字节

• 1、> < = != >= <=
  2、and or not
  3、like / between and / in / is null

• like 用通配符% 表示任意多的字符，包括0个，下划线表示一个

  • SELECT *
    FROM employees
    WHERE first_name LIKE ‘%a%’;

    SELECT *
    FROM employees
    WHERE first_name LIKE ‘__e__a%’;

    SELECT *
    FROM employees
    WHERE last_name LIKE ‘__%’

• between

  • 两边都是闭区间、有前后关系

    SELECT *
    FROM employees
    WHERE employee_id BETWEEN 100 AND 120;

• IN: 表示某字段是否满足in列表中的一项，in中的判断逻辑是：是否能找到直接相等？

  • SELECT *
    FROM employees
    WHERE job_id IN (‘IT_PROG’,‘AS_VD’);

• IS NULL:等号不能判断null， <=>安全等于符号

  • SELECT *
    FROM employees
    WHERE commission_pct IS NULL;
  • SELECT *
    FROM employees
    WHERE commission_pct IS NOT NULL;

• 排序查询

  • 默认为升序、ASC \ DESC

  • 一般放在查询的最后边

  • SELECT *,salary*12*(1+IFNULL(commission_pct,0)) AS money
    FROM employees
    ORDER BY money DESC;
    
    # 按照多个条件进行排序
    SELECT *
    FROM employees
    ORDER BY salary DESC, employee_id;
    

• 常见函数

  • 单行函数：concat / length / ifnull / UPPER / LOWER (变化大小写) / substr（截取字符串） / instr（返回第一次出现的起始索引），trim是去掉左右空格，lpad

    # 返回第一次出现的起始索引
    SELECT INSTR('zhuahdhqwdhqwo','hq');
    # 可以选择去前或者去后
    SELECT TRIM('a' FROM 'aaaazhiyaaaaaaa')；
    # lpad进行左长度填充，超过固定长度就会截断
    SELECT LPAD('yyy',10,'-');
    # rpad右填充
    SELECT RPAD('yyy',10,'-');
    # replace
    SELECT REPLACE('nihaowoshiczy','czy','dameinv');
    
    # round:四舍五入
    SELECT ROUND(4.5);
    SELECT ROUND(1.657,2);
    
    # ceil：向上取整，返回大于等于本身的最小整数
    
    # floor：向下取整，返回小于等于本身的最小整数
    
    # truncate 截断
    SELECT TRUNCATE(1.666666,3);
    
    #mod取余，结果只看被除数的正负号： mod(a,b)  =  a-a/b*b
    
    #时间转换 STR_TO_DATE
    SELECT STR_TO_DATE('7-31 2023','%c-%d %Y');
    SELECT DATE_FORMAT(NOW(),'%Y  %c_%d');
    
    select version();
    select database();
    
    # 查询时间之差
    SELECT DATEDIFF(NOW(),'2000-7-31');
    

  • 流程控制函数

    # if
    SELECT IF(commission_pct IS NULL,'A','B') FROM employees;
    
    # case当作switch使用
    SELECT salary, department_id, 
    CASE department_id
    WHEN 30 THEN salary*1.1
    WHEN 40 THEN salary*1.2
    WHEN 50 THEN salary*1.3
    ELSE salary
    END 
    FROM employees;
    
    # case当作if else使用
    SELECT salary,
    CASE
    WHEN salary >20000 THEN 'A'
    WHEN salary >15000 THEN 'B'
    WHEN salary >10000 THEN 'C'
    ELSE 'D'
    END
    FROM employees;
    

  • 分组函数：max / min / avg / sum / count 和分组函数一起查询的字段需要和分组函数结果一致

    sum和avg不计算考虑null
    max和min也忽略考虑null
    count计算的是非空字段的数量
    
    SELECT COUNT(*) FROM employees;
    SELECT COUNT(1) FROM employees;
    

• 查询

  • 分组查询

    SELECT MAX(salary),job_id
    FROM employees
    GROUP BY job_id;
    
    
    SELECT COUNT(*), department_id
    FROM employees
    GROUP BY department_id
    HAVING COUNT(*)>2;
    
    # 查询每个工种有奖金的最高工资大于12000的工种和最高工资
    SELECT job_id, MAX(salary)
    FROM employees
    WHERE commission_pct IS NOT NULL
    GROUP BY job_id
    HAVING MAX(salary)>12000;
    
    # 查询领导编号大于102的每个领导下最低工资大于5000的领导编号和最低工资
    SELECT manager_id, MIN(salary)
    FROM employees
    WHERE manager_id > 102
    GROUP BY manager_id
    HAVING MIN(salary)>5000;
    
    # 多个字段分组
    SELECT AVG(salary), department_id, job_id
    FROM employees
    GROUP BY department_id, job_id;
    
    # 多个字段分组查询
    SELECT AVG(salary), department_id, job_id
    FROM employees
    WHERE department_id IS NOT NULL
    GROUP BY department_id, job_id
    HAVING AVG(salary)>10000
    ORDER BY AVG(salary) DESC;
    

  • 连接查询：sql92和sql99语法

    # sql92语法内连接语法：
    
    # 等值连接：产生交集部分
    SELECT `name`,boyname FROM beauty, boys
    WHERE beauty.`boyfriend_id` =  boys.`id`;
    
    SELECT department_name,d.manager_id,MIN(salary)
    FROM departments d, employees e
    WHERE e.`department_id` = d.`department_id` AND e.`commission_pct` IS NOT NULL
    GROUP BY department_name,d.manager_id;
    
    # 非等值连接
    SELECT salary, grade_id
    FROM employees,job_grade
    WHERE salary BETWEEN lowerst_sal AND highest_sal;
    
    # 自连接
    SELECT a.`employee_id`, b.`employee_id`
    FROM employees a, employees b
    WHERE a.`manager_id` = b.`employee_id`;
    
    
    SELECT country_id, COUNT(*)
    FROM departments d,locations l
    WHERE d.`location_id` = l.`location_id`
    GROUP BY l.`country_id`
    HAVING COUNT(*)>2;
    

    # sql99内连接语法
    #inner join
    SELECT last_name,department_name
    FROM employees e
    INNER JOIN departments d
    ON e.`department_id` = d.`department_id`;
    
    # 三表连接
    SELECT last_name,department_name,job_title 
    FROM employees e
    INNER JOIN departments d
    ON e.`department_id` = d.`department_id`
    INNER JOIN jobs j
    ON e.`job_id` = j.`job_id`
    ORDER BY department_name;
    
    # 非等值连接
    # 自连接
    SELECT e.last_name,m.last_name
    FROM employees e
    INNER JOIN employees m
    ON e.`manager_id` = m.`employee_id`
    WHERE e.`last_name` LIKE '%k%';
    

    # 外连接，用于查询一个表中有另一个表中没有，结果为主表的全部记录
    # 包括两部分：内连接（只要有相等就会显示） + 主表有但是从表没有的显示为null（筛选的时候用从表的主键）
    # 左外和右外交换表的顺序，可以实现同样的结果
    # 一般是为了查询除了交集部分的不匹配的行
    
    # left outer  right outer
    SELECT `name`,boys.*
    FROM beauty
    LEFT OUTER JOIN boys 
    ON beauty.`boyfriend_id` = boys.`id`
    WHERE boys.id IS NOT NULL;
    
    SELECT department_name
    FROM departments d
    LEFT OUTER JOIN employees e
    ON d.department_id = e.department_id
    WHERE e.employee_id IS NULL;
    
    
    # 查询部门名为SAL或者IT的员工信息,防止出现SAL部门没有员工
    SELECT e.*
    FROM employees e
    RIGHT OUTER JOIN departments d
    ON e.`department_id` = d.`department_id`
    WHERE d.`department_name`= 'SAL' OR d.`department_name` = 'IT';
    
    # full outer join，除了内连接结果，保留左和右边没有匹配的部分
    
    # 交叉连接：就是笛卡尔乘积
    SELECT `name`, boyname
    FROM beauty
    CROSS JOIN boys;  
    

  • 子查询

    # 有了子查询后尽量不要用两表连接筛选了
    # 如果是select嵌套select那么成为外查询/主查询
    
    # 子查询，结果集为一个标量值
    # where后边加select
    SELECT last_name,job_id,salary
    FROM employees
    WHERE salary=(
    	SELECT MIN(salary)
    	FROM employees
    );
    
    # having后边加select
    SELECT department_id,MIN(salary)
    FROM employees
    GROUP BY department_id
    HAVING MIN(salary)>(
    	SELECT MIN(salary)
    	FROM employees
    	WHERE department_id = 50
    );
    
    in/not in:可以改写为 =any / !=all
    ANY
    ALL
    
    # select后边加select
    # 查询是一条一条进行遍历的，所以在select后边的查询必须是一行一列
    SELECT d.*,(
    	SELECT COUNT(*)
    	FROM employees e
    	WHERE e.department_id = d.department_id
    )FROM departments d;
    
    # from后边加select，必须给新表起别名
    SELECT ag_dep.*, jg.level
    FROM
    (
    	SELECT AVG(salary) ag,department_id
    	FROM employees
    	GROUP BY department_id
    ) ag_dep INNER JOIN job_grades jg
    ON ag_dep.ag BETWEEN jg.lowest_sal AND jg.highest_sal
    
    # exist
    SELECT d.department_name
    FROM departments d
    WHERE d.department_id IN (
    	SELECT e.department_id
    	FROM employees e
    );
    
    SELECT d.department_name
    FROM departments d
    WHERE EXISTS(
    	SELECT *
    	FROM employees e
    	WHERE e.`department_id` = d.`department_id`
    );
    
    SELECT bo.*
    FROM boys bo
    WHERE EXISTS(
    	SELECT *
    	FROM beauty b
    	WHERE b.`boyfriend_id`  = bo.`id`
    );
    
    SELECT bo.*
    FROM boys bo
    WHERE bo.`id`  IN (
    	SELECT b.`boyfriend_id`
    	FROM beauty b
    );
    

  • 分页查询

    # 分页查询
    limit offset,size;
    offset:要显示的起始索引，从0开始
    size：要显示的条目数
    
    # 分页公式：
    limit (page-1)*size,size;
    
    SELECT *
    FROM employees
    WHERE commission_pct IS NOT NULL
    ORDER BY salary DESC
    LIMIT 10;
    

    

    # 一个经典的样例
    # 查平均工资最高的部门的manager的信息
    # 两个表的不同拼接会是不同结果
    （
    如果是employee的department_id和department的department_id拼接，那么得到的结果是我们知道每个员工的部门详细信息
    但是如果是employee的employee_id和department的manager_id拼接，那么得到的是我们知道每个部门领导人的详细信息
    ）
    
    SELECT *
    FROM employees e
    INNER JOIN departments d
    ON e.`employee_id` = d.`manager_id`
    WHERE d.`department_id` = (
    	SELECT  department_id
    	FROM employees
    	GROUP BY department_id
    	ORDER BY AVG(salary) DESC
    	LIMIT 1
    );
    

  • 联合查询

    union 连接两个查询结果：查询结果来自于多个表，表之间没有连接关系，但是查询结果列数和列类型要一致 
    union all：可以包含两个表中的重复项