mysql 笔记2

梦里梦外; 2021-12-16 02:23 152阅读 0赞

-- 数据的准备
        -- 创建一个数据库
        CREATE DATABASE python_test CHARSET=utf8;
    
        -- 使用一个数据库
        USE python_test;
    
        -- 显示使用的当前数据是哪个?
        SELECT DATABASE();
    
        -- 创建一个数据表
        -- students表
        CREATE TABLE students(
            id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT NOT NULL,
            NAME VARCHAR(20) DEFAULT '',
            age TINYINT UNSIGNED DEFAULT 0,
            height DECIMAL(5,2),
            gender ENUM('男','女','中性','保密') DEFAULT '保密',
            cls_id INT UNSIGNED DEFAULT 0,
            is_delete BIT DEFAULT 0
        );
    
        -- classes表
        CREATE TABLE classes (
            id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY NOT NULL,
            NAME VARCHAR(30) NOT NULL
        );
    
    
    
    -- 查询
        -- 查询所有字段
        -- select * from 表名;
        SELECT * FROM students;
        SELECT * FROM classes;
        SELECT id, NAME FROM classes;
    
        -- 查询指定字段
        -- select 列1,列2,... from 表名;
        SELECT NAME, age FROM students;
        
        -- 使用 as 给字段起别名
        -- select 字段 as 名字.... from 表名;
        SELECT NAME AS 姓名, age AS 年龄 FROM students;
    
        -- select 表名.字段 .... from 表名;
        SELECT students.name, students.age FROM students;
    
        
        -- 可以通过 as 给表起别名
        -- select 别名.字段 .... from 表名 as 别名;
        SELECT students.name, students.age FROM students;
        SELECT s.name, s.age FROM students AS s;
        -- 失败的select students.name, students.age from students as s;
    
    
        -- 消除重复行
        -- distinct 字段
        SELECT DISTINCT gender FROM students;
    
    
    -- 条件查询
        -- 比较运算符
            -- select .... from 表名 where .....
            -- >
            -- 查询大于18岁的信息
            SELECT * FROM students WHERE age>18;
            SELECT id,NAME,gender FROM students WHERE age>18;
    
            -- <
            -- 查询小于18岁的信息
            SELECT * FROM students WHERE age<18;
    
            -- >=
            -- <=
            -- 查询小于或者等于18岁的信息
    
            -- =
            -- 查询年龄为18岁的所有学生的名字
            SELECT * FROM students WHERE age=18;
    
    
            -- != 或者 <>
    
    
        -- 逻辑运算符
            -- and
            -- 18到28之间的所以学生信息
            SELECT * FROM students WHERE age>18 AND age<28;
            -- 失败select * from students where age>18 and <28;
    
    
            -- 18岁以上的女性
            SELECT * FROM students WHERE age>18 AND gender="女";
            SELECT * FROM students WHERE age>18 AND gender=2;
    
    
            -- or
            -- 18以上或者身高查过180(包含)以上
            SELECT * FROM students WHERE age>18 OR height>=180;
    
    
            -- not
            -- 不在 18岁以上的女性 这个范围内的信息
            -- select * from students where not age>18 and gender=2;
            SELECT * FROM students WHERE NOT (age>18 AND gender=2);
    
            -- 年龄不是小于或者等于18 并且是女性
            SELECT * FROM students WHERE (NOT age<=18) AND gender=2;
    
    
        -- 模糊查询
            -- like 
            -- % 替换1个或者多个
            -- _ 替换1个
            -- 查询姓名中 以 "小" 开始的名字
            SELECT NAME FROM students WHERE NAME="小";
            SELECT NAME FROM students WHERE NAME LIKE "小%";
    
            -- 查询姓名中 有 "小" 所有的名字
            SELECT NAME FROM students WHERE NAME LIKE "%小%";
    
            -- 查询有2个字的名字
            SELECT NAME FROM students WHERE NAME LIKE "__";
    
            -- 查询有3个字的名字
            SELECT NAME FROM students WHERE NAME LIKE "__";
    
            -- 查询至少有2个字的名字
            SELECT NAME FROM students WHERE NAME LIKE "__%";
    
    
            -- rlike 正则
            -- 查询以 周开始的姓名
            SELECT NAME FROM students WHERE NAME RLIKE "^周.*";
    
            -- 查询以 周开始、伦结尾的姓名
            SELECT NAME FROM students WHERE NAME RLIKE "^周.*伦$";
    
    
        -- 范围查询
            -- in (1, 3, 8)表示在一个非连续的范围内
            -- 查询 年龄为18、34的姓名
            SELECT NAME,age FROM students WHERE age=18 OR age=34;
            SELECT NAME,age FROM students WHERE age=18 OR age=34 OR age=12;
            SELECT NAME,age FROM students WHERE age IN (12, 18, 34);
    
    
            
            -- not in 不非连续的范围之内
            -- 年龄不是 18、34岁之间的信息
            SELECT NAME,age FROM students WHERE age NOT IN (12, 18, 34);
    
    
            -- between ... and ...表示在一个连续的范围内
            -- 查询 年龄在18到34之间的的信息
            SELECT NAME, age FROM students WHERE age BETWEEN 18 AND 34;
    
            
            -- not between ... and ...表示不在一个连续的范围内
            -- 查询 年龄不在在18到34之间的的信息
            SELECT * FROM students WHERE age NOT BETWEEN 18 AND 34;
            SELECT * FROM students WHERE NOT age BETWEEN 18 AND 34;
            -- 失败的select * from students where age not (between 18 and 34);
    
    
        -- 空判断
            -- 判空is null
            -- 查询身高为空的信息
            SELECT * FROM students WHERE height IS NULL;
            SELECT * FROM students WHERE height IS NULL;
            SELECT * FROM students WHERE height IS NULL;
    
            -- 判非空is not null
            SELECT * FROM students WHERE height IS NOT NULL;
    
    
    
    -- 排序
        -- order by 字段
        -- asc从小到大排列，即升序
        -- desc从大到小排序，即降序
    
        -- 查询年龄在18到34岁之间的男性，按照年龄从小到到排序
        SELECT * FROM students WHERE (age BETWEEN 18 AND 34) AND gender=1;
        SELECT * FROM students WHERE (age BETWEEN 18 AND 34) AND gender=1 ORDER BY age;
        SELECT * FROM students WHERE (age BETWEEN 18 AND 34) AND gender=1 ORDER BY age ASC;
    
    
        -- 查询年龄在18到34岁之间的女性，身高从高到矮排序
        SELECT * FROM students WHERE (age BETWEEN 18 AND 34) AND gender=2 ORDER BY height DESC;
        
    
        -- order by 多个字段
        -- 查询年龄在18到34岁之间的女性，身高从高到矮排序, 如果身高相同的情况下按照年龄从小到大排序
        SELECT * FROM students WHERE (age BETWEEN 18 AND 34) AND gender=2 ORDER BY height DESC,id DESC;
    
    
        -- 查询年龄在18到34岁之间的女性，身高从高到矮排序, 如果身高相同的情况下按照年龄从小到大排序,
        -- 如果年龄也相同那么按照id从大到小排序
        SELECT * FROM students WHERE (age BETWEEN 18 AND 34) AND gender=2 ORDER BY height DESC,age ASC,id DESC;
    
        
        -- 按照年龄从小到大、身高从高到矮的排序
        SELECT * FROM students ORDER BY age ASC, height DESC;
    
    
    -- 聚合函数
        -- 总数
        -- count
        -- 查询男性有多少人，女性有多少人
        SELECT * FROM students WHERE gender=1;
        SELECT COUNT(*) FROM students WHERE gender=1;
        SELECT COUNT(*) AS 男性人数 FROM students WHERE gender=1;
        SELECT COUNT(*) AS 女性人数 FROM students WHERE gender=2;
    
    
        -- 最大值
        -- max
        -- 查询最大的年龄
        SELECT age FROM students;
        SELECT MAX(age) FROM students;
    
        -- 查询女性的最高 身高
        SELECT MAX(height) FROM students WHERE gender=2;
    
        -- 最小值
        -- min
    
        
        -- 求和
        -- sum
        -- 计算所有人的年龄总和
        SELECT SUM(age) FROM students;
    
        
        -- 平均值
        -- avg
        -- 计算平均年龄
        SELECT AVG(age) FROM students;
    
    
        -- 计算平均年龄 sum(age)/count(*)
        SELECT SUM(age)/COUNT(*) FROM students;
    
    
        -- 四舍五入 round(123.23 , 1) 保留1位小数
        -- 计算所有人的平均年龄，保留2位小数
        SELECT ROUND(SUM(age)/COUNT(*), 2) FROM students;
        SELECT ROUND(SUM(age)/COUNT(*), 3) FROM students;
    
        -- 计算男性的平均身高 保留2位小数
        SELECT ROUND(AVG(height), 2) FROM students WHERE gender=1;
        -- select name, round(avg(height), 2) from students where gender=1;
    
    -- 分组
    
        -- group by
        -- 按照性别分组,查询所有的性别
        SELECT NAME FROM students GROUP BY gender;
        SELECT * FROM students GROUP BY gender;
        SELECT gender FROM students GROUP BY gender;
        -- 失败select * from students group by gender;
    
        -- 计算每种性别中的人数
        SELECT gender,COUNT(*) FROM students GROUP BY gender;
    
    
        -- 计算男性的人数
        SELECT gender,COUNT(*) FROM students WHERE gender=1 GROUP BY gender;
    
    
        -- group_concat(...)
        -- 查询同种性别中的姓名
         SELECT gender,GROUP_CONCAT(NAME) FROM students WHERE gender=1 GROUP BY gender;
         SELECT gender,GROUP_CONCAT(NAME, age, id) FROM students WHERE gender=1 GROUP BY gender;
         SELECT gender,GROUP_CONCAT(NAME, "_", age, " ", id) FROM students WHERE gender=1 GROUP BY gender;
    
        -- having
        -- 查询平均年龄超过30岁的性别，以及姓名 having avg(age) > 30
        SELECT gender, GROUP_CONCAT(NAME),AVG(age) FROM students GROUP BY gender HAVING AVG(age)>30;
        
        -- 查询每种性别中的人数多于2个的信息
        SELECT gender, GROUP_CONCAT(NAME) FROM students GROUP BY gender HAVING COUNT(*)>2;
    
    
    
    -- 分页
        -- limit start, count
    
        -- 限制查询出来的数据个数
        SELECT * FROM students WHERE gender=1 LIMIT 2;
    
        -- 查询前5个数据
        SELECT * FROM students LIMIT 0, 5;
    
        -- 查询id6-10（包含）的书序
        SELECT * FROM students LIMIT 5, 5;
    
    
        -- 每页显示2个，第1个页面
        SELECT * FROM students LIMIT 0,2;
    
        -- 每页显示2个，第2个页面
        SELECT * FROM students LIMIT 2,2;
    
        -- 每页显示2个，第3个页面
        SELECT * FROM students LIMIT 4,2;
    
        -- 每页显示2个，第4个页面
        SELECT * FROM students LIMIT 6,2; -- -----> limit (第N页-1)*每个的个数, 每页的个数;
    
        -- 每页显示2个，显示第6页的信息, 按照年龄从小到大排序
        -- 失败select * from students limit 2*(6-1),2;
        -- 失败select * from students limit 10,2 order by age asc;
        SELECT * FROM students ORDER BY age ASC LIMIT 10,2;
    
        SELECT * FROM students WHERE gender=2 ORDER BY height DESC LIMIT 0,2;
    
    
    
    -- 连接查询
        -- inner join ... on
    
        -- select ... from 表A inner join 表B;
        SELECT * FROM students INNER JOIN classes;
    
        -- 查询 有能够对应班级的学生以及班级信息
        SELECT * FROM students INNER JOIN classes ON students.cls_id=classes.id;
    
        -- 按照要求显示姓名、班级
        SELECT students.*, classes.name FROM students INNER JOIN classes ON students.cls_id=classes.id;
        SELECT students.name, classes.name FROM students INNER JOIN classes ON students.cls_id=classes.id;
    
        -- 给数据表起名字
        SELECT s.name, c.name FROM students AS s INNER JOIN classes AS c ON s.cls_id=c.id;
    
        -- 查询 有能够对应班级的学生以及班级信息，显示学生的所有信息，只显示班级名称
        SELECT s.*, c.name FROM students AS s INNER JOIN classes AS c ON s.cls_id=c.id;
        
        -- 在以上的查询中，将班级姓名显示在第1列
        SELECT c.name, s.* FROM students AS s INNER JOIN classes AS c ON s.cls_id=c.id;
    
        -- 查询 有能够对应班级的学生以及班级信息, 按照班级进行排序
        -- select c.xxx s.xxx from student as s inner join clssses as c on .... order by ....;
        SELECT c.name, s.* FROM students AS s INNER JOIN classes AS c ON s.cls_id=c.id ORDER BY c.name;
    
        -- 当时同一个班级的时候，按照学生的id进行从小到大排序
        SELECT c.name, s.* FROM students AS s INNER JOIN classes AS c ON s.cls_id=c.id ORDER BY c.name,s.id;
    
        -- left join
        -- 查询每位学生对应的班级信息
        SELECT * FROM students AS s LEFT JOIN classes AS c ON s.cls_id=c.id;
    
        -- 查询没有对应班级信息的学生
        -- select ... from xxx as s left join xxx as c on..... where .....
        -- select ... from xxx as s left join xxx as c on..... having .....
        SELECT * FROM students AS s LEFT JOIN classes AS c ON s.cls_id=c.id HAVING c.id IS NULL;
        SELECT * FROM students AS s LEFT JOIN classes AS c ON s.cls_id=c.id WHERE c.id IS NULL;
    
        -- right join   on
        -- 将数据表名字互换位置，用left join完成
    
    -- 自关联
        -- 省级联动 url:http://demo.lanrenzhijia.com/2014/city0605/
    
        -- 查询所有省份
        SELECT * FROM areas WHERE pid IS NULL;
    
        -- 查询出山东省有哪些市
        SELECT * FROM areas AS province INNER JOIN areas AS city ON city.pid=province.aid HAVING province.atitle="山东省";
        SELECT province.atitle, city.atitle FROM areas AS province INNER JOIN areas AS city ON city.pid=province.aid HAVING province.atitle="山东省";
    
        -- 查询出青岛市有哪些县城
        SELECT province.atitle, city.atitle FROM areas AS province INNER JOIN areas AS city ON city.pid=province.aid HAVING province.atitle="青岛市";
        SELECT * FROM areas WHERE pid=(SELECT aid FROM areas WHERE atitle="青岛市")
    
    
    -- 子查询
        -- 标量子查询
        -- 查询出高于平均身高的信息
    
        -- 查询最高的男生信息
        SELECT * FROM students WHERE height = 188;
        SELECT * FROM students WHERE height = (SELECT MAX(height) FROM students);
    
        -- 列级子查询
        -- 查询学生的班级号能够对应的学生信息
        -- select * from students where cls_id in (select id from classes);

转载于:https://www.cnblogs.com/huazhou695/p/9899488.html