mysql的基础语句和外键查询及其语句详解(推荐)

2025-04-12 09:32 数据库作者：商bol45

思路：双指针思路可以吗，我就直接找G,如果后一个是1就cnt++，如果不是数字，用一个指针i指向G，另一个指针j移动，当不是G时停止，统计G的个数，如果是奇数个同时G的下一个是1，cnt++，如果为偶数同时G的下一个是1，就用那唯一一次机会G的国特抵消一次，cnt++，此后不再对偶数个G进行cnt++,同时把i移动到j指向G位置,就这样一直遍历字符串；

s = input()
cnt = 0
king_used = False
i = 0
while i < len(s):
    if s[i] == 'G':
        if i + 1 < len(s):
            if s[i + 1] == '1':
                cnt += 1
  php              i += 2
                continue
            elif s[i + 1] in ['2', '3']:
                i += 2
                continue
            else:
                j = i + 1
                while j < len(s) and s[j] == 'G':
                    j += 1
                g_count = j - i
                if g_count % 2 == 1:
                    cnt += 1
                elif g_count % 2 == 0 and not king_used:
                    cnt += 1
                    king_used = True
                i = j
        else:
            i += 1
    else:
        i += 1
print(cnt)
转c++

我不知道为啥我c++代码过不了，但是python嫩果

一、MySQL 基础语句

1. 数据库操作创建数据库

CREATE DATABASE school_db;

删除数据库

DROP DATABASE school_db;

选择数据库

USE school_db;

2. 表操作创建表

CREATE TABLE students (
    student_id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(50) NOT NULL,
    age INT,
    gender ENUM('男', '女'),
    enrollment_date DATE DEFAULT CURRENT_DATE
);

删除表

DROP TABLE students;

修改表结构

ALTER TABLE students ADD COLUMN email VARCHAR(100);
ALTER TABLE students MODIFY COLUMN age TINYINT;
ALTER TABLE students DROP COLUMN gender;

3. CRUD 操作

插入数据

INSERT INTO students (name, age, gender) 
VALUES ('张三', 18, '男'), ('李四', 19, '女');

查询数据

-- 基本查询
SELECT * FROM students;
-- 条件查询
SELECT name, age FROM students WHERE age > 18;
-- 排序
SELECT * FROM students ORDER BY age DESC;
-- 分组
SELECT gender, COUNT(*) FROM students GROUP BY gender;
-- 分页
SELECT * FROM students LIMIT 5 OFFSET 0;  -- 第一页，每页5条

更新数据

UPDATE students SET age = 20 WHERE name = '张三';

删除数据

DELETE FROM stupythondents WHERE student_id = 1;

二、外键与关联查询

1. 外键概念

外键(Foreign Key)用于建立表与表之间的关联关系，确保数据完整性。

2. 创建带外键的表

-- 班级表
CREATE TABLE classes (
    class_id INT AUTO_INCREMENT PRIMARY KEY,
    class_name VARCHAR(50) NOT NULL,
    teacher VARCHAR(50)
);
-- 学生表(带外键)
CREATE TABLE students (
    student_id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(50) NOT NULL,
    age INT,
    class_id INT,
    FOREIGN KEY (class_id) REFERENCES classes(class_id)
        ON DELETE SET NULL
        ON UPDATE CASCADE
);

3. 外键约束选项

ON DELETE CASCADE

: 主表记录删除时，从表相关记录自动删除

ON DELETE SET NULL

: 主表记录删除时，从表外键设为NULL

ON DELETE RESTRICT

: 拒绝删除主表记录(默认)

ON UPDATE CASCADE

: 主表主键更新时，从表外键同步更新

4. 关联查询

内连接(INNER JOIN)

SELECT s.name, s.age, c.class_name
FROM students s
INNER JOIN classes c ON s.class_id = c.class_id;

左连接(LEFT JOIN)

SELECT s.name, c.class_name
FROM students s
LEFT JOIN classes c ON s.class_id = c.class_id;

右连接(RIGHT JOIN)

SELECT s.name, c.class_name
FROM students s
RIGHT JOIN classes c ON s.class_id = c.class_id;

全连接(FULL JOIN) - MySQL通过UNION实现

SELECT s.name, c.class_name FROM students s LEFT JOIN classes c ON s.class_id = c.class_id
UNION
SELECT s.name, c.class_name FROM students s RIGHT JOIN classes c ON s.class_id = c.class_id;

三、完整示例

1. 创建数据库和表

-- 创建数据库
CREATE DATABASE school_management;
USE school_management;
-- 创建班级表
CREATE TABLE classes (
    class_id INT AUTO_INCREMENT PRIMARY KEY,
    class_name VARCHAR(50) NOT NULL,
    teacher VARCHAR(50),
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);
-- 创建学生表(带外键)
CREATE TABLE students (
    student_id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(50) NOT NULL,
    age INT CHECK (age BETWEEN 10 AND 30),
    gendjavascripter ENUM('男', '女'),
    class_id INT,
    FOREIGN KEY (class_id) REFERENCES classes(class_id)
        ON DELETE SET NULL
        ON UPDATE CASCADE,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);
-- 创建成绩表(多外键)
CREATE TABLE scores (
    score_id INT AUTO_INCREMENT PRIMARY KEY,
    student_id INT,
    subject VARCHAR(50),
    score DECIMAL(5,2),
    exam_date DATE,
    FOREIGN KEY (student_id) REFERENCES students(student_id)
        ON DELETE CASCADE
        ON UPDATE CASCADE
);

2. 插入测试数据

-- 插入班级数据
INSERT INTO classes (class_name, teacher) VALUES
('一年级一班', '王老师'),
('一年级二班', '李老师'),
('二年级一班', '张老师');
-- 插入学生数据
INSERT INTO students (name, age, gender, class_id) VALUES
('张三', 18, '男', 1),
('李四', 19, '女', 1),
('王五', 17, '男', 2),
('赵六', 20, '女', NULL);
-- 插入成绩数据
INSERT INTO scores (student_id, subject, score, exam_date) VALUES
(1, '数学', 90.5, '2023-06-15'),
(1, '语文', 85.0, '2023-06-16'),
(2, '数学', 92.0, '2023-06-15'),
(3, '英语', 88.5, '2023-06-17');

3. 复杂查询示例

查询每个班级的学生人数

SELECT c.class_name, COUNT(s.student_id) AS student_count
FROM classes c
LEFT JOIN students s ON c.class_id = s.class_id
GROUP BY c.class_id;

查询学生及其班级和成绩(多表连接)

SELECT s.name, s.age, c.class_name, sc.subject, sc.score
FROM students s
LEFT JOIN classes c ON s.class_id = c.class_编程id
LEFT JOIN scores sc ON s.student_id = sc.student_id
ORDER BY s.name, sc.subject;

查询平均分高于85的班级

SELECT c.class_name, AVG(sc.score) AS avg_score
FROM classes c
JOIN students s ON c.class_id = s.class_id
JOIN scores sc ON s.student_id = sc.student_id
GROUP BY c.class_id
HAVING avg_score > 85;

使用子查询

-- 查询没有参加任何考试的学生
SELECT name FROM students
WHERE student_id NOT IN (SELECT DISTINCT student_id FROM scores);

四、外键管理

1. 查看外键约束

SELECT 
    TABLE_NAME, COLUMN_NAME, CONSTRAINT_NAME, 
    REFERENCED_TABLE_NAME, REFERENCED_COLUMN_NAME
FROM
    INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHERE
    REFERENCED_TABLE_SCHEMA = 'school_management';

2. 删除外键约束

ALTER TABLE students DROP FOREIGN KEY students_ibfk_1;

3. 添加外键约束

ALTER TABLE students
ADD CONSTRAINT fk_class
FOREIGN KEY (class_id) REFERENCES classes(class_id)
ON DELETE SET NULL
ON UPDATE CASCADE;

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