本文主要是介绍mysql的基础语句和外键查询及其语句详解(推荐)，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
《mysql的基础语句和外键查询及其语句详解(推荐)》：本文主要介绍mysql的基础语句和外键查询及其语句详解(推荐),本文给大家介绍的非常详细,对大家的学习或工作具有一定的参考借鉴价值,需要的朋...
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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