mysql 修改添加主键

ALTER TABLE students ADD PRIMARY KEY (student_id);

-- 删除旧的主键
ALTER TABLE students DROP PRIMARY KEY;

-- 添加新的主键
ALTER TABLE students ADD PRIMARY KEY (email);

-- 检查是否有重复值
SELECT student_id, COUNT(*) 
FROM students 
GROUP BY student_id 
HAVING COUNT(*) > 1;

-- 删除重复值
DELETE FROM students 
WHERE student_id IN (
    SELECT student_id 
    FROM (
        SELECT student_id, ROW_NUMBER() OVER (PARTITION BY student_id ORDER BY id) AS rn 
        FROM students
    ) t 
    WHERE rn > 1
);

-- 检查是否有空值
SELECT * FROM students WHERE student_id IS NULL;

-- 删除空值
DELETE FROM students WHERE student_id IS NULL;

-- 检查外键约束
SHOW CREATE TABLE students;

-- 删除外键约束（如果有）
ALTER TABLE students DROP FOREIGN KEY fk_name;

-- 修改主键
ALTER TABLE students DROP PRIMARY KEY;
ALTER TABLE students ADD PRIMARY KEY (new_primary_key);