mysql为null替代

SELECT * FROM table WHERE column IS NULL;

ALTER TABLE table_name MODIFY column_name datatype NOT NULL;

SELECT * FROM table WHERE column IS NOT NULL;

SELECT COALESCE(column, 'default_value') AS new_column FROM table;

-- 创建一个表，其中某些字段允许NULL值
CREATE TABLE users (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    email VARCHAR(100) NULL,
    age INT NULL
);

-- 插入数据，某些字段为NULL
INSERT INTO users (id, name, email, age) VALUES (1, 'Alice', NULL, 30);

-- 查询包含NULL值的记录
SELECT * FROM users WHERE email IS NULL;

-- 修改表结构，将字段设置为NOT NULL，并指定默认值
ALTER TABLE users MODIFY email VARCHAR(100) NOT NULL DEFAULT 'default@example.com';

-- 使用COALESCE函数处理NULL值
SELECT id, name, COALESCE(email, 'default@example.com') AS email, age FROM users;