MySQL判断非空和非空函数

-- 创建一个测试表
CREATE TABLE customers (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  age INT
);

-- 插入示例数据
INSERT INTO customers (id, name, age) VALUES 
(1, 'John Doe', NULL),
(2, 'Jane Smith', 25),
(3, NULL, 30);

-- 判断字段是否为空
SELECT name FROM customers WHERE age IS NULL;
SELECT name FROM customers WHERE age IS NOT NULL;

-- 测试IS NULL
SELECT CASE 
  WHEN (age IS NULL) THEN 'Age is NULL'
  ELSE 'Age is not NULL'
END AS result FROM customers;

-- 测试IS NOT NULL
SELECT CASE 
  WHEN (age IS NOT NULL) THEN 'Age is not NULL'
  ELSE 'Age is NULL'
END AS result FROM customers;

-- 使用COALESCE判断字段是否为空
SELECT name FROM customers WHERE COALESCE(age, '') = '';

-- 返回第一个非NULL参数的值
SELECT COALESCE(NULL, 'Hello', 'World'); -- 输出Hello
SELECT COALESCE(NULL, NULL, NULL); -- 输出NULL

-- 使用COALESCE函数进行非空判断
SELECT CASE 
  WHEN COALESCE(age, '') = '' THEN 'Age is NULL'
  ELSE 'Age is not NULL'
END AS result FROM customers;

-- 使用IFNULL函数进行非空处理
SELECT IFNULL(name, 'N/A') FROM customers;

-- 使用IFNULL函数进行非空处理
SELECT IFNULL(name, 'N/A') FROM customers;

-- 使用NULLIF函数进行非空判断
SELECT NULLIF(age, 0) FROM customers;

-- 使用NULLIF函数进行非空判断
SELECT CASE 
  WHEN NULLIF(age, 0) IS NULL THEN 'Age is NULL'
  ELSE 'Age is not NULL'
END AS result FROM customers;