-- 使用 IS NULL
SELECT * FROM table_name WHERE column_name IS NULL;
 
-- 使用 IS NOT NULL
SELECT * FROM table_name WHERE column_name IS NOT NULL;

-- 等于空字符串
SELECT * FROM table_name WHERE column_name = '';
 
-- 不等于空字符串
SELECT * FROM table_name WHERE column_name != '';

-- 判斷為 NULL 或空字符串
SELECT * FROM table_name 
WHERE column_name IS NULL OR column_name = '';
 
-- 判斷不為 NULL 且不為空字符串
SELECT * FROM table_name 
WHERE column_name IS NOT NULL AND column_name != '';

-- 判斷為 NULL 或空字符串
SELECT * FROM table_name 
WHERE COALESCE(column_name, '') = '';
 
-- 判斷不為 NULL 且不為空字符串
SELECT * FROM table_name 
WHERE COALESCE(column_name, '') != '';

-- 判斷為 NULL 或空字符串
SELECT * FROM table_name 
WHERE NULLIF(column_name, '') IS NULL;
 
-- 判斷不為 NULL 且不為空字符串
SELECT * FROM table_name 
WHERE NULLIF(column_name, '') IS NOT NULL;

-- 創(chuàng)建示例表
CREATE TABLE users (
    id SERIAL PRIMARY KEY,
    name VARCHAR(100),
    email VARCHAR(100)
);
 
-- 插入測(cè)試數(shù)據(jù)
INSERT INTO users (name, email) VALUES 
('張三', 'zhangsan@example.com'),
('', 'lisi@example.com'),
(NULL, 'wangwu@example.com'),
('趙六', '');
 
-- 查詢 name 字段為 NULL 或空字符串的記錄
SELECT * FROM users 
WHERE name IS NULL OR name = '';
 
-- 查詢 email 字段不為 NULL 且不為空字符串的記錄
SELECT * FROM users 
WHERE email IS NOT NULL AND email != '';
 
-- 使用 COALESCE 查詢有效名稱
SELECT * FROM users 
WHERE COALESCE(name, '') != '';

-- 排除 NULL、空字符串和只包含空格的字符串
SELECT * FROM table_name 
WHERE COALESCE(TRIM(column_name), '') != '';
 
-- 或者使用正則表達(dá)式
SELECT * FROM table_name 
WHERE column_name IS NULL OR column_name ~ '^[[:space:]]*$';