假设我们有两个表t1并t2具有以下结构和数据：

CREATE TABLE t1 (
    id INT PRIMARY KEY
);
 
CREATE TABLE t2 (
    id INT PRIMARY KEY
);
 
INSERT INTO t1 VALUES (1),(2),(3);
INSERT INTO t2 VALUES (2),(3),(4); 

首先执行SQL

SELECT 
    t1.id, t2.id 
FROM 
    t1
    LEFT JOIN t2 ON t1.id=t2.id

得到结果

mysql> SELECT 
    ->     t1.id, t2.id 
    -> FROM 
    ->     t1
    ->     LEFT JOIN t2 ON t1.id=t2.id;
+----+------+
| id | id   |
+----+------+
|  1 | NULL |
|  2 |    2 |
|  3 |    3 |

在加上查询条件

SELECT 
    t1.id, t2.id 
FROM 
    t1
    LEFT JOIN t2 ON t1.id=t2.id
WHERE 
    t2.id IS NULL; 

参考资料

https://www.begtut.com/mysql/mysql-minus.html