INNER JOIN在MySQL

Question

有我的两个表：

表名：FRIENDS

+-------+---------+---------+-------------------+ 
| id | firstName | lastName | city  | 
+-------+--------------+------------+-----------+ 
| 1  | dudi   | edri  | london | 
| 2  | maor   | azulay  | madrid | 
| 3  | batel  | azulay  | tel aviv | 
| 4  | nir   | cohen  | barcelona | 
| 5  | evia   | perez  | miami  | 
| 6  | neria  | perez  | new-york | 
| 7  | nevo   | kakoun  | roma  | 
+-------+---------+---------+-------------------+ 

表名：订单

+-------+---------+---------+-----------------+ 
| id | firstName | amount | status | 
+-------+--------------+----------+-----------+ 
| 1  | dudi   | 5684  | shipped | 
| 2  | maor   | 4896  | shipped | 
| 3  | batel  | 2496  | delay  | 
+-------+--------------+----------+-----------+ 

我的问题是： 我想没有订单的朋友。 答案：

| 4  | nir   | cohen  | barcelona | 
| 5  | evia   | perez  | miami  | 
| 6  | neria  | perez  | new-york | 
| 7  | nevo   | kakoun  | roma  | 
+-------+---------+---------+-------------------+ 

我怎么写的查询与内部联接。 谢谢。

Answer 1

您不应该在ORDERS中使用firstName作为外键。外键应引用主键。使用LEFT OUTER JOIN订单

+-------+---------+---------+-----------------+ 
| id | friendID | amount | status | 
+-------+--------------+----------+-----------+ 
| 1  | 1   | 5684  | shipped | 
| 2  | 2   | 4896  | shipped | 
| 3  | 3   | 2496  | delay  | 
+-------+--------------+----------+-----------+ 

和查询：使用ID从FRIENDS，如：

表名

SELECT f.* 
FROM FRIENDS f 
LEFT JOIN ORDERS o 
    ON f.ID = o.friendID 
WHERE o.ID IS NULL; 

LiveDemo

另一种可能性是使用correlated subquery：

SELECT f.* 
FROM FRIENDS f 
WHERE NOT EXISTS (SELECT 1 
        FROM orders o 
        WHERE o.friendID = f.ID); 

LiveDemo2

---

表：

CREATE TABLE friends(
    id  INTEGER NOT NULL PRIMARY KEY  -- you can add AUTO_INCREMENT if needed 
    ... 
); 

CREATE TABLE orders(
    id  INTEGER NOT NULL PRIMARY KEY  
    ,friendID INTEGER NOT NULL 
    ,FOREIGN KEY (friendID) REFERENCES friends(id) 
    ... 
);