如何使用join子句来检查行是否存在？

Question

我有两个表是这样的：

// Posts 
+----+---------+-----------------+----------+ 
| id | title | content  | amount | 
+----+---------+-----------------+----------+ 
| 1 | title1 | content1  | NULL  | 
| 2 | title2 | content2  | 1000  | 
| 3 | title3 | content3  | 5000  | 
| 4 | title4 | content4  | NULL  | 
| 5 | title5 | content5  | 2000  | 
+----+---------+-----------------+----------+ 
//        ^NULL means that question is free 


// Money_Paid 
+----+---------+---------+ 
| id | user_id | post_id | 
+----+---------+---------+ 
| 1 | 123  | 2  | 
| 2 | 345  | 5  | 
| 3 | 123  | 5  | 
+----+---------+---------+ 

所有我想要做的事：我的一些帖子是不是免费的，任何人希望看到他们要他们的成本。现在我需要一个查询来检查当前用户是否支付了该帖子的费用？ （只针对非免费后）

SELECT * FROM Posts p WHERE id = :post_id 
LEFT JOIN Money_Paid mp ON p.amount IS NOT NULL AND ... 

---

下面是几个例子：（基于当前表）

$stm->bindValue(":post_id", 2, PDO::PARAM_INT); 
$stm->bindValue(":user_id", 123, PDO::PARAM_INT); 
//=> TRUE (the user of '123' can see this post because he paid post's cost) 

$stm->bindValue(":post_id", 2, PDO::PARAM_INT); 
$stm->bindValue(":user_id", 345, PDO::PARAM_INT); 
//=> FALSE (the user of '345' cannot see this post because he didn't pay post's cost) 

$stm->bindValue(":post_id", 5, PDO::PARAM_INT); 
$stm->bindValue(":user_id", 345, PDO::PARAM_INT); 
//=> TRUE 

---

所有我想要的的查询：我想要一列包含true或false（仅适用于非免费帖子）告诉我当前用户支付了当前帖子的费用。换句话说：

有一个在Money_paid表中的一行其中user_id = :user_id =>true

没有任何行 - >false

Answer 1

不是100％肯定，我理解你想要的结果，但这里有添加一个新列，可以返回true或false取决于用户是否已支付方式：

select *, 
    case when mp.id is not null then 'true' else 'false' end paid 
from Posts p 
    left join Money_Paid mp ON mp.post_id = p.id and mp.user_id = :user_id 
where p.id = :post_id 

Answer 2

查询应该是这样的：

SELECT * 
FROM Posts p 
JOIN Money_Paid mp ON p.id = mp.post_id 
WHERE mp.user_id = :user_id 
AND p.id = :post_id 

如果用户支付了内容，这将返回该行。如果用户没有支付内容，将不会返回任何内容。

Answer 3

正如我评论你可以试试（检查加入的地方）

SELECT * FROM Posts p 
INNER JOIN Money_Paid mp ON p.amount IS NOT NULL AND 
JOIN Money_Paid mp ON 
mp.post_id = p.id AND p.amount IS NOT NULL AND mp.user_id = :user_id 
WHERE 
id = :post_id 

另外，你可以列出它们全部。

SELECT * FROM Posts p 
WHERE 
p.amount IS NULL 
UNION 
SELECT * FROM Posts p 
INNER JOIN Money_Paid mp ON 
p.amount IS NOT NULL AND 
mp.post_id = p.id AND 
mp.user_id = :user_id 

OR

SELECT * FROM Posts p 
WHERE 
p.amount IS NULL 
OR p.id in ( 
SELECT post_id FROM Money_Paid mp 
WHERE 
p.amount IS NOT NULL AND 
mp.user_id = :user_id 
) 

Answer 4

如果你想列会告诉你，如果你可以查看后或不尝试这样的事情（左连接和if语句）

select 
    p.*, 
    if(p.amount is not null, if(mp.user_id is not null, 1, 0), 1) as canView 
from Posts p 
left join Money_Paid mp on mp.post_id = p.id and mp.user_id = :user_id; 

如果列

canView是1，那么用户可以查看帖子，否则他不能。您还可以检查具体的帖子只需添加where p.id = :post_id