SELECT查詢IF條件

Question

我的博客頁面中存在一個SELECT查詢問題。

當評論狀態= 1時，我想評論每個博客的評論數。

我申請以下查詢..

SELECT CONCAT(u.first_name," ",u.last_name) name,r.*,IF(c.status=1,COUNT(c.id)) as comment 
FROM users u RIGHT JOIN resources r ON u.id = r.created_by 
LEFT JOIN comments c ON r.id = c.resource_id 
WHERE r.type = 1 
AND r.status=1 
GROUP BY r.id 
ORDER BY r.created_date DESC 
LIMIT 0,5 

，但它給SYNTEX錯誤..

Error: SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ') as comment FROM users u RIGHT JOIN resources r ON u.id = r.created_by LEFT JOI' at line 1 

請告訴我，我錯了。

由於

Answer 1

如果語句包含三個表達式。首先，表達式，其次是條件爲真時返回的值，如果條件爲假，則返回第三個，因此您缺少第三個表達式。 嘗試下面的代碼

SELECT CONCAT(u.first_name," ",u.last_name) name,r.*,IF(c.status=1,COUNT(c.id), 0) as comment 
FROM users u RIGHT JOIN resources r ON u.id = r.created_by 
LEFT JOIN comments c ON r.id = c.resource_id 
WHERE r.type = 1 
AND r.status=1 
GROUP BY r.id 
ORDER BY r.created_date DESC 
LIMIT 0,5 

Answer 2

Select concat(u.first_name," ",u.last_name) name,r.*,  
case when c.status=1 then COUNT(c.id) end as comment 
FROM users u RIGHT JOIN resources r ON u.id = r.created_by  
LEFT JOIN comments c ON r.id = c.resource_id  
WHERE r.type = 1  
AND r.status=1  
GROUP BY r.id  
ORDER BY r.created_date DESC  
LIMIT 0,5  

https://www.w3schools.com/sql/func_mysql_case.asp

Answer 3

if函數需要3個參數被傳遞給它。 IF(expression ,expr_true, expr_false)是應該如何使用的。

看一看https://www.w3resource.com/mysql/control-flow-functions/if-function.php