Question

我的博客页面中的SELECT查询存在一个问题。

当评论状态= 1时，我想要每个博客的评论数。

我申请以下查询..

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

但它给出了SYNTEX ERROR ..

 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

请告诉我错在哪里。

由于

Answer 1

if语句包含三个表达式。首先是表达式，第二个是条件为真时返回的值，第三个条件是假，因此你缺少第三个表达式。请尝试以下代码

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函数需要传递3个参数。 IF(expression ,expr_true, expr_false)是应该如何使用的。

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

SELECT查询IF CONDITION

3 个答案: