Question

假设这个

用户表： [id] [mail] [pass]

发生表： [id] [uid] [日期] [内容]

投票表 [uid] [hid] [type] [datetime]

1个用户可以有0个或更多发生，1个发生可以有0个或更多的投票..

我想获得特定事件的总票数

SELECT
 H.*,
 SUM(CASE WHEN V.type='C' THEN 1 ELSE 0 END) AS upvotes,
 SUM(CASE WHEN V.type='R' THEN 1 ELSE 0 END) AS downvotes
FROM
 happens H
LEFT JOIN
 votes AS V
 ON V.hid = H.id
WHERE
 H.uid = :uid

问题是，如果没有关联的投票我从mysql得到一个空行，而不是EMPTY但是NULL

如何避免此空结果？

[编辑]

尝试一下：

基本

SELECT H.* FROM happens H LEFT JOIN votes AS V ON V.hid = H.id WHERE H.uid = '178d937'

结果 - ＆gt;空

所有其他尝试使用SUM，COUNT，HAVING等提供：

[id] [uid] [what] [latitude] [longitude] [date] [time] [hide] [upvotes] [downvotes]
NULL NULL   NULL    NULL    NULL    NULL    NULL    NULL    0   0

Answer 1

使用HAVING检查空值

SELECT
 H.*,
 SUM(CASE WHEN V.type='C' THEN 1 ELSE 0 END) AS upvotes,
 SUM(CASE WHEN V.type='R' THEN 1 ELSE 0 END) AS downvotes
FROM
 happens H
LEFT JOIN
 votes AS V
 ON V.hid = H.id
WHERE
 H.uid = :uid
HAVING upvotes IS NOT NULL

Answer 2

SELECT H.*,
    SUM(IFNULL(V.type = 'C', 0)) AS upvotes,
    SUM(IFNULL(V.type = 'R', 0)) AS downvotes

Answer 3

如果您将LEFT JOIN更改为INNER JOIN（或只是JOIN），您将只获得在投票表中至少有1条相关记录的已发生记录：

SELECT
 H.*,
 SUM(CASE WHEN V.type='C' THEN 1 ELSE 0 END) AS upvotes,
 SUM(CASE WHEN V.type='R' THEN 1 ELSE 0 END) AS downvotes
FROM
 happens H
INNER JOIN
 votes AS V
 ON V.hid = H.id
WHERE
 H.uid = :uid

查看http://www.w3schools.com/sql/sql_join.asp以获取有关不同联接类型的信息。

修改

好的，所以要把所有事情都发生，但对于那些没有投票的人来说是0（加上COALESCE）：

SELECT H.*, COALESCE(SUM(CASE WHEN V.type='C' THEN 1 ELSE 0 END),0) AS upvotes, COALESCE(SUM(CASE WHEN V.type='R' THEN 1 ELSE 0 END),0) AS downvotes FROM happens H LEFT JOIN votes AS V ON V.hid = H.id WHERE H.uid = :uid

Answer 4

SELECT
 H.*,
 COALESCE(SUM(CASE WHEN V.type='C' THEN 1 ELSE 0 END), 0) AS upvotes,
 COALESCE(SUM(CASE WHEN V.type='R' THEN 1 ELSE 0 END), 0) AS downvotes

Answer 5

如果ON条件不匹配，则Left Join将返回左侧的所有行和右侧的NULL值。使用Inner Join。

Answer 6

使用2语句解决

首先选择所有发生
第二次选择每次发生的所有选票并合并值

这是我的php happenModel类的解决方案：

public function getUserHappensWithVotes() {
$selectQuery = <<<QUERY
SELECT
 SUM(CASE WHEN V.type='C' THEN 1 ELSE 0 END) AS upvotes,
 SUM(CASE WHEN V.type='R' THEN 1 ELSE 0 END) AS downvotes
FROM
 votes V
WHERE
 V.uid = :uid
AND
 V.hid = :hid
QUERY;

    // get all happen as array of class
    $userHappens = $this->userHappens;
    // empty container
    $happenWithVotes = array();

    foreach( $userHappens AS $happen ) {
        $sql = $this->pdo->prepare( $selectQuery );
        $sql->execute( array( ':uid'=>$happen->uid, ':hid'=>$happen->id ) );

        // fetching the count of up and down votes of every happen
        $votes = $sql->fetch( PDO::FETCH_ASSOC );
        // merging happen class as array with votes
        $happen = array_merge( (array) $happen, $votes );

        // rebuild the userHappen array
        $happenWithVotes[] = (object) $happen;
    }

    if ( $this->encoder ) {
        return $this->encoder->setData( $this->parse( $happenWithVotes ) )->encode();
        } else {
        return $this->parse( $happenWithVotes );
    }
}

避免查询的null结果

6 个答案: