Php 发布PDO集MySQL查询

我在尝试执行以下查询时出错！我一直在犯“一般性错误”，不知道自己做错了什么

尝试1：

$conn = pdoClass::getConnection();

$sql = "set @total := 0;
        SELECT bank_guarantee_id, 
        @total := amount + @total AS TOTAL,
        @total % 800 AS REMAINDER
            FROM bank_guarantee
            WHERE @total < 800;";
$stmt = $conn->query($sql);
$companyname = $stmt->fetchAll(PDO::FETCH_ASSOC);

尝试2：

try{
    $sql = "set @total := 0;";
    $stmt = $conn->query($sql);
    $stmt->execute();
}
catch(PDOException $e) 
{ 
    echo $e->getMessage(); 
} 
try{
    $sql = "SELECT bank_guarantee_id, @total := amount + @total AS TOTAL, @total-800 AS REMAINDER
    FROM bank_guarantee
    WHERE @total < 800;";
    $stmt = $conn->query($sql);
    $stmt->execute();
    $companyname = $stmt->fetchAll(PDO::FETCH_ASSOC);
}
catch(PDOException $e) 
{ 
    echo $e->getMessage(); 
}

任何意见都将不胜感激

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SQL FIDDLE LINK:

将连接代码放入异常处理并检查捕获输出（PDOException$e）{echo$e->getMessage（）；}它说的是execption:SQLSTATE[HY000]：General error@HarshalMahajanTry with prepare&execute statement我尝试过，但返回空数组…使用它们时错误消失了？

try{
    $sql = "set @total := 0;";
    $stmt = $conn->query($sql);
    $stmt->execute();
}
catch(PDOException $e) 
{ 
    echo $e->getMessage(); 
} 
try{
    $sql = "SELECT bank_guarantee_id, @total := amount + @total AS TOTAL, @total-800 AS REMAINDER
    FROM bank_guarantee
    WHERE @total < 800;";
    $stmt = $conn->query($sql);
    $stmt->execute();
    $companyname = $stmt->fetchAll(PDO::FETCH_ASSOC);
}
catch(PDOException $e) 
{ 
    echo $e->getMessage(); 
}

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