PDO：使用子字符串作为标准来获取行数[重复]

Question

我可以使用以下代码从字段连接中提取正确的状态：“城市，州 - 名称县 - 邮政编码”：

$sql="SELECT trim(substring(SUBSTRING_INDEX(county_city,',',-1),1,3)) as state
          FROM
            " . $this->table_name . "
          WHERE
            admid = 45";

如果我使用以下代码尝试获取行数，最终会找到 0 行：

$sql="SELECT trim(substring(SUBSTRING_INDEX(county_city,',',-1),1,3)) as state,
            COUNT(*) as total_rows
          FROM
            " . $this->table_name . "
          WHERE
            DATE_FORMAT(refer_date,'%m') = ?
          AND
            DATE_FORMAT(refer_date, '%Y') = ?
          AND
            unit = ?
          AND
            state = ?";

            // prepare query statement
            $stmt = $this->conn->prepare( $sql );

            // bind variable values
            $stmt->bindParam(1, $refer_month);
            $stmt->bindParam(2, $refer_year);
            $stmt->bindParam(3, $unit);
            $stmt->bindParam(4, $state);

            $stmt->execute();
            $row = $stmt->fetch(PDO::FETCH_ASSOC);

            return $row['total_rows'];

知道为什么吗？

Answer 1

https://dev.mysql.com/doc/refman/8.0/en/problems-with-alias.html

标准 SQL 不允许您在 WHERE 子句中引用列别名。施加此限制是因为在执行 WHERE 代码时，可能尚未确定列值。

你需要做的：

$sql="SELECT trim(substring(SUBSTRING_INDEX(county_city,',',-1),1,3)) as state,
        COUNT(*) as total_rows
      FROM
        " . $this->table_name . "
      WHERE
        DATE_FORMAT(refer_date,'%m') = ?
      AND
        DATE_FORMAT(refer_date, '%Y') = ?
      AND
        unit = ?
      AND
        trim(substring(SUBSTRING_INDEX(county_city,',',-1),1,3)) = ?";

或者，或者类似：

"SELECT * FROM (SELECT trim(substring(SUBSTRING_INDEX(county_city,',',-1),1,3)) as state,
        COUNT(*) as total_rows
      FROM
        " . $this->table_name . "
      WHERE
        DATE_FORMAT(refer_date,'%m') = ?
      AND
        DATE_FORMAT(refer_date, '%Y') = ?
      AND
        unit = ?
   ) as q1 WHERE q1.state = ?";

我的子选择语法可能有点偏离，但你明白了。