使用子查询与 LEFT JOIN 一起选择 MAX 值

Question

我有一个获取搜索结果的查询，效果很好。

查询成功示例：

 SELECT
    individuals.individual_id,
    individuals.unique_id,
    TIMESTAMPDIFF(YEAR,individuals.day_of_birth,CURDATE()) AS age,
    individuals_dynamics.id,
    individuals_achievements.degree
  FROM
    individuals as individuals
  LEFT JOIN
    individuals_dynamics AS individuals_dynamics ON individuals.unique_id = individuals_dynamics.individual_id
  LEFT JOIN
    individuals_achievements AS individuals_achievements ON individuals.unique_id = individuals_achievements.individual_id
  WHERE
    $uuid_access_status $display_type $detailed_search_query
  ORDER BY 
    $search_sort $search_order

从现在开始，每个人在individuals_achievements 中都有不止一条记录，这是我想要获取 MAX 值（最新 id）的地方。

我尝试了许多不同的查询，但总是收到错误在非对象上调用成员函数 rowCount()。

我理解这个错误的含义，但我不知道我在哪里犯了这个错误以及一般是什么错误。

我不成功的尝试示例：

  SELECT
    individuals.individual_id,
    individuals.unique_id,
    TIMESTAMPDIFF(YEAR,individuals.day_of_birth,CURDATE()) AS age,
    individuals_dynamics.id,
    individuals_achievements.degree
  FROM
    individuals as individuals
  LEFT JOIN
    individuals_dynamics AS individuals_dynamics ON individuals.unique_id = individuals_dynamics.individual_id
  INNER JOIN
  (
       SELECT
            degree, MAX(id) AS latest_record
       FROM
            individuals_achievements
       GROUP BY
            latest_record
  ) individuals_achievements AS individuals_achievements ON individuals.unique_id = individuals_achievements.individual_id
  WHERE
    $uuid_access_status $display_type $detailed_search_query
  ORDER BY 
    $search_sort $search_order

我在这里缺少什么？请问有什么帮助吗？

Answer 1

如果在处理查询时出现错误，PDO::query 将返回 FALSE，并将 rowCount 作为成员函数应用到 FALSE 将导致您看到的错误。

检查您的查询是否有语法错误并解析PDO 会返回给您的任何错误消息。

Answer 2

INNER JOIN
  (
       SELECT
            degree, MAX(id) AS latest_record,individual_id
       FROM
            individuals_achievements
       GROUP BY
            latest_record
  ) individuals_achievements AS individuals_achievements_1 ON individuals.unique_id = individuals_achievements_1.individual_id

应该是

INNER JOIN (SELECT blah) AS something ON individuals.unique_id=something.individual_id

Answer 3

您必须在INNER JOIN 中选择individual_id，因为您在ON 子句中使用的那部分，并且您只选择了degree, MAX(id) AS latest_record 并尝试加入individual_id，这在新生成的未定义内联表

individuals_achievements AS individuals_achievements 
ON individuals.unique_id = individuals_achievements.individual_id

试试这个

SELECT
    individuals.individual_id,
    individuals.unique_id,
    TIMESTAMPDIFF(YEAR,individuals.day_of_birth,CURDATE()) AS age,
    individuals_dynamics.id,
    individuals_achievements.degree
  FROM
    individuals as individuals
  LEFT JOIN
    individuals_dynamics AS individuals_dynamics ON individuals.unique_id = individuals_dynamics.individual_id
  INNER JOIN
  (
       SELECT
            degree, MAX(id) AS latest_record,individual_id
       FROM
            individuals_achievements
       GROUP BY
            latest_record
  ) individuals_achievements AS individuals_achievements_1 ON individuals.unique_id = individuals_achievements_1.individual_id
  WHERE
    $uuid_access_status $display_type $detailed_search_query
  ORDER BY 
    $search_sort $search_order

确保您在表 individuals_achievements 中有一个列 individual_id

Answer 4

这是您的from 子句：

  FROM
    individuals as individuals
  LEFT JOIN
    individuals_dynamics AS individuals_dynamics ON individuals.unique_id = individuals_dynamics.individual_id
  INNER JOIN
  (
       SELECT
            degree, MAX(id) AS latest_record
       FROM
            individuals_achievements
       GROUP BY
            latest_record
  ) individuals_achievements AS individuals_achievements ON individuals.unique_id = individuals_achievements.individual_id;

我至少可以发现三个问题。第一个是individuals_achievements AS individuals_achievements；第二个是对不在子查询中的individuals_achievements.individual_id 的引用。第三个是group by latest_record。

  FROM individuals LEFT JOIN
       individuals_dynamics
       ON individuals.unique_id = individuals_dynamics.individual_id LEFT JOIN
       individuals_achievements
       ON individuals.unique_id = individuals_achievements.individual_id JOIN
       (SELECT ia.individual_id, MAX(ia.id) AS latest_record
        FROM individuals_achievements ia
        GROUP BY ia.individual_id
       ) iamax
       ON individuals.unique_id = iamax.individual_id and
          individuals_achievements.id = iamax.latest_record

这会添加一个额外的子查询，带有最新记录的 id。

顺便说一句，表别名与表名同名是多余的。这只会使查询变得混乱。此外，对别名使用表缩写是个好主意，例如 ia 代表 individuals_achievements。因为这个答案只关注from 子句，所以我没有做出改变。

Answer 5

首先，您并没有真正在 SQL 中使用 AS 关键字来使您的查询更具可读性，这会有所帮助。我想知道WHERE 子句的（php？）变量中有什么。

另外，您的子查询没有返回字段individual_id，但您尝试加入它。我想你可能想要这个：

SELECT
  ind.individual_id,
  ind.unique_id,
  TIMESTAMPDIFF(YEAR,ind.day_of_birth,CURDATE()) AS age,
  ind_dyn.id,
  ind_ach.degree
FROM
  individuals as ind
LEFT JOIN
  individuals_dynamics AS ind_dyn ON ind.unique_id = ind_dyn.individual_id
INNER JOIN
  individuals_achievements AS ind_ach 
     ON ind_ach.individual_id = ind.unique_id
    AND ind_ach.id =
    (
      SELECT MAX(id) FROM individuals_achievements
      WHERE individuals_achievements.individual_id = ind.unique_id
    )
WHERE
  $uuid_access_status $display_type $detailed_search_query
ORDER BY 
  $search_sort $search_order