Question

我需要帮助编写查询以获取一些信息，但我在编写时遇到了问题。

[table_People]
int id
var name

[table_Tools]
int id
var name

[table_Activity1]
int person_id
int tool_id
date delivery_date

[table_Activity2]
int person_id
int tool_id
date installation_date

查询需要返回所有人员的列表以及他们在活动1或2中使用的最新工具的名称（两者之间发生的最新活动）。

SELECT
    people.id   AS personId,
    people.name AS personName,
    (
        SELECT
            tools.name AS toolName
        FROM
            activity1
        JOIN
            tools ON tools.id=activity1.tool_id
        WHERE
            activity1.id=people.id
        UNION ALL
        SELECT
            tools.name AS toolName
        FROM
            activity2
        JOIN
            tools ON tools.id=activity2.tool_id
        WHERE
            activity2.id=people.id
        ORDER BY
            installationDate,deliveryDate
    ) AS toolName
FROM
    people
ORDER BY
    people.name
ASC

我遇到的问题是我无法按日期排序（交付或安装），因为它们是不同的列名称。

Answer 1

在子查询中使用UNION会创建派生的临时表。未选中的列不在结果集中，因此您不能对不在SELECT子句中的列进行ORDER。

使用UNION时，SELECT子句中使用的第一个列名称将用于结果集中（类似于别名，但您也可以使用别名）。

请务必在SELECT子句中命名该列。

您还需要LIMIT子句将子查询限制为单行：

SELECT
    people.id   AS personId,
    people.name AS personName,
    (
        SELECT
            tools.name AS toolName, delivery_date
        FROM
            activity1
        JOIN
            tools ON tools.id=activity1.tool_id
        WHERE
            activity1.id=people.id
        UNION ALL
        SELECT
            tools.name AS toolName, installation_date
        FROM
            activity2
        JOIN
            tools ON tools.id=activity2.tool_id
        WHERE
            activity2.id=people.id
        ORDER BY
            deliveryDate
        LIMIT 1
    ) AS toolName
FROM
    people
ORDER BY
    people.name
ASC

这是一个更简单的例子来说明这个问题：

SELECT fish FROM sea
UNION
SELECT dog FROM land
ORDER BY fish

与：

相同

SELECT fish AS animal FROM sea
UNION
SELECT dog AS animal FROM land
ORDER BY animal

将结果放入派生的临时表中，您可以根据需要为列命名，但您使用的第一个名称仍然存在。

Answer 2

我的解决方案将联合放在一个子查询中，然后由它们命令。您只需要第一行，因此您需要一个limit子句（或Oracle中的rownum = 1或MSSQL中的top 1）：

SELECT people.id   AS personId,
       people.name AS personName, 
       (SELECT toolname
        FROM ((SELECT tools.name AS toolName, delivery_date as thedate
               FROM activity1 a
               WHERE a.PersonId = people.id
              ) union all
              (SELECT tools.name AS toolName, installation_date as thedate
               FROM activity2 a
               WHERE a.PersonId = people.id
              )
             ) a join
             tools t
             on a.toolsid = t.toolsid
        order by 2 desc
        limit 1
       ) AS toolName
FROM people
ORDER BY people.name ASC

为了简化查询，我还删除了最里面的连接到工具。

Answer 3

如果没有在select。

之后指定列，则无法对列进行排序

例如：

select name from people order by name

select a1.delivery_date, t.name from activity1 a1, tools t 
order by a1.delivery_date,t.name

投影的所有选定列也必须在order by定义中定义。在您的示例中，两个select语句都只是tools.name as toolname，但您希望按其他列排序。

Answer 4

待办事项

SELECT
    people.id   AS personId,
    people.name AS personName,
    IF (
        (SELECT
            deliveryDate AS dDate
        FROM
            activity1
        WHERE
            person_id=personId) --assuming you have only one row returned here, else limit by some condition
        >
        (SELECT
            installationDate AS iDate
         FROM
            activity2
         WHERE
            person_id=personId) --assuming you have only one row returned here, else limit by some condition
         , (SELECT
                tools.name AS toolName
            FROM
                activity1
            JOIN
                tools ON tools.id=activity1.tool_id
            WHERE
                activity1.person_id=personId)
         , (SELECT
                tools.name AS toolName
            FROM
                activity2
            JOIN
                tools ON tools.id=activity2.tool_id
            WHERE
                activity2.person_id=personId)
           ) AS toolName
FROM
    people        
ORDER BY
    people.name
ASC

此查询假定活动表中每人只有记录。如果还有更多，你需要根据总和最大条件限制你的选择结果集，只有你知道。

Answer 5

MySQL版本（可能你可以从中获得更紧凑的代码）：

Select * from 
(
  Select toolName, person_id, Max(TargetDate) as MaxDate
  From
        (
            SELECT tools.name AS toolName, activity1.person_id, activity1.delivery_date as targetDate
            FROM
                activity1
            JOIN
                tools ON tools.id=activity1.tool_id
            UNION ALL
            SELECT
                tools.name AS toolName, activity2.person_id, activity2.installation_date as TargetDate
            FROM
                activity2
            JOIN
                tools ON tools.id=activity2.tool_id
        )
 Group by toolName, person_id

) preselect
    join
    (
    Select toolName, person_id, Max(TargetDate)
    From
            (
                SELECT tools.name AS toolName, activity1.person_id, activity1.delivery_date as targetDate
                FROM
                    activity1
                JOIN
                    tools ON tools.id=activity1.tool_id
                UNION ALL
                SELECT
                    tools.name AS toolName, activity2.person_id, activity2.installation_date as TargetDate
                FROM
                    activity2
                JOIN
                    tools ON tools.id=activity2.tool_id
            )) result on result.toolName = preselect.toolName and result.person_id = preselect.person_id and result.TargetDate = preselect.MaxDate

Answer 6

SELECT
    p.id             AS person_id
  , p.name           AS person_name
  , CASE WHEN COALESCE(a1.delivery_date, '1000-01-01')
              > COALESCE(a2.installation_date, '1000-01-01')
           THEN t1.name
           ELSE t2.name
    END              AS tool_name
FROM 
    People AS p
  LEFT JOIN 
    Activity1 AS a1
        ON (a1.tool_id, a1.delivery_date) =
           ( SELECT tool_id, delivery_date
             FROM Activity1 AS a
             WHERE a.person_id = p.id
             ORDER BY delivery_date DESC
             LIMIT 1
           )
  LEFT JOIN 
    Tools AS t1
        ON t1.id = a1.tool_id
  LEFT JOIN 
    Activity2 AS a2
        ON (a2.tool_id, a2.installation_date) =
           ( SELECT tool_id, installation_date
             FROM Activity2 AS a
             WHERE a.person_id = p.id
             ORDER BY installation_date DESC
             LIMIT 1
           )
  LEFT JOIN 
    Tools AS t2
        ON t2.id = a2.tool_id

Answer 7

选择people.id为person_id，people.name为person_name，tools.name为toolsname 从table_people人们离开加入（选择例如，在installation_date＆gt; delivery_date然后是act2.tool_id时 else act1.tool_id以recent_most_tool_id结尾，例如，在installation_date＆gt; delivery_date然后是act2.person_id 否则act1.person_id以recent_most_person_id结尾 from table_activity1 act1 inner join table_activity2 act2 在act1.person_id = act2.person_id）X on people.id = X.recent_most_person_id 内连接table_tools工具 on tools.id = X.recent_most_tool_id

联合的嵌套查询和分组（SQL）

7 个答案: