通过联合嵌套查询和分组（SQL）

Question

我需要帮助编写查询以获取一些信息，但我在编写它时遇到困难。

[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 

查询需要返回所有的人的名单，他们在任何活动1或2（两者之间发生的最近期的活动）最近使用的工具的名称。

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（发送或安装），因为我收到错误，因为它们是不同的列名称。

Answer 1

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 

用于投影的所有选定的列必须在order by定义被定义为好。在你的例子中，两条select语句只是取tools.name as toolname，但你想按其他列排序。

Answer 2

做

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 

这个查询假设有在活动表每人只记录。如果还有更多，你需要根据总和最大条件限制你选择的结果集，或者只有你知道。

Answer 3

我的解决方案将联合放在一个子查询中，然后通过它们进行排序。您只需要在第一行，所以你需要（在MSSQL或ROWNUM = 1 Oracle或顶部1）限制条款：

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，我也去掉了最里面的加盟工具。

Answer 4

MySQL版本（可能是你可以从它那里得到更紧凑的代码）：

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 5

在子查询中使用UNION创建一个派生临时表。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。

当使用UNION时，在SELECT子句中使用的第一个列名在结果集中使用（类似于别名，尽管您也可以使用别名）。

只要确保在SELECT子句中命名该列即可。

你还需要一个LIMIT子句的子查询限制为单行：

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 

这里有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 

是一样的：

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

结果放入派生临时表中，您可以根据需要命名列，但是你使用棍子的第一个名字。

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如为person_id，people.name为PERSON_NAME，tools.name作为toolsname 从table_people人们留下加入 （ 选择 情况下INSTALLATION_DATE> DELIVERY_DATE然后act2.tool_id 其他act1.tool_id结束，因为recent_most_tool_id， 情况下，当INSTALLATION_DATE> DELIVERY_DATE然后act2.person_id 别的act1.person_id端从table_activity1 ACT1 recent_most_person_id 内上people.id = X.recent_mo加入table_activity2 ACT2 上act1.person_id = act2.person_id）X st_person_id 内部连接table_tools工具 on tools.id = X.recent_most_tool_id