Sql 如何有条件地转换select查询中的数据

我有一个select查询，它按如下方式返回我的数据：

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PartCode  | ParentCode  |  Flag  |
---------------------------------------
ABC       |  XYZ        |  null  |
---------------------------------------
PQR       |  XYZ        |   Y    |
---------------------------------------

现在，我想根据以下标志将其转置：

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PartCode  | ParentCode  |  Flag  |
---------------------------------------
ABC       |  XYZ        |  null  |
---------------------------------------
PQR       |  XYZ        |   Y    |
---------------------------------------

-标记为Y的零件代码应标记为criticalPart，其他标记为SecondaryPart

-总是有两行我想应用这个条件

---------------------------------------------
ParentCode | CriticalPart  |  SecondaryPart |
---------------------------------------------
XYZ        |  PQR          |    ABC         |
---------------------------------------------

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我将使用两个子查询和父代码上的联接

SELECT  critical.ParentCode, 
        critical.PartCode as CriticalPart, 
        secondary.PartCode as SecondaryPart
    FROM (
        SELECT  ParentCode, PartCode 
            FROM Table 
            WHERE Flag IS NOT NULL
    ) critical
    INNER JOIN (
        SELECT ParentCode, PartCode 
            FROM Table 
            WHERE Flag IS NULL
    ) secondary ON critical.ParentCode = secondary.ParentCode

---

我将使用两个子查询和父代码上的联接

SELECT  critical.ParentCode, 
        critical.PartCode as CriticalPart, 
        secondary.PartCode as SecondaryPart
    FROM (
        SELECT  ParentCode, PartCode 
            FROM Table 
            WHERE Flag IS NOT NULL
    ) critical
    INNER JOIN (
        SELECT ParentCode, PartCode 
            FROM Table 
            WHERE Flag IS NULL
    ) secondary ON critical.ParentCode = secondary.ParentCode

---

使用下面的代码来实现结果

  select ParentCode
    , Max(case when flag = 'Y' then PartCode end) as CriticalPart
    , Max(case when flag is null then PartCode end) as SecondaryPart
    from table 
    group by ParentCode

---

使用下面的代码来实现结果

  select ParentCode
    , Max(case when flag = 'Y' then PartCode end) as CriticalPart
    , Max(case when flag is null then PartCode end) as SecondaryPart
    from table 
    group by ParentCode

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聚合函数，智能！聚合函数，智能！