Question

我有一张类似这样的表：

State_id | County_id|  City_id |     Name      |    Additional_Name
-----------------------------------------------------------------
   1     |     0    |     0    |   California  |        State
   1     |     1    |     0    |   Los Angeles |        County
   1     |     1    |     1    |   Los Angeles |        City
   1     |     2    |     0    |   San Diego   |        County
   1     |     2    |     1    |   San Diego   |        City
   2     |     0    |     0    |   Texas       |        State
   2     |     1    |     0    |   Harris      |        County
   2     |     1    |     1    |   Houston     |        City

持续10,000行。我想要完成的是构建一个SELECT语句，该语句将导致：

  State       |   County     |     City
 -------------------------------------------
  California  |  Los Angeles |    Los Angeles
  California  |  San Diego   |    San Diego
  Texas       |  Harris      |    Houston

如你所见，我想选择每个城市并展示它的州和县。 state_id，county_id，city_id和Additonal_Name列应该是解决此问题的必要条件，但我不知道如何使用它们。

Answer 1

这将使用您当前的表结构获取您的数据：

SELECT t2.[Name] AS [State]
    ,t3.[Name] AS County
    ,t1.[Name] AS City
FROM MyTable t1
JOIN MyTable t2 -- get state
    ON t2.State_id = t1.State_id
       AND t2.County_id = 0
       AND t2.City_id = 0
JOIN MyTable t3 -- get county
    ON t3.County_id = t1.County_id
       AND t3.State_id = t1.State_id
       AND t3.City_id = 0
WHERE t1.City_id > 0 --(or use t1.Additional_Name = 'City'

但是真的：

您应该将数据库规范化为三个单独的表：

City
State
County

这将使您的数据和编写查询变得更简单，更易读。

表格看起来像这样：

<强>国家

ID Name
 1 California
 2 Texas

<强>县

ID Name
 1 Los Angeles
 2 San Diego
 3 Harris

<强>城市

ID StateID CountyID Name
 1 1       1        Los Angeles
 2 1       2        San Diego
 3 2       3        Houston

希望你能看到管理事物有多容易。您可以选择将StateID添加到County表格以进一步规范化。但是我的答案很简短。

使用类似查询从这些新表中选择相同的数据：

SELECT state.[Name] AS [State]
    ,county.[Name] AS County
    ,city.[Name] AS City
FROM MyCity city
JOIN MyState state ON state.ID = city.StateID -- get state
JOIN MyCounty county ON county.ID = city.CountyID -- get county

带有返回分层数据的SQL SELECT Query

1 个答案:

但是真的：