当列的顺序不同时创建 UNION ALL 查询

Question

我有 600 tables 来执行 UNION ALL 查询。不幸的是，每个 table 中的列顺序各不相同，但它们将始终具有相同的名称 - 例如：

Table 1

    Item, Cost, ID, Code, Location

Table 2

    Cost, Id, Code, Location, Item

Table 3

    Id, Code, Cost, Item, Location

有没有办法编写 Union 查询，使其与列名匹配，而不管原始顺序如何 table？

Answer 1

唉，没有。 UNION ALL 按位置而不是按名称。但是，您可以生成列：

select string_agg(column_name, ', ')
from information_schema.columns
where table_name = ? and
      table_schema = ?;

然后您可以将该列表插入您的代码。

Answer 2

以下代码将以任意顺序查找具有指定列的所有 table。然后它将使用该列表 assemble 一个查询，该查询将所有 table 的数据与每个 table.

的列以相同的顺序合并

declare @Query as NVarChar(max);
-- Quote column names here as needed:
declare @Prefix as NVarChar(64) = N'select Id, Item, Code, Location, Cost from ';
declare @Suffix as NVarChar(64) = NChar( 13 ) + NChar( 10 ) + N'union all' + NChar( 13 ) + NChar( 10 );

with
  TargetTables as (
    -- All of the table which have the specified list of columns, regardless of column order.
    select T.Table_Schema, T.Table_Name
      from Information_Schema.Tables as T inner join
        Information_Schema.Columns as C on C.Table_Schema = T.Table_Schema and C.Table_Name = T.Table_Name
    where C.Column_Name in ( 'Id', 'Item', 'Code', 'Location', 'Cost' ) -- Do not quote column names here.
    group by T.Table_Schema, T.Table_Name
    having Count( Distinct C.Column_Name ) = 5
    )
  -- Build the query by inserting   @Prefix   and   @Suffix   around each properly quoted table schema and name.
  select @Query = (
    select @Prefix + QuoteName( Table_Schema ) + '.' + QuoteName( Table_Name ) + @Suffix
      from TargetTables
      order by Table_Schema, Table_Name
      for XML path(''), type).value('.[1]', 'VarChar(max)' );

-- Clean up the tail end of the query.
select @Query = Stuff( @Query, DataLength( @Query ) / DataLength( N'-' ) - DataLength( @Suffix ) / DataLength( N'-' ) + 1, DataLength( @Suffix ) / DataLength( N'-' ), N';' );

-- Display the resulting query.
--   In SSMS use Results To Text (Ctrl-T) to see the query on multiple lines.
select @Query as Query;

-- Execute the query. NB: The parentheses are required.
execute ( @Query );

根据您的需要，您可以运行这一次获取查询并将生成的语句剪切并粘贴到某个适当的位置，例如存储过程或视图，或者您可以让它生成动态 SQL 并执行它。

额外的验证，例如排除系统tables，留给reader.

当列的顺序不同时创建 UNION ALL 查询

Create a UNION ALL query when the columns are in different order

sql

tsql

union-all