当列的排列顺序不同时，创建UNION ALL查询

Question

我有600个表要执行UNION ALL查询。不幸的是，每个表中列的顺序各不相同，但是它们始终具有相同的名称-例如：

表1

    Item, Cost, ID, Code, Location

表2

    Cost, Id, Code, Location, Item

表3

    Id, Code, Cost, Item, Location

是否有写联合查询的方法，无论原始表中的顺序如何，它都将与列名匹配？

Answer 1

A，不。 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 = ?;

然后您可以将列表插入代码中。

Answer 2

以下代码将以任何顺序查找具有指定列的所有表。然后，它将使用该列表组装一个查询，该查询将所有表中的数据与每个表中的列以相同的顺序进行合并。

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    where C.Column_Name in ( 'Id', 'Item', 'Code', 'Location', 'Cost' ) -- Do not quote column names here.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 );

根据您的需求，您可以运行一次以获取查询，然后将结果语句剪切并粘贴到适当的位置，例如存储过程或视图，也可以让它生成动态SQL并执行它。

[其他验证，例如不包括系统表，留给读者。