使用可以基于参数设置的变量的 SELECT 案例

Question

我想从一个表中选择一个特定的值，同时使用另一个数据库中的信息，该信息是基于当前数据库的值设置的。

所以选择案例来查找操作员代码并设置数据库路径..然后使用相同的路径并整理结果。

DECLARE @DB varchar (1000)

CASE 
WHEN @Operator= 1 THEN SET @DB = '{SERVERNAME\ENTITY\DBNAME}'
WHEN @Operator= 2 THEN SET @DB = '{SERVERNAME2\ENTITY2\DBNAME2}'
WHEN @Operator= 3 THEN SET @DB = '{SERVERNAME3\ENTITY3\DBNAME3}'

Select transItem_item collate SQL_Latin1General_CI_AS
FROM Group_Transactions
INNER JOIN @DB.Table_Trans
ON (transItem.item_id collate SQL_Latin1General_CI-AS = Table_Trans.item_id)
Where ---Condition

Answer 1

控制流方法（可能是最有效的）：

IF @Operator = 1
  BEGIN
    SELECT stuff
    FROM   Group_Transactions
     INNER
      JOIN "Server1\Instance1".Database1.Schema.Table_Trans
        ON Group_Transactions... = Table_Trans...
    WHERE  things...
    ;
  END
ELSE IF @Operator = 2
  BEGIN
    SELECT stuff
    FROM   Group_Transactions
     INNER
      JOIN "Server2\Instance2".Database2.Schema.Table_Trans
        ON Group_Transactions... = Table_Trans...
    WHERE  things...
    ;
  END
ELSE IF @Operator = 3
  BEGIN
    SELECT stuff
    FROM   Group_Transactions
     INNER
      JOIN "Server3\Instance3".Database3.Schema.Table_Trans
        ON Group_Transactions... = Table_Trans...
    WHERE  things...
    ;
  END
;

单[条件]查询方式：

SELECT Group_Transactions.stuff
     , trans1.other_thing As other_thing1
     , trans2.other_thing As other_thing2
     , trans3.other_thing As other_thing3
     , Coalesce(trans1.other_thing, trans2.other_thing, trans3.other_thing) As other_thing
FROM   Group_Transactions
 LEFT
  JOIN "Server1\Instance1".Database1.Schema.Table_Trans As trans1
    ON trans1... = Group_Transactions...
   AND trans1.things...
   AND @Operator = 1
 LEFT
  JOIN "Server2\Instance2".Database2.Schema.Table_Trans As trans2
    ON trans2... = Group_Transactions...
   AND trans2.things...
   AND @Operator = 2
 LEFT
  JOIN "Server3\Instance3".Database3.Schema.Table_Trans As trans3
    ON trans3... = Group_Transactions...
   AND trans3.things...
   AND @Operator = 3
;

Answer 2

如果这是 TSQL（我从您的排序规则名称中猜测），那么您最好尝试使用 OPENQUERY 来针对另一台数据库服务器运行您的联接。如果您在同一台服务器上查询数据库，您可以将查询构建为参数，然后使用EXEC 运行它。

Answer 3

Gvee 的控制流方法可能很冗长，但它会起作用。如果你有一堆数据库，你可能想创建一个像我的 @tbl_Databases 一样的查找表。这是一个动态 SQL 解决方案：

DECLARE @Operator INT = 1,
        @DB VARCHAR(1000);

DECLARE @tbl_Databases TABLE (ID INT IDENTITY(1,1),DB VARCHAR(1000))
INSERT INTO @tbl_Databases(DB)
VALUES ('{SERVERNAME\ENTITY\DBNAME}'),('{SERVERNAME2\ENTITY2\DBNAME2}'),('{SERVERNAME3\ENTITY3\DBNAME3}');

SELECT @DB = DB
FROM @tbl_Databases
WHERE ID = @Operator

SELECT @DB

SELECT
(
'SELECT transItem_item COLLATE SQL_Latin1General_CI_AS
FROM Group_Transactions
INNER JOIN ' + @DB + '.dbo.Table_Trans
ON (transItem.item_id collate SQL_Latin1General_CI-AS = Table_Trans.item_id)
Where 1 = 1'
)