T-SQL Recipes之 Table Variables and Temporary Tables

Problem

许多时候， 我们想要Table Variables在动态SQL中执行，但现实是很骨感的。比如这个示例：

DECLARE @sql_command NVARCHAR(MAX);
DECLARE @parameter_list NVARCHAR(MAX);
DECLARE @last_names TABLE ( last_name NVARCHAR(50) );
SELECT  @sql_command = '
SELECT DISTINCT
FirstName
FROM Person.Person
WHERE LastName IN (SELECT last_name FROM @last_names)'
EXEC sp_executesql @sql_command;

有人看到这里，也许第一直觉就会问，为什么不在动态SQL中申明@last_names，其实然并卵。

这条路走不通，那我们换条路吧，比如Type Table:

CREATE TYPE last_name_table AS TABLE
(last_name NVARCHAR(50));
GO
--DROP TYPE last_name_table
DECLARE @sql_command NVARCHAR(MAX);
DECLARE @parameter_list NVARCHAR(MAX);
DECLARE @first_name_calling_sql NVARCHAR(50) = 'Edward';
DECLARE @last_names AS last_name_table;

SELECT  @sql_command = '
INSERT  INTO @last_names
        ( last_name
        )
        SELECT  LastName
        FROM    Person.Person
        WHERE   FirstName = @first_name_calling_sql;

SELECT DISTINCT
FirstName
FROM Person.Person
WHERE LastName IN (SELECT last_name FROM @last_names)
'
SELECT  @parameter_list = '@first_name_calling_sql NVARCHAR(50),
@last_names last_name_table READONLY'
EXEC sp_executesql @sql_command, @parameter_list, @first_name_calling_sql,@last_names;

如果把Insert语句拿出来，当然是可以执行的，不过有些业务就是要动态处理，摊手中……

Solution

这时候该我们的临时表登场了。先看示例：

DECLARE @sql_command NVARCHAR(MAX);
DECLARE @parameter_list NVARCHAR(MAX);
IF OBJECT_ID('tempdb..#last_names') IS NOT NULL
BEGIN
    DROP TABLE #last_names;
END;

CREATE TABLE #last_names ( last_name NVARCHAR(50) );

INSERT  INTO #last_names
        ( last_name )
        SELECT  'Thomas'
SELECT  @sql_command = '
INSERT INTO #last_names
(last_name)
SELECT ''Smith'';
SELECT DISTINCT
FirstName
FROM Person.Person
WHERE LastName IN (SELECT last_name FROM #last_names);

'
EXEC sp_executesql @sql_command;
SELECT  *
FROM    #last_names;

IF OBJECT_ID('tempdb..#last_names') IS NOT NULL
BEGIN
    DROP TABLE #last_names;
END;

从运行结果判定可以得出结论：临时表不仅可以在动态SQL中访问，还可以进行写操作。当然另外还有一种用永久表做临时存储的，这里就不提供示例了。

最后送一个小彩蛋，假设业务场景，我们传入参数是BusinessEntityId的序列，以逗号分隔。然后动态SQL执行，获取想要的person。这时候应该怎么做呢？