在SQL Server 2005中，有两种方法可以创建临时表:
declare @tmp table (Col1 int, Col2 int);
or
create table #tmp (Col1 int, Col2 int);
这两者之间有什么区别?关于@tmp是否仍然使用tempdb，或者是否所有事情都发生在内存中，我读过相互矛盾的意见。
在哪些情况下，一个会优于另一个?
当前回答
在SQL中，临时表存储在TempDB中，本地临时表只在当前会话中可见，在另一个会话中不可见。这可以在嵌套存储过程调用之间共享。全局临时表对所有其他会话可见，当最后一个连接引用表关闭时，它们将被销毁。例如,
Select Dept.DeptName, Dept.DeptId, COUNT(*) as TotalEmployees
into #TempEmpCount
from Tbl_EmpDetails Emp
join Tbl_Dept Dept
on Emp.DeptId = Dept.DeptId
group by DeptName, Dept.DeptId
表变量类似于tempTables，表变量也是在TempDB中创建的。表变量的作用域是声明表变量的批处理、存储过程或语句块。它们可以作为过程之间的参数传递。同样的查询可以使用Table变量by来编写
Declare @tblEmployeeCount table
(DeptName nvarchar(20),DeptId int, TotalEmployees int)
Insert @tblEmployeeCount
Select DeptName, Tbl_Dept.DeptId, COUNT(*) as TotalEmployees
from Tbl_EmpDetails
join Tbl_Dept
on Tbl_EmpDetails.DeptId = Tbl_Dept.DeptId
group by DeptName, Tbl_Dept.DeptId
其他回答
只要看看公认答案中的声明，即表变量不参与日志记录。
通常情况下，似乎不存在日志记录数量上的差异(至少对于表本身的插入/更新/删除操作而言是如此，尽管后来我发现由于额外的系统表更新，存储过程中缓存的临时对象在这方面存在一些小差异)。
在下面的操作中，我观察了@table_variable和#temp表的日志记录行为。
成功的插入 多行插入语句因违反约束而回滚的位置。 更新 删除 释放
所有操作的事务日志记录几乎相同。
表变量版本实际上有一些额外的日志条目，因为它将一个条目添加到sys. log中(然后从sys. log中删除)。Syssingleobjrefs基本表，但总的来说记录的字节更少，因为表变量的内部名称比#temp表少消耗236个字节(少118个nvarchar字符)。
要重现的完整脚本(最好在单用户模式下启动并使用sqlcmd模式的实例上运行)
:setvar tablename "@T"
:setvar tablescript "DECLARE @T TABLE"
/*
--Uncomment this section to test a #temp table
:setvar tablename "#T"
:setvar tablescript "CREATE TABLE #T"
*/
USE tempdb
GO
CHECKPOINT
DECLARE @LSN NVARCHAR(25)
SELECT @LSN = MAX([Current LSN])
FROM fn_dblog(null, null)
EXEC(N'BEGIN TRAN StartBatch
SAVE TRAN StartBatch
COMMIT
$(tablescript)
(
[4CA996AC-C7E1-48B5-B48A-E721E7A435F0] INT PRIMARY KEY DEFAULT 0,
InRowFiller char(7000) DEFAULT ''A'',
OffRowFiller varchar(8000) DEFAULT REPLICATE(''B'',8000),
LOBFiller varchar(max) DEFAULT REPLICATE(cast(''C'' as varchar(max)),10000)
)
BEGIN TRAN InsertFirstRow
SAVE TRAN InsertFirstRow
COMMIT
INSERT INTO $(tablename)
DEFAULT VALUES
BEGIN TRAN Insert9Rows
SAVE TRAN Insert9Rows
COMMIT
INSERT INTO $(tablename) ([4CA996AC-C7E1-48B5-B48A-E721E7A435F0])
SELECT TOP 9 ROW_NUMBER() OVER (ORDER BY (SELECT 0))
FROM sys.all_columns
BEGIN TRAN InsertFailure
SAVE TRAN InsertFailure
COMMIT
/*Try and Insert 10 rows, the 10th one will cause a constraint violation*/
BEGIN TRY
INSERT INTO $(tablename) ([4CA996AC-C7E1-48B5-B48A-E721E7A435F0])
SELECT TOP (10) (10 + ROW_NUMBER() OVER (ORDER BY (SELECT 0))) % 20
FROM sys.all_columns
END TRY
BEGIN CATCH
PRINT ERROR_MESSAGE()
END CATCH
BEGIN TRAN Update10Rows
SAVE TRAN Update10Rows
COMMIT
UPDATE $(tablename)
SET InRowFiller = LOWER(InRowFiller),
OffRowFiller =LOWER(OffRowFiller),
LOBFiller =LOWER(LOBFiller)
BEGIN TRAN Delete10Rows
SAVE TRAN Delete10Rows
COMMIT
DELETE FROM $(tablename)
BEGIN TRAN AfterDelete
SAVE TRAN AfterDelete
COMMIT
BEGIN TRAN EndBatch
SAVE TRAN EndBatch
COMMIT')
DECLARE @LSN_HEX NVARCHAR(25) =
CAST(CAST(CONVERT(varbinary,SUBSTRING(@LSN, 1, 8),2) AS INT) AS VARCHAR) + ':' +
CAST(CAST(CONVERT(varbinary,SUBSTRING(@LSN, 10, 8),2) AS INT) AS VARCHAR) + ':' +
CAST(CAST(CONVERT(varbinary,SUBSTRING(@LSN, 19, 4),2) AS INT) AS VARCHAR)
SELECT
[Operation],
[Context],
[AllocUnitName],
[Transaction Name],
[Description]
FROM fn_dblog(@LSN_HEX, null) AS D
WHERE [Current LSN] > @LSN
SELECT CASE
WHEN GROUPING(Operation) = 1 THEN 'Total'
ELSE Operation
END AS Operation,
Context,
AllocUnitName,
COALESCE(SUM([Log Record Length]), 0) AS [Size in Bytes],
COUNT(*) AS Cnt
FROM fn_dblog(@LSN_HEX, null) AS D
WHERE [Current LSN] > @LSN
GROUP BY GROUPING SETS((Operation, Context, AllocUnitName),())
结果
+-----------------------+--------------------+---------------------------+---------------+------+---------------+------+------------------+
| | | | @TV | #TV | |
+-----------------------+--------------------+---------------------------+---------------+------+---------------+------+------------------+
| Operation | Context | AllocUnitName | Size in Bytes | Cnt | Size in Bytes | Cnt | Difference Bytes |
+-----------------------+--------------------+---------------------------+---------------+------+---------------+------+------------------+
| LOP_ABORT_XACT | LCX_NULL | | 52 | 1 | 52 | 1 | |
| LOP_BEGIN_XACT | LCX_NULL | | 6056 | 50 | 6056 | 50 | |
| LOP_COMMIT_XACT | LCX_NULL | | 2548 | 49 | 2548 | 49 | |
| LOP_COUNT_DELTA | LCX_CLUSTERED | sys.sysallocunits.clust | 624 | 3 | 624 | 3 | |
| LOP_COUNT_DELTA | LCX_CLUSTERED | sys.sysrowsets.clust | 208 | 1 | 208 | 1 | |
| LOP_COUNT_DELTA | LCX_CLUSTERED | sys.sysrscols.clst | 832 | 4 | 832 | 4 | |
| LOP_CREATE_ALLOCCHAIN | LCX_NULL | | 120 | 3 | 120 | 3 | |
| LOP_DELETE_ROWS | LCX_INDEX_INTERIOR | Unknown Alloc Unit | 720 | 9 | 720 | 9 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysallocunits.clust | 444 | 3 | 444 | 3 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysallocunits.nc | 276 | 3 | 276 | 3 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.syscolpars.clst | 628 | 4 | 628 | 4 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.syscolpars.nc | 484 | 4 | 484 | 4 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysidxstats.clst | 176 | 1 | 176 | 1 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysidxstats.nc | 144 | 1 | 144 | 1 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysiscols.clst | 100 | 1 | 100 | 1 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysiscols.nc1 | 88 | 1 | 88 | 1 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysobjvalues.clst | 596 | 5 | 596 | 5 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysrowsets.clust | 132 | 1 | 132 | 1 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysrscols.clst | 528 | 4 | 528 | 4 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysschobjs.clst | 1040 | 6 | 1276 | 6 | 236 |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysschobjs.nc1 | 820 | 6 | 1060 | 6 | 240 |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysschobjs.nc2 | 820 | 6 | 1060 | 6 | 240 |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.sysschobjs.nc3 | 480 | 6 | 480 | 6 | |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.syssingleobjrefs.clst | 96 | 1 | | | -96 |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | sys.syssingleobjrefs.nc1 | 88 | 1 | | | -88 |
| LOP_DELETE_ROWS | LCX_MARK_AS_GHOST | Unknown Alloc Unit | 72092 | 19 | 72092 | 19 | |
| LOP_DELETE_ROWS | LCX_TEXT_MIX | Unknown Alloc Unit | 16348 | 37 | 16348 | 37 | |
| LOP_FORMAT_PAGE | LCX_HEAP | Unknown Alloc Unit | 1596 | 19 | 1596 | 19 | |
| LOP_FORMAT_PAGE | LCX_IAM | Unknown Alloc Unit | 252 | 3 | 252 | 3 | |
| LOP_FORMAT_PAGE | LCX_INDEX_INTERIOR | Unknown Alloc Unit | 84 | 1 | 84 | 1 | |
| LOP_FORMAT_PAGE | LCX_TEXT_MIX | Unknown Alloc Unit | 4788 | 57 | 4788 | 57 | |
| LOP_HOBT_DDL | LCX_NULL | | 108 | 3 | 108 | 3 | |
| LOP_HOBT_DELTA | LCX_NULL | | 9600 | 150 | 9600 | 150 | |
| LOP_INSERT_ROWS | LCX_CLUSTERED | sys.sysallocunits.clust | 456 | 3 | 456 | 3 | |
| LOP_INSERT_ROWS | LCX_CLUSTERED | sys.syscolpars.clst | 644 | 4 | 644 | 4 | |
| LOP_INSERT_ROWS | LCX_CLUSTERED | sys.sysidxstats.clst | 180 | 1 | 180 | 1 | |
| LOP_INSERT_ROWS | LCX_CLUSTERED | sys.sysiscols.clst | 104 | 1 | 104 | 1 | |
| LOP_INSERT_ROWS | LCX_CLUSTERED | sys.sysobjvalues.clst | 616 | 5 | 616 | 5 | |
| LOP_INSERT_ROWS | LCX_CLUSTERED | sys.sysrowsets.clust | 136 | 1 | 136 | 1 | |
| LOP_INSERT_ROWS | LCX_CLUSTERED | sys.sysrscols.clst | 544 | 4 | 544 | 4 | |
| LOP_INSERT_ROWS | LCX_CLUSTERED | sys.sysschobjs.clst | 1064 | 6 | 1300 | 6 | 236 |
| LOP_INSERT_ROWS | LCX_CLUSTERED | sys.syssingleobjrefs.clst | 100 | 1 | | | -100 |
| LOP_INSERT_ROWS | LCX_CLUSTERED | Unknown Alloc Unit | 135888 | 19 | 135888 | 19 | |
| LOP_INSERT_ROWS | LCX_INDEX_INTERIOR | Unknown Alloc Unit | 1596 | 19 | 1596 | 19 | |
| LOP_INSERT_ROWS | LCX_INDEX_LEAF | sys.sysallocunits.nc | 288 | 3 | 288 | 3 | |
| LOP_INSERT_ROWS | LCX_INDEX_LEAF | sys.syscolpars.nc | 500 | 4 | 500 | 4 | |
| LOP_INSERT_ROWS | LCX_INDEX_LEAF | sys.sysidxstats.nc | 148 | 1 | 148 | 1 | |
| LOP_INSERT_ROWS | LCX_INDEX_LEAF | sys.sysiscols.nc1 | 92 | 1 | 92 | 1 | |
| LOP_INSERT_ROWS | LCX_INDEX_LEAF | sys.sysschobjs.nc1 | 844 | 6 | 1084 | 6 | 240 |
| LOP_INSERT_ROWS | LCX_INDEX_LEAF | sys.sysschobjs.nc2 | 844 | 6 | 1084 | 6 | 240 |
| LOP_INSERT_ROWS | LCX_INDEX_LEAF | sys.sysschobjs.nc3 | 504 | 6 | 504 | 6 | |
| LOP_INSERT_ROWS | LCX_INDEX_LEAF | sys.syssingleobjrefs.nc1 | 92 | 1 | | | -92 |
| LOP_INSERT_ROWS | LCX_TEXT_MIX | Unknown Alloc Unit | 5112 | 71 | 5112 | 71 | |
| LOP_MARK_SAVEPOINT | LCX_NULL | | 508 | 8 | 508 | 8 | |
| LOP_MODIFY_COLUMNS | LCX_CLUSTERED | Unknown Alloc Unit | 1560 | 10 | 1560 | 10 | |
| LOP_MODIFY_HEADER | LCX_HEAP | Unknown Alloc Unit | 3780 | 45 | 3780 | 45 | |
| LOP_MODIFY_ROW | LCX_CLUSTERED | sys.syscolpars.clst | 384 | 4 | 384 | 4 | |
| LOP_MODIFY_ROW | LCX_CLUSTERED | sys.sysidxstats.clst | 100 | 1 | 100 | 1 | |
| LOP_MODIFY_ROW | LCX_CLUSTERED | sys.sysrowsets.clust | 92 | 1 | 92 | 1 | |
| LOP_MODIFY_ROW | LCX_CLUSTERED | sys.sysschobjs.clst | 1144 | 13 | 1144 | 13 | |
| LOP_MODIFY_ROW | LCX_IAM | Unknown Alloc Unit | 4224 | 48 | 4224 | 48 | |
| LOP_MODIFY_ROW | LCX_PFS | Unknown Alloc Unit | 13632 | 169 | 13632 | 169 | |
| LOP_MODIFY_ROW | LCX_TEXT_MIX | Unknown Alloc Unit | 108640 | 120 | 108640 | 120 | |
| LOP_ROOT_CHANGE | LCX_CLUSTERED | sys.sysallocunits.clust | 960 | 10 | 960 | 10 | |
| LOP_SET_BITS | LCX_GAM | Unknown Alloc Unit | 1200 | 20 | 1200 | 20 | |
| LOP_SET_BITS | LCX_IAM | Unknown Alloc Unit | 1080 | 18 | 1080 | 18 | |
| LOP_SET_BITS | LCX_SGAM | Unknown Alloc Unit | 120 | 2 | 120 | 2 | |
| LOP_SHRINK_NOOP | LCX_NULL | | | | 32 | 1 | 32 |
+-----------------------+--------------------+---------------------------+---------------+------+---------------+------+------------------+
| Total | | | 410144 | 1095 | 411232 | 1092 | 1088 |
+-----------------------+--------------------+---------------------------+---------------+------+---------------+------+------------------+
@wcm -实际上nit选择表变量不只是Ram -它可以部分存储在磁盘上。
临时表可以有索引，而表变量只能有主索引。如果速度是一个问题，表变量可以更快，但显然如果有很多记录，或者需要搜索聚集索引的临时表，那么临时表将更好。
好的背景文章
另一个主要区别是表变量没有列统计信息，而临时表有。这意味着查询优化器不知道表变量中有多少行(它猜测是1)，如果表变量实际上有大量行，这可能导致生成高度非优化的计划。
临时表(##temp/#temp)和表变量(@table)之间的区别如下:
Table variable (@table) is created in the memory. Whereas, a Temporary table (##temp/#temp) is created in the tempdb database. However, if there is a memory pressure the pages belonging to a table variable may be pushed to tempdb. Table variables cannot be involved in transactions, logging or locking. This makes @table faster then #temp. So table variable is faster then temporary table. Temporary table allows Schema modifications unlike Table variables. Temporary tables are visible in the created routine and also in the child routines. Whereas, Table variables are only visible in the created routine. Temporary tables are allowed CREATE INDEXes whereas, Table variables aren’t allowed CREATE INDEX instead they can have index by using Primary Key or Unique Constraint.
还要考虑到，您通常可以用派生表来替换这两个表，这也可能更快。但是，与所有性能调优一样，只有针对实际数据的实际测试才能告诉您特定查询的最佳方法。
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