全局临时表： ##T1
局部临时表： #T1
表变量： @T1
临时表 vs 表变量：可见性,持久性,性能（具体见下表）
临时表有统计信息可以支持优化
表变量不需要重新编译
| Functionality/Object Type | Local Temp Table | Table Variable | Table Expression |
|---|---|---|---|
| Scope/Visibility | Current and inner levels | Local Batch | Derived Table/CTE: Current statement View/Inline UDF: Global |
| Physical representation in tempdb | Yes | Yes | No |
| Part of outer transaction/Affected by outer transaction rollback | Yes | No | Yes |
| Logging | To support transaction rollback | To support statement rollback | Yes |
| Locking | Yes | No | Yes |
| Statistics/recompilations/efficient plans | Yes | No | Yes |
| Table size | Any | Typically recommended for small tables | Any |