使用双 FROM 子句删除

Question

我很惊讶这能奏效。我不确定我明白为什么。

create table #tempt(something int)

DELETE FROM #tempt -- works fine
FROM #tempt EM

我希望需要在 DELETE 中使用别名，而且我特别希望首先不需要那个 FROM。为什么第一个FROM可以存在？

DELETE EM --what I would expect to work
FROM #tempt EM

Answer 1

因为如果您查看文档 msdn:

[ WITH <common_table_expression> [ ,...n ] ]
DELETE 
    [ TOP (expression ) [ PERCENT ] ] 
    [ FROM ] 
    { { table_alias
      | <object> 
      | rowset_function_limited 
      [ WITH (table_hint_limited [ ...n ] ) ] } 
      | @table_variable
    }
    [ <OUTPUT Clause> ]
    [ FROM table_source [ ,...n ] ] 
    [ WHERE { <search_condition> 
            | { [ CURRENT OF 
                   { { [ GLOBAL ] cursor_name } 
                       | cursor_variable_name 
                   } 
                ]
              }
            } 
    ] 
    [ OPTION ( <Query Hint> [ ,...n ] ) ] 
[; ]    
<object> ::=
{ 
    [ server_name.database_name.schema_name. 
      | database_name. [ schema_name ] . 
      | schema_name.
    ]
    table_or_view_name 
}

有两个FROM！

第一个是可选的，用于标识目标 table。

第二个（不是标准的）它可用于通过连接而不是存在对目标 table 实现额外过滤。

经过进一步调查，第一个 FROM 似乎是编写 DELETE FROM 语句的标准方式。但是使用第二个 FROM 会使语句看起来不自然（如您所指出的），因此这可能是它是可选的背后的原因。

Answer 2

来自 MSDN（摘录）：

DELETE 
    [ FROM ] 
    { { table_alias
      | table_or_view_name       
    }
    [ FROM table_source [ ,...n ] ]

第一个FROM:

FROM: An optional keyword that can be used between the DELETE keyword and the target table_or_view_name

第一个 FROM 的参数：

table_alias: The alias specified in the FROM table_source clause representing the table or view from which the rows are to be deleted.

table_or view_name: The name of the table or view from which the rows are to be removed.

第二个FROM:

FROM table_source: Specifies an additional FROM clause. This Transact-SQL extension to DELETE allows specifying data from and deleting the corresponding rows from the table in the first FROM clause.

所以，简而言之：

第一个 FROM 是 optional 并且用于指定 target table 从哪个行开始被删除。您可以省略它，正如您在 OP 中指出的那样，只需使用第二个 FROM 中指定的 table_alias。
第二个FROM是一个T-SQL扩展，用来指定要从target中删除的行table.

使用双 FROM 子句删除

Delete with double FROM clause

sql-server

sql-server-2008-r2