为什么正常 SELECT 和 SELECT INTO 的“受影响的行数”不同？答案

【问题标题】：Why does the number of 'Rows affected' differ between a normal SELECT and a SELECT INTO?为什么正常 SELECT 和 SELECT INTO 的“受影响的行数”不同？
【发布时间】：2021-01-26 14:13:03
【问题描述】：

我的查询如下所示：

SELECT  
    OBJECT_SCHEMA_NAME(T.[object_id],DB_ID()) AS [Schema]
    ,T.[name] AS [Table], AC.[name] AS [Column]
    ,TY.[name] AS [Data Type]
    ,sep.[value] AS [Description]
    ,AC.[max_length] AS [Maximum Field Length]
    ,create_date AS [Date Created]
    ,modify_date AS [Date Modified]
    ,CASE WHEN is_identity = 1 THEN 'Yes' ELSE 'No' END AS [Primary Key]
    ,CASE WHEN AC.[is_nullable] = 1 THEN 'Yes' ELSE 'No' END AS [Allows Nulls]  
--  INTO 
--  tmp_desc
FROM sys.[tables] AS T   
    INNER JOIN sys.[all_columns] AC ON T.[object_id] = AC.[object_id]  
    INNER JOIN sys.[types] TY ON AC.[system_type_id] = TY.[system_type_id] AND AC.[user_type_id] = TY.[user_type_id]   
    left join sys.extended_properties sep on t.object_id = sep.major_id
                                         and ac.column_id = sep.minor_id
                                         and sep.name = 'MS_Description'
WHERE
    T.[is_ms_shipped] = 0

...列出我的表格和字段中的一些属性。

所以当我执行以下步骤时，会发生奇怪的事情：

我运行 SELECT，但尚未创建临时表（我运行了一个删除表以防万一）：1018 条记录返回
我运行 SELECT INTO（在上面的代码中取消注释）：1028 条记录受影响
我再次注释掉 INTO，暂时不要删除临时表，并且只选择：1028 条记录返回
我运行 'select * from tmp_desc'：1028 条记录返回
我删除了临时表并再次运行 SELECT ONLY：1018 条记录返回

如果 tmp_desc 不是来自查询的一部分，那么创建临时表的任何想法都会影响从查询返回的行？

【问题讨论】：

标签： sql sql-server tsql ssms

【解决方案1】：

tmp_desc 中存储的数据包括自身的详细信息；显然，当您只运行SELECT（没有INTO）时，它不会。因此，在运行纯 SELECT 后，INTO 会增加 10 行，因为表 tmp_desc 中有 10 列。

db<>fiddle

如果您希望查询“相同”，请使用 INTO 从查询中排除 tmp_desc：

SELECT S.[name] AS [Schema],
       T.[name] AS [Table],
       AC.[name] AS [Column],
       TY.[name] AS [Data Type],
       sep.[value] AS [Description],
       AC.[max_length] AS [Maximum Field Length],
       create_date AS [Date Created],
       modify_date AS [Date Modified],
       CASE WHEN is_identity = 1 THEN 'Yes' ELSE 'No' END AS [Primary Key],
       CASE WHEN AC.[is_nullable] = 1 THEN 'Yes' ELSE 'No' END AS [Allows Nulls]
INTO tmp_desc
FROM sys.schemas S
     INNER JOIN sys.[tables] T ON S.schema_id = T.schema_id
     INNER JOIN sys.[all_columns] AC ON T.[object_id] = AC.[object_id]
     INNER JOIN sys.[types] TY ON AC.[system_type_id] = TY.[system_type_id]
                              AND AC.[user_type_id] = TY.[user_type_id]
     LEFT JOIN sys.extended_properties sep ON T.object_id = sep.major_id
                                          AND AC.column_id = sep.minor_id
                                          AND sep.name = 'MS_Description'
WHERE T.[is_ms_shipped] = 0
  AND NOT(S.[name] = N'dbo' AND T.[name] = 'tmp_desc');

【讨论】：