sp_executesql 未在 SQL Server 2012 的动态 sql 查询中正确设置变量

Question

在下面的查询中，我尝试使用在 SQL Server 2012 中由 sp_executesql 执行的动态查询来设置 @productsExist 的值。问题是即使表 @tableName 存在并包含记录，动态查询执行后productsExist的值始终为null。

问题：为什么即使表存在且有记录，@productsExist 的查询也会返回 null？

DECLARE @productsExist INT;
DECLARE @countQuery NVARCHAR(MAX) = 'IF OBJECT_ID(@tableName, N''U'') IS NOT NULL 
                     begin  select top(1) @productsExist = 1  from ' + @tableName + ' end';

EXECUTE sp_executesql @countQuery, N'@tableName varchar(500),@productsExist INT',
              @tableName = @tableName,
              @productsExist = @productsExist;

select @productsExist as ProductsExist--returns always a NULL value for ProductsExist

Answer 1

您需要将@productsExist 参数声明为OUTPUT：

[ 输出 |输出]

表示参数为输出参数

DECLARE @productsExist INT
        ,@tableName SYSNAME = 'tab';

DECLARE @countQuery NVARCHAR(MAX) = 
N'IF OBJECT_ID(@tableName, N''U'') IS NOT NULL 
  begin  select top(1) @productsExist = 1  from ' + QUOTENAME(@tableName) + ' end';

EXECUTE dbo.sp_executesql 
        @countQuery,
        N'@tableName SYSNAME ,@productsExist INT OUTPUT',     -- here
        @tableName = @tableName,
        @productsExist = @productsExist OUTPUT;               -- here

SELECT @productsExist as ProductsExist;

SqlFiddleDemo

---

如果指定表中没有记录，@productsExist 将返回NULL。如果您想要 1 表示现有记录，0 表示没有记录，请使用：

DECLARE @countQuery NVARCHAR(MAX) = 
N'IF OBJECT_ID(@tableName, N''U'') IS NOT NULL 
  BEGIN
    IF EXISTS (SELECT 1 FROM '+ QUOTENAME(@tableName) + ')
       SELECT @productsExist = 1
    ELSE 
       SELECT @productsExist = 0
  END'; 

SqlFiddleDemo2

结果：

table not exists          => NULL
table exists no records   => 0
table exists >= 1 records => 1