sql-server - 访问未声明的变量

Question

为什么这段代码给出一行 NULL 值而不是错误消息 必须声明标量变量“@i”。。为什么 Microsoft 在 t-sql 中使用这种行为？

if 1 = 0
begin
    declare @i int = 1;
end;

select @i;

Answer 1

来自 MSDN :

The scope of a variable is the range of Transact-SQL statements that can reference the variable. The scope of a variable lasts from the point it is declared until the end of the batch or stored procedure in which it is declared.

因此在您的示例中，@i 变量的范围是定义它们的批处理或过程。

所以下一个查询:-

if 1 = 0
begin
    declare @i int = 1
end
else
begin
    declare @i int = 3
end

select @i

正在检索下一个错误:-

The variable name '@i' has already been declared. Variable names must be unique within a query batch or stored procedure.

但您的查询没有。

更新

Microsoft 表示他们不会解决此问题:-

Make it possible to declare variables that are only visible within a block.