sql - SQL查询存储过程参数

Question

我想知道原因，当我们在更新、删除或插入中创建存储过程时

update TABLE_NAME set column_name = @variable_name  

没事。

为什么我们不能传递参数或变量来选择喜欢

select @column_variable from @table_variable

我知道作为一种解决方法，您需要使用动态 SQL，但它不起作用的原因是什么？

Answer 1

If this is about SQL Server, then the reason why you cannot parametrise column names and table names with this statement,

select @column_variable from @table_variable

is because this can already be a valid statement and interpreted in a different way:

• @name would be interpreted as a reference to a scalar variable whose value is to be returned as a dataset column;

• @name would be interpreted as a table variable name, i.e. the name of a variable of a table type.

In each of these cases, the use of @name to denote a parameter holding the name of an actual column or table to select from would simply be very confusing.

On the other hand, one might think that a different syntax could have been devised for this (i.e. specifically for parametrisation of names) and yet it hasn't.

My opinion (and I have to admit that it's just my opinion) why there isn't such a syntax is that by building parametrisable names into SQL you would probably end up with less efficient query planner. If at the time of query compilation you don't know what and whence the query is trying to select, you can't really build a really efficient plan for it, can you.

Of course, building a query plan could have been delayed until the time when the name parameters have been evaluated, but the planner would have had to build a plan every time such a query is invoked, unlike now, when the query plan is stored once and then used many times.