Dynamic table in SQL Server - sql-server

Question

I have a really weird and complex requirement that I need help with. I have a table let's say Tasks that contains all the tasks for a user/system. I need to filter out the tasks per user and show it in UI. But here is the scene, the Tasks table contains a column base_table that stores the table name (real SQL Server table) on which it is based. It also stores the base table id which navigates to a particular record in the base table. Now I need to add some filter in the base table and if it satisfies the task would get retrieved.
I did try to put up a procedure which would hit a select query against base table and also check conditions.
CREATE PROCEDURE gautam_dtTable_test
(#TableName AS nvarchar(max))
AS
BEGIN try
declare #sql nvarchar(max)
declare #ret tinyint
set #ret = 0
set #sql = 'select 1 where exists (Select top 1 Id from ' + #TableName+' where some_condition)';
Exec sp_executesql #sql, N'#var tinyint out', #ret out
return #ret
end try
begin catch
return 0
end catch
I have used the procedure to input table name and hit some conditions and return a flag, 1/0 kind of thing. I also want to use try catch so that if there is any error, it would return false.
That's why I have used the procedure, not function. But seems like we can use this procedure into sql statement. Overall what I have in my mind is
Select *
from tasks
where some_conditions
and procedure/function_to_check(tasks.base_table)
Key issues with my approach
The base_table name could be invalid, so are some columns in it. So, I would love to use a try-catch.
Need to Embed it as sub-query to avoid parallel operations. But it seems tough when your procedure/function have EXEC and sp_executesql defined.
Any kind of help is appreciated. Thanks in advance!

Answer 1

The question as stated is a bit unclear so I am going to make some assumptions here. It looks like you are trying achieve the following:
First it seems you are trying to only return task in your task table where the ‘base_table’ column value references a valid SQL Server table.
Secondly if I understand the post correctly, based on the where clause condition passed to the tasks table you are trying to determine if the same columns exists in your base table.
The first part is certainly doable. However, the second part is not since it would require the query to somehow parse itself to determine what columns are being filtered on.
The following query show how you can retrieve only tasks for which there is a valid corresponding table.
SELECT *
FROM [dbo].[tasks] ts
CROSS APPLY (
SELECT [name]
FROM sys.objects WHERE object_id = OBJECT_ID('[dbo].' + QUOTENAME(ts.base_table)) AND type in (N'U')
) tb
If the field(s) you are trying to filter on is known up front (i.e. you are not trying to parse based of the tasks table) then you can modify the above query to pass the desired columns you want to check as follow:
DECLARE #columnNameToCheck NVARCHAR(50) = 'col2'
SELECT ts.*
FROM [dbo].[tasks] ts
CROSS APPLY (
SELECT [name]
FROM sys.objects WHERE object_id = OBJECT_ID('[dbo].' + QUOTENAME(ts.base_table)) AND type in (N'U')
) tb
CROSS APPLY (
SELECT [name]
FROM sys.columns WHERE object_id = OBJECT_ID('[dbo].' + QUOTENAME(ts.base_table)) AND [name] = #columnName