Handling brackets created by QUOTENAME - sql

Question

I have a SQL script like below. I am using QUOTENAME to construct the name of the table to query. It is giving me an Invalid object name old.tb2018 error because QUOTENAME is generating the table name with brackets like so - [old.tb2018].
I have considered passing tablename as a parameter to sp_executesql, but I am using #old_table_name in other places in the code. I fear that will create a bunch of duplicate code because of all the other places that is being used in. How do I pass the tablename to #trunc_success without it generating an error?
DECLARE #YEAR NVARCHAR(4) = year(DATEADD(YEAR, - 1, getdate()))
DECLARE #old_table_name NVARCHAR(500)
DECLARE #new_table_name NVARCHAR(500)
DECLARE #trunc_success NVARCHAR (3000)
SET #old_table_name = QUOTENAME('old.tb' + #year);
SET #new_table_name = QUOTENAME('new.tb' + #year);
SET #trunc_success = '
SELECT
CASE
WHEN EXISTS
(
--This should no records
SELECT
top 1
*
FROM ' + #old_table_name + '
) THEN ''Failed''
ELSE ''Passed''
END result
UNION ALL
SELECT
CASE
WHEN EXISTS
(
--This should return no records
SELECT top 1 *
FROM ' + #new_table_name + '
) THEN ''Failed''
ELSE ''Passed''
END'
print #trunc_success
EXECUTE sp_executesql #trunc_success

Answer 1

Your table's name isn't old.tb2018 it's tb2018 and it's on the schema old. It should be:
DECLARE #old_table_name sysname; --Correct data type for object names
DECLARE #new_table_name sysname; --Correct data type for object names
...
SET #old_table_name = N'old.' + QUOTENAME(N'tb' + #year);
SET #new_table_name = N'new.' + QUOTENAME(N'tb' + #year);
...
If you want to quote the schema name too (good when not using a literal value) it would be:
QUOTENAME(N'new') + N'.' + QUOTENAME(N'tb' + #year);
Although I've used sysname here, on second though, the idea should be different. It should really be
DECLARE #old_table_name sysname; --Correct data type for object names
DECLARE #new_table_name sysname; --Correct data type for object names
...
SET #old_table_name = QUOTENAME(N'tb' + #year);
SET #new_table_name = QUOTENAME(N'tb' + #year);
...
And then in your query you'd have:
'...FROM old.' + QUOTENAME(#old_table_name) + '...'
The reason for this is that sysname is a synonym for nvarchar(128) NOT NULL, however, QUOTENAME returns an nvarchar(258); therefore quoting 2 objects, schema and table with a delimitor (.), could potentially result in an nvarchar(517) (258 + 1 + 258). It's therefore better to split the individual parts into their own parameters/variables and use QUOTENAME at the point of injection. This will avoid any unexpected truncation if you do have any overly long object names, or have (foolishly) used lots of ] characters in it's name.
Of course, the only way you could get get a 258 character value for a real object's name from QUOTENAME would be if the object's name was made up of 128 ] characters ([ for the start and ] for the end, and 128 ] characters escaped to ]]):
SELECT LEN(QUOTENAME(REPLICATE(']',128)));
If you really did have an object named that, I would honestly be questioning your sanity.