In SSMS copied string has different behaviour to original string - tsql

Question

I am attempting to semi automate creation of my databases
As part of this I want to add extended properties of column descriptions.
When I try to run sp_sqlexec in my script ( or even just Exec(#mystring) I get an error. However, if while debugging, I copy the dynamic sql string from the watch window and then run sp_sqlexec on the copied string in a seperate window I get no errors and the extended properties are added correctly.
The following script demonstrates the problem:
--Create a table to apply column descriptions to
Create table dbo.table1 (id int, name nvarchar(20));
--Create the table that contains our column descriptions
Create table dbo.column_descs_table (schemaname nvarchar(20), tablename nvarchar(20), columnname nvarchar(20), column_description nvarchar(20))
Insert into column_descs_table (schemaname, tablename, columnname, column_description)
values ('dbo', 'table1', 'id', 'the id column'), ('dbo' , 'table1', 'name', 'the name column');
--Dynamic sql string varaible to hold the commands
Declare #dyn_sql nvarchar(max);
Set #dyn_sql = 'N'''; --Set to opening quote
--now create the string containing commands to add column escriptions
SELECT #dyn_sql = #dyn_sql + N' EXEC sp_addextendedproperty ''''Col Desc'''', ''''' + column_description + N''''', ''''SCHEMA'''', ' + schemaname + N', ''''TABLE'''', ' + tablename + N', ''''COLUMN'''', ' + columnname + N' ;'
FROM dbo.column_descs_table
Set #dyn_sql = #dyn_sql + ''''; --add the closing quote
Print #dyn_sql --If I copy the contents of #dyn_sql here and run seperately it works OK
Exec sp_sqlexec #dyn_sql -- this line causes error
The error I get is
Msg 102, Level 15, State 1, Line 1
Incorrect syntax near ' EXEC sp_addextendedproperty 'Col Desc', 'the id column', 'SCHEMA', dbo, 'TABLE', table1, 'COLUMN', id ; EXEC sp_addextendedprope'.
Yet if I step through the code and copy the contents of #dyn_sql then paste this as follows:
Exec sp_sqlexec N' EXEC sp_addextendedproperty ''Col Desc'', ''the id column'', ''SCHEMA'', dbo, ''TABLE'', table1, ''COLUMN'', id ; EXEC sp_addextendedproperty ''Col Desc'', ''the name column'', ''SCHEMA'', dbo, ''TABLE'', table1, ''COLUMN'', name ;'
Then the above works fine and the column descriptions are added as expected.
Any help on this specific copying problem is much appreciated. I do understand the security issues with dynamic sql ( this script will be removed from the database once my setup is complete)
Thanks in advance
Jude

Answer 1

It looks like it's because your leading N is included within the string to execute; you don't need it at all. In other words, you are ending up with something like this:
exec sp_execsql 'N'' exec sp_addextendedproperty /* etc. */ '''
But it should be like this:
exec sp_execsql N'exec sp_addextendedproperty /* etc. */ '
But why are you even using dynamic SQL here? All values passed to sp_addextendedproperty can be passed as parameters so there is no obvious reason to use dynamic SQL, unless you've simplified something for the question.
Finally, you should be using sp_executesql, it's the preferred way to execute dynamic SQL.

Answer 2

I believe that I have resolved my string copying problem. SQL was detecting double quotes in by concatenated string as empty strings and removing them. A simple example showing the problem and my solution is below:
--Example to Select 'simple string' and then 'concat string' into results sets
DECLARE
#Simplestring nvarchar( max ) = '' ,
#Concatstring nvarchar( max ) = '' ,
#Stringvar nvarchar( 10 ) = 'string';
--The double quotes in next line are the quotemark we want plus a quotemark acting
--as an escape character
--#simplestring will be set to 'Select 'simple string' '
SET #Simplestring = 'Select ''simple string'' ';
--Similarly we need #concatstring to be set to 'Select 'Concat string' '
SET #Concatstring = 'Select '' concat' + #Stringvar + ''; -- this wont work the last
--double quote will be removed
--Add a character that cannot appear in any othe part of the concatenation - I've used *
SET #Concatstring = 'Select '' Concat ' + #Stringvar + '*';
--Now replace the * with a quote mark
SET #Concatstring = REPLACE( #Concatstring , '*' , '''' ); -- This will work
EXEC sp_executesql #Simplestring;
EXEC sp_executesql #Concatstring;
There may be a simpler solution than mine.
Many thanks for the advice on using sp_executesql. I am working on changing my code to use this ( with variables passed in as parametrs).
Jude