[This was originally posted at http://timstall.dotnetdevelopersjournal.com/refactoring_sql_code_with_file_includes_and_variables.htm
Everyone who maintains code knows that duplicated code is bad. While OOP languages like C# (and even xml "languages" like MSBuild) provide ways to refactor, SQL seems lacking in such features. If you run a similarity-analyzer (like Simian), you can probably see large amounts of duplicated code. Two common refactoring techniques to help with this would be:
- Dynamic include – Include one snippet within another. For example, we could have a chunk of code that is duplicated in a stored proc, and a table function.
- Variables – We can abstract any SQL code (table names, sql commands) to a variable.
Note that mere stored procs or user-defined-functions are insufficient, as they can’t handle all snippets (like declaring variables which are used in the calling function), or they have awful performance in the where clause.
We can use a technology “SqlCmds” to accomplish this. (http://msdn.microsoft.com/en-us/library/aa833281.aspx).
How to enable Sql Commands in SqlStudio:
- Single query window – “On the Query menu, click SQLCMD Mode.”
- For all windows – “To turn SQLCMD scripting on by default, on the Tools menu select Options, expand Query Execution, and SQL Server, click the General page, and then check the By default open new queries in SQLCMD Mode box.”
How to enable Sql Commands in Visual Studio
This requires the database edition of Visual Studio. Click the allow "SqlCmd" button on the tool bar.
Basic variable test
--set variable, and then use it - use the ":setvar" command
:setvar SomeTable TestCodeGen
select * from $(SomeTable)
-- environmental variables too!
select '$(COMPUTERNAME)' --returns my comp name (like 'TimStall2')
This means we could have an external script set the environmental variables (like the PrimaryDataBase), and then easily re-run those in the SQL Editor. Note that you can use the free tool SetX.exe to set environmental variables.
File Include – Basic (use the “:r” command)
--File 1 (var_def.inc):
:setvar PrimaryDB MyDatabase70
select * from $(PrimaryDB).MySchema.TestCodeGen
For example, we could have a “header” file that includes a bunch of variable definitions (like all the PrimaryDB, ReportDB, etc…), and then include it wherever needed. Or, we could include any other SQL snippet. For example, we could use this to effectively make private functions (instead of only have global functions) that are encapsulated to a single stored proc.
File Include – avoid function in a where clause
--File 1 (myProc_func1.sql):
--some reusable snippet (note how is uses the variable '@myNum")
co = '1234' or NumberInteger > @myNum
declare @myNum integer
select @myNum = 10
select * from TestCodeGen
and LastChangedBy < GetDate()
One catch to all of this is that if you have your own database installation code via ADO.Net, you need to manually parse it yourself. However, that should be easy enough to do given the strict syntax of the SqlCmds.
Note that this is substituted “at compile time”. If you run the SQL Profiler, you won’t see the “:setvar” or “:r”, but rather the content already parsed. These techniques could be used to help refactor SQL code, just like similar techniques help refactor the code in other languages.