if else within CTE? - sql-server

Question

I want to execute select statement within CTE based on a codition. something like below
;with CTE_AorB
(
if(condition)
select * from table_A
else
select * from table_B
),
CTE_C as
(
select * from CTE_AorB // processing is removed
)
But i get error on this. Is it possible to have if else within CTEs? If not is there a work around Or a better approach.
Thanks.

Answer 1

try:
;with CTE_AorB
(
select * from table_A WHERE (condition true)
union all
select * from table_B WHERE NOT (condition true)
),
CTE_C as
(
select * from CTE_AorB // processing is removed
)
the key with a dynamic search condition is to make sure an index is used, Here is a very comprehensive article on how to handle this topic:
Dynamic Search Conditions in T-SQL by Erland Sommarskog
it covers all the issues and methods of trying to write queries with multiple optional search conditions. This main thing you need to be concerned with is not the duplication of code, but the use of an index. If your query fails to use an index, it will preform poorly. There are several techniques that can be used, which may or may not allow an index to be used.
here is the table of contents:
Introduction
The Case Study: Searching Orders
The Northgale Database
Dynamic SQL
Introduction
Using sp_executesql
Using the CLR
Using EXEC()
When Caching Is Not Really What You Want
Static SQL
Introduction
x = #x OR #x IS NULL
Using IF statements
Umachandar's Bag of Tricks
Using Temp Tables
x = #x AND #x IS NOT NULL
Handling Complex Conditions
Hybrid Solutions – Using both Static and Dynamic SQL
Using Views
Using Inline Table Functions
Conclusion
Feedback and Acknowledgements
Revision History
if you are on the proper version of SQL Server 2008, there is an additional technique that can be used, see: Dynamic Search Conditions in T-SQL Version for SQL 2008 (SP1 CU5 and later)
If you are on that proper release of SQL Server 2008, you can just add OPTION (RECOMPILE) to the query and the local variable's value at run time is used for the optimizations.
Consider this, OPTION (RECOMPILE) will take this code (where no index can be used with this mess of ORs):
WHERE
(#search1 IS NULL or Column1=#Search1)
AND (#search2 IS NULL or Column2=#Search2)
AND (#search3 IS NULL or Column3=#Search3)
and optimize it at run time to be (provided that only #Search2 was passed in with a value):
WHERE
Column2=#Search2
and an index can be used (if you have one defined on Column2)

Answer 2

Never ever try to put conditions like IF inside a single query statements. Even if you do manage to pull it off, this is the one sure-shot way to kill performance. Remember, a single statement means a single plan, and the plan will have to be generated in a way to satisfy both cases, when condition is true and when condition is false, at once. This usually result in the worse possible plan, since the 'condition' usually creates mutually exclusive access path for the plan and the union of the two results in always end-to-end table scan.
Your best approach, for this and many many other reasons, is to pull the IF outside of the statement:
if(condition true)
select * from table_A
else
select * from table_B

Answer 3

I think the IF ELSE stuff might have poor caching if your branch condition flips. Maybe someone more knowledgeable can comment.
Another way would be to UNION ALL with the WHERE clauses as suggested by others. The UNION ALL would replace the IF ELSE

Answer 4

If you are using a parameter, then you only need one statement.
#ID (Some parameter)
;with CTE
(
select * from table_A WHERE id = #ID
union all
select * from table_B WHERE (id = #ID and condition)
)