How to prevent CTE execution until reached OR NOT EXISTS - sql-server

Question

I have an SQL Server query that uses some CTEs (Common Table Expressions). It has two tables that it selects the data from. The two tables are identical in structure, but not necessary data. The query will first select from table_a and if no rows are fetched, it then selects from table_b. The query is something like this:
;WITH cte_a AS (
...
), cte_b AS (
...
)
SELECT *
FROM table_a
INNER JOIN cte_a ON condition_a
OR NOT EXISTS (
SELECT *
FROM table_b
INNER JOIN cte_b ON condition_b
)
The current problem that I have is that cte_b will always be executed regardless of whether table_a returns any rows. This is not very ideal for me; I would like to have cte_b execute if and only if the subquery for table_a returns no rows.
I tried moving the cte_b to be just before the subquery for table_b as
;WITH cte_a AS (
...
)
SELECT *
FROM table_a
INNER JOIN cte_a ON condition_a
OR NOT EXISTS (
;WITH cte_b AS (
...
)
SELECT *
FROM table_b
INNER JOIN cte_b ON condition_b
)
However, the IDE complains. I think that this wasn't the way CTEs are supposed to be used.

Answer 1

Create temporary table for storing the data - split the query to two separate INSERT statements, where the second is executed only in no data is populated after first query is completed. Something like this:
CREATE TABLE #TEmp
(
);
;WITH cte_a AS (
...
)
INSERT INTO #TEmp
SELECT *
FROM table_a
INNER JOIN cte_a ON condition_a
IF NOT EXISTS(SELECT 1 FROM #TEmp)
BEGIN;
INSERT INTO #TEmp
SELECT *
FROM table_a
WHERE NOT EXISTS (
SELECT *
FROM table_b
INNER JOIN cte_b ON condition_b
)

Answer 2

This is the nature of CTE's.
The best thing would be to not use a CTE and use a join, but I assume you can't.
I'd suggest you try using a sub-query so the engine can process it all at once.

Answer 3

I would phrase this using a union, and then excluding the second half of the union on cte_b should the first CTE have any results.
;WITH cte_a AS (
...
),
cte_b AS (
...
)
SELECT *
FROM table_a
INNER JOIN cte_a ON condition_a
UNION ALL
SELECT *
FROM table_b
INNER JOIN cte_b ON condition_b
WHERE NOT EXISTS (
SELECT 1
FROM table_a
INNER JOIN cte_a ON condition_a
)

Answer 4

;WITH cte1 AS
(
....
),
cte2 AS
(
...
)
SELECT *
FROM table_a
INNER JOIN cte_a ON condition_a ---Choose Your Condition...
UNION ALL
SELECT *
FROM table_b
INNER JOIN cte_b ON condition_b ---Choose Your Condtion....
WHERE NOT EXISTS (SELECT * FROM cte2)
Note:-
A CTE must be followed by a single SELECT, INSERT, UPDATE, or DELETE statement that references some or all the CTE columns.