Question

This query returns all days between date range defined with variables and result set is 31 distinct values. But, recursive CTE works in such a way that first query executed only once, and second query works on result set created before. So, it seems that there will be duplicates, but it returned distinct result set. Whether CTE apply DISTINCT clause internally or is it something else? How I get DISTINCT values?

DECLARE 
    @DateFrom DATE = '20130101' ,
    @DateTo DATE = '20130131'

WITH Days
AS ( SELECT CAST(@DateFrom AS DATETIME) AS dt
     UNION ALL
     SELECT DATEADD(dd, 1, dt)
     FROM Days s
     WHERE DATEADD(dd, 1, dt) <= CAST(@DateTo AS DATETIME)
   )

SELECT dt 
FROM Days

Answer 1

Each time a CTE recurses, it uses the rows produced by the previous recursion (or the initial set of rows if this is the first time it recurses).

So, for your query, the first run produces a single row – '20130101' converted to a datetime.

For the first recursive step, we take all the rows produced by the last recursion or initial set ('20130101') and use it to produce one new row, which contains '20130102'.

For the next recursive step, we take all rows produced by the last recursion ('20130102') and use it to produce one new row ('20130103')

For the next recursive step, we take all rows produced by the last recursion ('20130103') and use it to produce one new row ('20130104')

I don’t see anywhere where duplicates are going to arise.

Read more here:

The semantics of the recursive execution is as follows:

1. Split the CTE expression into anchor and recursive members.

2. Run the anchor member(s) creating the first invocation or base result set (T0).

3. Run the recursive member(s) with Ti as an input and Ti+1 as an output.

4. Repeat step 3 until an empty set is returned.

5. Return the result set. This is a UNION ALL of T0 to Tn.