Question

I want to display 12 months name from sql server. i though to accomplish insert month name into temp table and then fire select statement on that table. so i had to write 12 insert table to insert 12 months name. so i search google to find better solution and i got it.

here is the sql statement

WITH R(N) AS
(
    SELECT 0
    UNION ALL
    SELECT N+1 
    FROM R
    WHERE N < 12
)

SELECT LEFT(DATENAME(MONTH,DATEADD(MONTH,-N,GETDATE())),3) AS [month]
FROM R

the above script works perfectly but my problem is i just do not understand how it works. i never work with CTE.

so tell me what is the meaning of WITH R(N) AS

and see this sql

SELECT LEFT(DATENAME(MONTH,DATEADD(MONTH,-N,GETDATE())),3) AS [month] FROM R
when above sql execute how it is getting value for -N ??
because here i have not set anything for -N ??

so please anyone help me to understand how whole thing works. thaks

My Second Phase of Question

just have look a and tell me

;WITH months(MonthNumber) AS
(
    SELECT 0
    UNION ALL
    SELECT MonthNumber+1 
    FROM months
    WHERE MonthNumber < 12
)

onething is not clear to me that why only first time the below part execute

SELECT 0
UNION ALL
SELECT MonthNumber+1 
FROM months
WHERE MonthNumber < 12

and from the 2nd time only this below part execute

SELECT MonthNumber+1 
FROM months
WHERE MonthNumber < 12

whenever we write two sql statement using Union and execute then always it return data from two sql state but specially in this case from the 2nd time why only this below part
execute

SELECT MonthNumber+1 
FROM months
WHERE MonthNumber < 12

basically i not familiar with CTE with recursion technique and that is why things is not getting clear to me. if possible please discuss how CTE recursion works.

DECLARE @TotaDays SMALLINT
DECLARE @Month VARCHAR(15)
DECLARE @Year SMALLINT
DECLARE @date DATETIME 
SET @Month = 'January'
SET @Year = 2015
SET @date = '01 ' + @Month + ' ' + CONVERT(VARCHAR(4),@Year)
SET @TotaDays = 0
SELECT @TotaDays = DATEDIFF(DAY, @date, DATEADD(MONTH, 1, @date))

;WITH months(MonthNumber) AS
(
    SELECT 1
    UNION ALL
    SELECT MonthNumber+1 
    FROM months
    WHERE MonthNumber < @TotaDays
)

select * from months;

Answer 1

The With R(N) is a Common Table Expression. From MDSN:

A common table expression (CTE) can be thought of as a temporary
result set that is defined within the execution scope of a single
SELECT, INSERT, UPDATE, DELETE, or CREATE VIEW statement. A CTE is
similar to a derived table in that it is not stored as an object and
lasts only for the duration of the query. Unlike a derived table, a
CTE can be self-referencing and can be referenced multiple times in
the same query.

The R is the name of the result set (or table) that you are generating. And the N is the month number.

This CTE in particular is a Recursive Common Table Expression. From MSDN:

A common table expression (CTE) provides the significant advantage of
being able to reference itself, thereby creating a recursive CTE. A
recursive CTE is one in which an initial CTE is repeatedly executed to
return subsets of data until the complete result set is obtained.

When using CTE my suggestion would to be more descriptive with the names. So for your example you could use the following:

;WITH months(MonthNumber) AS
(
    SELECT 0
    UNION ALL
    SELECT MonthNumber+1 
    FROM months
    WHERE MonthNumber < 12
)
select *
from months;

In my version the months is the name of the result set that you are producing and the monthnumber is the value. This produces a list of the Month Numbers from 0-12 (See Demo).

Result:

| MONTHNUMBER |
---------------
|           0 |
|           1 |
|           2 |
|           3 |
|           4 |
|           5 |
|           6 |
|           7 |
|           8 |
|           9 |
|          10 |
|          11 |
|          12 |

Then the SELECT statement immediately after is using the values of the CTE result set to get you the Month Names.

Final query (See Demo):

;WITH months(MonthNumber) AS
(
    SELECT 0
    UNION ALL
    SELECT MonthNumber+1 
    FROM months
    WHERE MonthNumber < 12
)
SELECT LEFT(DATENAME(MONTH,DATEADD(MONTH,-MonthNumber,GETDATE())),3) AS [month]
FROM months;