SQL Server 2012

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CREATE TABLE T
	([Month] int, [MonthName] varchar(8), [Value] numeric)
;
	
INSERT INTO T
	([Month], [MonthName], [Value])
VALUES
	(3, 'March', 136.00),
	(4, 'April', 306.00),
	(7, 'July', 476.00),
	(12, 'December', 510.48)
;
4 rows affected

select t1.month
    ,datename(month,datefromparts(2020, t1.month, 1)) monthname
    ,t2.value 
from (
    select top 12 number + 1 as month from master..spt_values 
    where type = 'p'
) t1
left join t t2 on t2.month = (select max(month) from t tmp where tmp.month < = t1.month)
where t2.month is not null


month
monthname
value


3
March
136

4
April
306

5
May
306

6
June
306

7
July
476

8
August
476

9
September
476

10
October
476

11
November
476

12
December
510


/*
Month | MonthName | Value
3     | March     | 136.00
4     | April     | 306.00
5     | May       | 306.00 -- added data
6     | June      | 306.00 -- added data
7     | July      | 476.00
8     | August    | 476.00 -- added data
9     | September | 476.00 -- added data
10    | October   | 476.00 -- added data
11    | November  | 476.00 -- added data
12    | December  | 510.48
*/

month	monthname	value
3	March	136
4	April	306
5	May	306
6	June	306
7	July	476
8	August	476
9	September	476
10	October	476
11	November	476
12	December	510