Add extra column in sql to show ratio with previous row - sql

Question

I have a SQL table with a format like this:
SELECT period_id, amount FROM table;
+--------------------+
| period_id | amount |
+-----------+--------+
| 1 | 12 |
| 2 | 11 |
| 3 | 15 |
| 4 | 20 |
| .. | .. |
+-----------+--------+
I'd like to add an extra column (just in my select statement) that calculates the growth ratio with the previous amount, like so:
SELECT period_id, amount, [insert formula here] AS growth FROM table;
+-----------------------------+
| period_id | amount | growth |
+-----------+-----------------+
| 1 | 12 | |
| 2 | 11 | 0.91 | <-- 11/12
| 3 | 15 | 1.36 | <-- 15/11
| 4 | 20 | 1.33 | <-- 20/15
| .. | .. | .. |
+-----------+-----------------+
Just need to work out how to perform the operation with the line before. Not interested in adding to the table. Any help appreciated :)
** also want to point out that period_id is in order but not necessarily increasing incrementally

Answer 1

The window function Lag() would be a good fit here.
You may notice that we use (amount+0.0). This is done just in case AMOUNT is an INT, and NullIf() to avoid the dreaded divide by zero
Declare #YourTable table (period_id int,amount int)
Insert Into #YourTable values
( 1,12),
( 2,11),
( 3,15),
( 4,20)
Select period_id
,amount
,growth = cast((amount+0.0) / NullIf(lag(amount,1) over (Order By Period_ID),0) as decimal(10,2))
From #YourTable
Returns
period_id amount growth
1 12 NULL
2 11 0.92
3 15 1.36
4 20 1.33

Answer 2

If you are using SQL Server 2012+ then go for John Cappelletti answer.
And If you are also less blessed like me then this below code work for you in the 2008 version too.
Declare #YourTable table (period_id int,amount int)
Insert Into #YourTable values
( 1,12),
( 2,11),
( 3,15),
( 4,20)
;WITH CTE AS (
SELECT ROW_NUMBER() OVER (
ORDER BY period_id
) SNO
,period_id
,amount
FROM #YourTable
)
SELECT C1.period_id
,C1.amount
,CASE
WHEN C2.amount IS NOT NULL AND C2.amount<>0
THEN CAST(C1.amount / CAST(C2.amount AS FLOAT) AS DECIMAL(18, 2))
END AS growth
FROM CTE C1
LEFT JOIN CTE C2 ON C1.SNO = C2.SNO + 1
Which works same as LAG.
+-----------+--------+--------+
| period_id | amount | growth |
+-----------+--------+--------+
| 1 | 12 | NULL |
| 2 | 11 | 0.92 |
| 3 | 15 | 1.36 |
| 4 | 20 | 1.33 |
+-----------+--------+--------+