SQL Server: how to create sequence number column - sql-server

Question

I have a Sales table with the following data:
| SalesId | CustomerId | Amount |
|---------|------------|--------|
| 1 | 1 | 100 |
| 2 | 2 | 75 |
| 3 | 1 | 30 |
| 4 | 3 | 49 |
| 5 | 1 | 93 |
I would like to insert a column into this table that tells us the number of times the customer has made a purchase. So it'll be like:
| SalesId | CustomerId | Amount | SalesNum |
|---------|------------|--------|----------|
| 1 | 1 | 100 | 1 |
| 2 | 2 | 75 | 1 |
| 3 | 1 | 30 | 2 |
| 4 | 3 | 49 | 1 |
| 5 | 1 | 93 | 3 |
So I can see that in salesId = 5, that is the 3rd transaction for customerId = 1. How can I write such a query to insert / update such column? I am on MS SQL but I am also interested in the MYSQL solution should I need to do this there in the future.
Thank you.
ps. Apology for the table formatting. Couldn't figure out how to format it nicely.

Answer 1

You need ROW_NUMBER() to assign a sequence number. I'd strongly advise against storing this value though, since you will need to recalculate it with every update, instead, you may be best off creating a view if you need it regularly:
CREATE VIEW dbo.SalesWithRank
AS
SELECT SalesID,
CustomerID,
Amount,
SalesNum = ROW_NUMBER() OVER(PARTITION BY CustomerID ORDER BY SalesID)
FROM Sales;
GO
SQL Server Example on SQL Fiddle
ROW_NUMBER() will not assign duplicates in the same group, e.g. if you were assigning the rows based on Amount and you have two sales for the same customer that are both 100, they will not have the same SalesNum, in the absence of any other ordering criteria in your ROW_NUMBER() function they will be randomly sorted. If you want Sales with the same amount to have the same SalesNum, then you need to use either RANK or DENSE_RANK. DENSE_RANK will have no gaps in the sequence, e.g 1, 1, 2, 2, 3, whereas RANK will start at the corresponding position, e.g. 1, 1, 3, 3, 5.
If you must do this as an update then you can use:
WITH CTE AS
( SELECT SalesID,
CustomerID,
Amount,
SalesNum,
NewSalesNum = ROW_NUMBER() OVER(PARTITION BY CustomerID ORDER BY SalesID)
FROM Sales
)
UPDATE CTE
SET SalesNum = NewSalesNum;
SQL Server Update Example on SQL Fiddle
MySQL Does not have ranking functions, so you need to use local variables to achieve a rank by keeping track of the value from the previous row. This is not allowed in views so you would just need to repeat this logic wherever you needed the row number:
SELECT s.SalesID,
s.Amount,
#r:= CASE WHEN #c = s.CustomerID THEN #r + 1 ELSE 1 END AS SalesNum,
#c:= CustomerID AS CustomerID
FROM Sales AS s
CROSS JOIN (SELECT #c:= 0, #r:= 0) AS var
ORDER BY s.CustomerID, s.SalesID;
The order by is critical here, which means in order to order the results without affecting the ranking you need to use a subquery:
SELECT SalesID,
Amount,
CustomerID,
SalesNum
FROM ( SELECT s.SalesID,
s.Amount,
#r:= CASE WHEN #c = s.CustomerID THEN #r + 1 ELSE 1 END AS SalesNum,
#c:= CustomerID AS CustomerID
FROM Sales AS s
CROSS JOIN (SELECT #c:= 0, #r:= 0) AS var
ORDER BY s.CustomerID, s.SalesID
) AS s
ORDER BY s.SalesID;
MySQL Example on SQL Fiddle
Again, I would recommend against storing the value, but if you must in MySQL you would use:
UPDATE Sales
INNER JOIN
( SELECT s.SalesID,
#r:= CASE WHEN #c = s.CustomerID THEN #r + 1 ELSE 1 END AS NewSalesNum,
#c:= CustomerID AS CustomerID
FROM Sales AS s
CROSS JOIN (SELECT #c:= 0, #r:= 0) AS var
ORDER BY s.CustomerID, s.SalesID
) AS s2
ON Sales.SalesID = s2.SalesID
SET SalesNum = s2.NewSalesNum;
MySQL Update Example on SQL Fiddle

Answer 2

Using Subquery,
Select *, (Select count(customerid)
from ##tmp t
where t.salesid <= s.salesid
and t.customerid = s.customerid)
from ##tmp s

Answer 3

Try this -
SELECT SalesId, CustomerId, Amount,
SalesNum = ROW_NUMBER() OVER (PARTITION BY CustomerId ORDER BY SalesId)
FROM YOURTABLE