MySQL Add rownumbers per ID

Question

I have a table test with sessionIDs. I want to group the IDs and add rownumbers to each record per ID. I used MSSQL Row_Number() over(order by) in MySql and added an if-statement (to group per session_ID). However, it doesn't work. Does anyone know why this doesn't work?

SET @row:=0
SET @sessionID:=0

SELECT 
    session_ID
    @row:=if(@sessionID = session_ID, @row+1, 1) AS row_num
    @sessionID := session_ID
FROM test  

Answer 1

You need a comma after row_num and you need an order by:

SET @row:=0;
SET @sessionID:=0;

SELECT 
    session_ID,
    @row:=if(@sessionID = session_ID, @row+1, 1) AS row_num,
    @sessionID := session_ID
FROM test
ORDER BY session_ID;

You can write this as a single query:

SELECT session_ID,
       @row:=if(@sessionID = session_ID, @row+1, 1) AS row_num,
       @sessionID := session_ID
FROM test cross join
     (select @row := 0, @sessionID := 0)
ORDER BY session_ID;