Question

I have this table where I only want to look at AB

ID     CODE       COUNT   
102    AB         9
101    AB         8
100    AC         23    //not important!!!!
99     AB         7
98     AB         6
97     AB         5
96     AB         0

Conversed to this

ID    NEWID     CODE       COUNT   
102   102       AB         9
101   101       AB         8
99    100       AB         7
98    99        AB         6
97    98        AB         5
96    97        AB         0

Using this query:

SELECT t.ID, @NEWID := COALESCE(@NEWID - 1, t.ID) AS NEWID, t.CODE, t.COUNT
FROM
(SELECT ID, CODE, COUNT FROM some_table WHERE CODE = 'AB' ORDER BY ID DESC) t,
(SELECT @NEWID := NULL) _uv;

http://sqlfiddle.com/#!2/e0b8b/1/0

And now I want to count the difference of each consecutive NEWID.

So

Step 1: 9 - 8 = 1
Step 2: 8 - 7 = 1
Step 3: 7 - 6 = 1
Step 4: 6 - 5 = 1
Step 5: 5 - 0 = 5

I’m used to doing this with

LEFT OUTER JOIN some_table t2 ON t.ID = ( t2.ID + 1 ) 

And then taking the difference between t2.count and t.count,
but now when I’m using COALESCE, I cannot select this NEWID, so the code below does not work.

LEFT OUTER JOIN some_table t2 ON t.NEWID = ( t2.NEWID + 1 ) 

So how should I resolve this issue?

Answer 1

Oh, so that’s why you needed sequential IDs. Well, you can use user variables for this, too, and you don’t even need NEWID! Since you’re doing stuff like this, you’d do well to read up on how user variables work.

SELECT
    t.ID, t.CODE, t.COUNT,
    @PREVCOUNT - t.COUNT DIFFERENCE,
    @PREVCOUNT := t.COUNT  -- Updates for the next iteration, so it
                           -- must come last!
FROM
    (SELECT ID, CODE, COUNT FROM some_table WHERE CODE = 'AB' ORDER BY ID DESC) t,
    (SELECT @PREVCOUNT := NULL) _uv;

SQL Fiddle