using a LEFT OUTER JOIN on something COALESCED - mysql

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

Answer 2

Try this:
SELECT
t1.ID as ID1,
t2.ID as ID2,
t1.CODE as CODE,
t1.COUNT as C1,
t2.COUNT as C2,
t2.COUNT - t1.COUNT as DIFF
FROM
some_table t1
INNER JOIN some_table t2 ON t1.ID < t2.ID AND t1.CODE = t2.CODE
WHERE
t1.CODE='AB'
AND NOT EXISTS
(SELECT * FROM some_table t3
INNER JOIN some_table t4 ON t3.ID < t4.ID and t3.CODE = t4.CODE
WHERE
t3.CODE='AB'
AND t1.ID = t3.ID
AND t4.ID < t2.ID
)
ORDER BY t1.ID
Another way will be to use LIMIT:
SELECT
t.ID1 AS ID,
t.CODE as CODE,
t.C2-t.C1 AS DIFF
FROM
(
SELECT
t1.ID as ID1,
t1.CODE as CODE,
t1.COUNT as C1,
(SELECT t.COUNT
FROM some_table t
WHERE t.ID > t1.ID AND t.CODE=t1.CODE
ORDER BY t.ID
LIMIT 1) as C2
FROM
some_table t1
WHERE
t1.CODE='AB'
ORDER BY t1.ID) t
ORDER BY t.ID1
http://sqlfiddle.com/#!2/e0b8b/24