Group tandem records

Question

I have a table like this:

|     A |     B |
-----------------
|  22.1 |  15.8 |
| 12.15 |  4.55 |  <- Duplicate record
| 12.15 |  4.55 |  <- Duplicate record
| 12.15 |  4.55 |  <- Duplicate record
|  30.4 | 44.12 |
|  10.5 |  7.58 |
|  31.2 |  65.1 |  <- Duplicate record
|  31.2 |  65.1 |  <- Duplicate record
|   9.4 |   7.8 |
| 12.15 |  4.55 |  <- Same Duplicate record
| 12.15 |  4.55 |  <- Same Duplicate record
| 12.15 |  4.55 |  <- Same Duplicate record
|  31.2 |  65.1 |  <- Same Duplicate record
|  31.2 |  65.1 |  <- Same Duplicate record

Here I want to group A and B. But I want to group tandem values.
So if there is a different record between two same record then both record(same) should come.

For e.g. my expected output should be:

|     A |     B |
-----------------
|  22.1 |  15.8 |
| 12.15 |  4.55 |  <- Group 1
|  30.4 | 44.12 |
|  10.5 |  7.58 |
|  31.2 |  65.1 |  <- Group 2
|   9.4 |   7.8 |
| 12.15 |  4.55 |  <- Group 3 (Second Time)
|  31.2 |  65.1 |  <- Group 4 (Second Time)

What I am trying is:

SELECT * FROM MyTable
GROUP BY A,B

but it gives me wrong result:

|     A |     B |
-----------------
|  22.1 |  15.8 |
| 12.15 |  4.55 |
|  30.4 | 44.12 |
|  10.5 |  7.58 |
|  31.2 |  65.1 |
|   9.4 |   7.8 |
Here `12.15` and `31.2` is skipped second time. But I want it.

Note, I read this question but the solution is given in PHP while I want it in MySQL.
I am trying to solve the issue in this SQLFiddle.

Answer 1

Do you have a sequential ID on the column? If so, you can try something like:

SELECT t.A, t.B
FROM myTable t
WHERE NOT EXISTS
(
    SELECT 1
    FROM myTable t2
    WHERE t2.A = t.A
    AND t2.B = t.B
    AND t2.Id = (SELECT MIN(t3.Id) FROM myTable t3 WHERE t3.Id > t.Id)
)

SQL Fiddle example

---

If you don't have an ID, you can try the following, but remember there is no guarantee that you will get the results you want; the server can return results in any order it wishes unless you specify a column to order by:

SELECT t.A, t.B
FROM
(
    SELECT @curRow := @curRow + 1 AS Id, A, B
    FROM myTable
    JOIN (SELECT @curRow := 0) r ON 1=1
) t
WHERE NOT EXISTS
(
    SELECT 1
    FROM (
        SELECT @curRow2 := @curRow2 + 1 AS Id, A, B
        FROM myTable
        JOIN (SELECT @curRow2 := 0) r ON 1=1
    ) t2
    WHERE t2.A = t.A
    AND t2.B = t.B
    AND t2.Id = (
        SELECT MIN(t3.Id) 
        FROM (
            SELECT @curRow3 := @curRow3 + 1 AS Id, A, B
            FROM myTable
            JOIN (SELECT @curRow3 := 0) r ON 1=1
        ) t3
        WHERE t3.Id > t.Id
   )
)

SQL Fiddle example