Selecting records that appear several times in a row - sql

Question

My problem is that I would like to select some records which appears in a row.
For example we have table like this:
x
x
x
y
y
x
x
y
Query should give answer like this:
x 3
y 2
x 2
y 1

Answer 1

SQL tables represent unordered sets. Your question only makes sense if there is a column that specifies the ordering. If so, you can use the difference-of-row-numbers to determine the groups and then aggregate:
select col1, count(*)
from (select t.*,
row_number() over (order by <ordering col>) as seqnum,
row_number() over (partition by col1 order by <ordering col>) as seqnum_2
from t
) t
group by col1, (seqnum - seqnum_2)

Answer 2

I made a SQL Fiddle
http://sqlfiddle.com/#!18/f8900/5
CREATE TABLE [dbo].[SomeTable](
[data] [nchar](1) NULL,
[id] [int] IDENTITY(1,1) NOT NULL
);
INSERT INTO SomeTable
([data])
VALUES
('x'),
('x'),
('x'),
('y'),
('y'),
('x'),
('x'),
('y')
;
select * from SomeTable;
WITH SomeTable_CTE (Data, total, BaseId, NextId)
AS
(
SELECT
Data,
1 as total,
Id as BaseId,
Id+1 as NextId
FROM SomeTable
where not exists(
Select * from SomeTable Previous
where Previous.Id+1 = SomeTable.Id
and Previous.Data = SomeTable.Data)
UNION ALL
select SomeTable_CTE.Data, SomeTable_CTE.total+1, SomeTable_CTE.BaseId as BaseId, SomeTable.Id+1 as NextId
from SomeTable_CTE inner join SomeTable on
SomeTable.Data = SomeTable_CTE.Data
and
SomeTable.Id = SomeTable_CTE.NextId
)
SELECT Data, max(total) as total
FROM SomeTable_CTE
group by Data, BaseId
order by BaseId

Answer 3

The elephant in the room is the missing column(s) to establish the order of rows.
SELECT col1, count(*)
FROM (
SELECT col1, order_column
, row_number() OVER (ORDER BY order_column)
- row_number() OVER (PARTITION BY col1 ORDER BY order_column) AS grp
FROM tbl
) t
GROUP BY col1, grp
ORDER BY min(order_column);
To exclude partitions with only a single row, add a HAVING clause:
SELECT col1, count(*)
FROM (
SELECT col1, order_column
, row_number() OVER (ORDER BY order_column)
- row_number() OVER (PARTITION BY col1 ORDER BY order_column) AS grp
FROM tbl
) t
GROUP BY col1, grp
HAVING count(*) > 1
ORDER BY min(order_column);
db<>fiddle here
Add a final ORDER BY to maintain original order (and a meaningful result). You may want to add a column like min(order_column) as well.
Related:
Find the longest streak of perfect scores per player
Select longest continuous sequence
Group by repeating attribute