Filter out orphan table entries - mysql

Question

Suppose there is a table with only two columns (an example is shown below). Every '1' entry should be followed (in the sorted order given below) by a '0'. However, as you can see, in the table, there are some 'orphans' where there are two consecutive '1's.
How can I create a query that returns all the rows, except for the first of any consecutive '1's? (This would reduce the example below from 16 rows to 14)
1 E
0 A
1 T
0 S
1 R
0 E
1 F
0 T
1 G
1 T
0 R
1 X
1 R
0 R
1 E
0 T
I'm going to try and clarify my problem, I think that above I simplified it too much. Imagine one table called logs, with four columns:
user (a string containing a username)
machine (a string uniquely identifying various PCs)
type (event's type: a 1 for login and a 0 for logout)
time (the time of the event being logged)
[The machine/time pair provides a unique key, as no machine can be logged in or out of twice at the same instant. Presumably an 'ID' column could be artificially created based on machine/time sort if needed.]
The idea is that every login event should be accompanied by a logout event. In an ideal word it would be fairly easy to match logins to logouts, and hence analyse the time spent logged in.
However, in the case of a power cut, the logout will not be recorded. Therefore (considering only one machine's data, sorted by time) if there are two login events in a row, we want to ignore the first login, because we don't have any reliable data from it. This is the problem I am trying to solve.

Answer 1

Provided, that
only 1's are dupes, never 0's
You want to get rid of all the first 1's if there are more.
Your text says "except for the first of any consecutive", but I think, this is what you want. Or there can only ever be 2, then it is the same.
SELECT x.*
FROM x
LEFT JOIN x y on y.id = (x.id + 1)
WHERE (x.nr = y.nr) IS NOT TRUE -- OR x.nr = 0
ORDER BY x.id
If you want to preserve double 0's, use the commented clause additionally, but probably not needed.
Edit after question edit:
You may want to add an auto-increment column to your data to make this simpler:
Generate (i.e. write) a row number index column in MySQL
Other RDBMS (PostgreSQL, Oracle, SQL Server, ..) have window functions like row_number() or lag() and lead() that make such an operation much easier.

Answer 2

Assuming you get an id (add column, set column id = record number in database) use:
select a.*
from the_table a
left join the_table b on b.id = a.id + 1
and b.col1 = 0
where a.col1 = 1
and b.id is null

Answer 3

Try:
select l.*
from logs l
where l.type = 0 or
not (select type
from (select * from logs order by `time` desc) n
where n.machine = l.machine and
n.user = l.user and
n.time > l.time)
group by () )

Answer 4

USING a CTE to separate the lag-logic from the selection criteria.
DROP TABLE tmp.bits;
CREATE TABLE tmp.bits
( id SERIAL NOT NULL
, bit INTEGER NOT NULL
, code CHAR(1)
);
INSERT INTO tmp.bits(bit, code) VALUES
(1, 'T' )
, (0, 'S' )
, (1, 'R' )
, (0, 'E' )
, (1, 'F' )
, (0, 'T' )
, (1, 'G' )
, (1, 'T' )
, (0, 'R' )
, (1, 'X' )
, (1, 'R' )
, (0, 'R' )
, (1, 'E' )
, (0, 'T' )
;
SET search_path='tmp';
SELECT * FROM bits;
-- EXPLAIN ANALYZE
WITH prevnext AS (
SELECT
bt.id AS thisid
, bt.bit AS thisbit
, bt.code AS thiscode
, bp.bit AS prevbit
, bp.code AS prevcode
FROM bits bt
LEFT JOIN bits bp ON (bt.id > bp.id)
AND NOT EXISTS ( SELECT * FROM bits nx
WHERE nx.id > bp.id
AND nx.id < bt.id
)
)
SELECT thisid, thisbit, thiscode
FROM prevnext
WHERE thisbit=0
OR prevbit IS NULL OR thisbit <> prevbit
;
EDIT:
for those poor soals that cannot use CTEs, it is easy to create a view instead:
CREATE VIEW prevnext AS (
SELECT
bt.id AS thisid
, bt.bit AS thisbit
,bt.code AS thiscode
, bp.bit AS prevbit
, bp.code AS prevcode
FROM bits bt
LEFT JOIN bits bp ON (bt.id > bp.id)
AND NOT EXISTS ( SELECT * FROM bits nx
WHERE nx.id > bp.id
AND nx.id < bt.id
)
)
;
SELECT thisid, thisbit, thiscode
FROM prevnext
WHERE thisbit=0
OR prevbit IS NULL OR thisbit <> prevbit
;