mysql - SQL: selecting rows where column value changed from previous row -


let's have (mysql) database, sorted increasing timestamp:

timestamp   system statusa statusb  2011-01-01          ok     ok       2011-01-02     b      ok     ok      2011-01-03         fail   fail      2011-01-04     b      ok    fail      2011-01-05         fail    ok       2011-01-06          ok     ok       2011-01-07     b     fail   fail

how select rows statusa changed previous row system? statusb doesn't matter (i show in question illustrate there may many consecutive rows each system statusa doesn't change). in example above, query should return rows 2011-01-03 (statusa changed between 2011-01-01 , 2011-01-03 systema), 2011-01-06, 2011-01-07.

the query should execute table having tens of thousands of records.

thanks

select a.* tablex a.statusa <>       ( select b.statusa         tablex b         a.system = b.system           , a.timestamp > b.timestamp         order b.timestamp desc         limit 1       )

but can try (with index on (system,timestamp):

select system, timestamp, statusa, statusb   ( select (@statuspre <> statusa , @systempre=system) statuschanged          , system, timestamp, statusa, statusb          , @statuspre := statusa          , @systempre := system     tablex        , (select @statuspre:=null, @systempre:=null) d     order system            , timestamp   ) statuschanged ;

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