Question

I have a PHP program w/MySQL database which contains many records. Two columns of particular relevance are incidentnumber and date. These both move forward only. However, sometimes a user enters data which is out of sequence; eg:

Incident Date
1        Jan 1 2000
2        Jan 1 2010
3        Jan 1 2002

It appears that incident 2 was entered with the wrong date, it should be Jan 1 2001.

Is there any way to query for records where the date is out of sequence? Or do I have to iterate through all records tracking last date to find the error?

ADDED NOTE: The incidents are not sequential (they might go 1,3,6,123, etc). Nor are the dates sequential. And these are columns in the same table.

Answer 1

This command selects any records for which there exists in the same table a record with a lower Incident number but a higher Date.

 SELECT * FROM TableName T1 WHERE EXISTS 
   (SELECT * FROM TableName T2 
      WHERE T2.Incident < T1.Incident AND T2.Date > T1.Date)

This slightly more complex command will find only records for which are out of order in “both directions”, meaning they have an later dated record earlier in the file and an earlier dated record later in the file. This avoids the situation in which making a mistake in a very early record in the file will make all the subsequent records appear out of order. However, it will not catch a problem in the two records with the lowest or highest incident numbers.

 SELECT * FROM TableName T1 WHERE EXISTS 
   (SELECT * FROM TableName T2 
      WHERE T2.Incident < T1.Incident AND T2.Date > T1.Date)
 AND EXISTS
   (SELECT * FROM TableName T2 
      WHERE T2.Incident > T1.Incident AND T2.Date < T1.Date)

Finally, as ruakh points out in the comments, the above query gives you ALL the out-of-order records. Although that is, technically, what you wanted it makes it difficult to find the “point of breakage” in the chain of dates. The following query will give you only the records where the chain gets messed up, does not require IncidentID to increase monotonically, and allows deletions of incidents.

 SELECT * FROM TableName T1 WHERE 
    Date < (SELECT Date FROM TableName T2 WHERE T2.IncidentID = 
   (SELECT MAX(IncidentID) FROM TableName T3 WHERE T3.IncidentID < T1.IncidentID))
 OR Date > (SELECT Date FROM TableName T2 WHERE T2.IncidentID = 
   (SELECT MAX(IncidentID) FROM TableName T3 WHERE T3.IncidentID > T1.IncidentID))

(Not tested, since I don’t have a copy of MySQL handy).