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# Validating a pair of columns with range tests

I have two fields "zipfrom" and "zipto" in one table. I want to compare another two fields (user-entered) with...

these fields to validate that the user-entered zip range is unique in the table. For example,

```Table
zipfrom  zipto
51000    52000```

If the user enters 51150-51175 then we should return error that this range is already entered.

In the example you've given, the codes 51150-51175 represent a range of numbers that lie completely within the range 51000-52000. I shall assume that you also want to issue an error if the user-entered range overlaps an existing range at either end. These three conditions are illustrated below, where X------X is an existing range already in the database, and A----A, B----B, and C----C are ranges for which you want to issue an error --

```     X----------X
A----A
B-------B
C------C```

You can check all three of these conditions by noting that at least one of the endpoints of a "bad" user-entered range is within the existing range --

```create table zipranges
( zipfrom char(5)
, zipto char(5)
, constraint rangecheck1
check ( not exists
( select * from zipranges XX
where zipfrom between XX.zipfrom and XX.zipto
or zipto   between XX.zipfrom and XX.zipto )
)
);```

However, not every database is going to let you declare a constraint that way, with a subquery. At worst you would have to write some programming logic to perform the NOT EXISTS test before deciding to insert. Another problem with this type of check constraint, as pointed out by Joe Celko on page 160 of SQL Puzzles & Answers (ISBN 1-55860-453-7), is that you will have trouble inserting an initial row into an empty table.

There's one more problem, too. What if the user enters a range that spans an existing range?

```     X----------X
A---------------A```

This, too, can be covered by a check constraint --

```  , constraint rangecheck2
check ( not exists
( select * from zipranges YY
where zipfrom < YY.zipfrom
and zipto   > YY.zipto )
)```

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This was last published in March 2002

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