You don't say what technology you are using to define the schema. But let's
assume it's XSDL 1.1 with assertions (and with no limits on the XPath syntax
allowed in assertions) - as implemented in Saxon 9.0.

Then I think you can do it with

<xs:complexType name="type-of-data-element">
  ...
  <xs:assert test="e1[1]/@val=20 and 
  every $e in e1[position()>1] satisfies $e/@val =
$e/preceding-sibling::e1/@val + 1"/>
</xs:complexType>

I'm not sure this is quite right according to your description - perhaps you
allow gaps in the numbering sequence. In that case it's:

<xs:assert test="e1[1]/@val=20 and 
every $e in e1[position()>1] satisfies $e/+@val >
$e/preceding-sibling::e1/+@val"/>

(The odd little "+" signs are a quirky way of forcing conversion to a number
in XPath 2.0)

Give it a try and let us know if it works.

Michael Kay
http://www.saxonica.com/

> 	
> I need to define the schema for "val" attribute in the below XML:
> 
> <data term="EXP">
> <el val="20">
> <ch> some data </ch>
> </el>
> <el val="21">
> <ch> some data </ch>
> </el>
> .....similarly for val=22,23,24,....N
> </data>
> 
> <data term="SUPP">
> <el val="20">
> <ch> some data </ch>
> </el>
> <el val="21">
> <ch> some data </ch>
> </el>
> .....similarly for val=22,23,24,....N
> </data>
> 
> Here, am more interested in the "val" attribute of "el" 
> element. What is the Specific schema for "val"
> attribute which satisfies the following conditions:
> 1. allowed values for "val" are 20,21,22,..... N (no limit on 
> N although it's typically < 100.) 2. "val" always begins with 
> 20 in each "data" element scope.
> 3. "val" must be in ascending order (20,21,22...etc) 4. "val" 
> must be unique in each "data" element scope.
> 
> appreciate any pointers.
> +D
> 
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