SQL substring question

Hi,

 

I need to write a 'quick' interactive SQL statement to update a file
based on the conditional field being a substring and the result field
being a substring.

 

FileA

Field1 (7,A)   position 1,3 contains many values, but I need only 'TRD'
records and position 4-7 contains a customer number value  e.g.
TRD1100, TRD5555, TRD6789 etc

 

Field2 (243,A)  contains many values, but I need to extract positions
89-90

 

I need to update all Field1 'TRD' records where field2 position 89-90 =
'AA' to be value 'XX'

 

Any help would be very appreciated.

 

Thanks in advance.  

 

Sheri Rowe