How to use SET FILTER TO in FoxPro to limit the results to matched records in another cursor

Question

Say, I've got a cursor with data records, with an identifying foreign key in csrData:

|  ID  |  Foreign_ID  |  
|  1   |  1           |  
|  2   |  1           |  
|  3   |  2           |  
|  4   |  2           |  
|  5   |  2           |  
|  6   |  3           |  

And a list of foreign IDs in a separate cursor csrMatches:

|  Match_Foreign_ID  |  
|  1                 |  
|  3                 |  

Is there a SET FILTER TO command I can issue against csrData to get:

|  ID  |  Foreign_ID  |  
|  1   |  1           |  
|  2   |  1           |  
|  6   |  3           |  

I know this can be achieved using a SQL query, but the reporting framework I'm using limits me to SET FILTER TO statements. The INLIST() function works similarly, but I'd like to do filtering against a set of unknown size.

Answer 1

You can create a filter condition that will look up a given csrData.Foreign_ID in csrMatches and return true if it's found, false otherwise.

First, create an index on the Match_Foreign_ID field in csrMatches. Then, create a filter that uses indexseek() to test each Foreign_ID for inclusion in csrMatches.

select csrMatches
index on Match_Foreign_ID tag Foreign_ID

select csrData
set filter to indexseek(csrData.Foreign_ID, .f., "csrMatches", "Foreign_ID")

Using indexseek() is slightly faster than seek() because indexseek() simply uses the index to check if the key is found, and does not move the active record pointer.