What is the WITH statement doing in this example? I'm trying to randomly generate data

Question

INSERT INTO files (fileUID, filename)
WITH fileUIDS(fileUID) AS
( VALUES(1) UNION ALL
  SELECT fileUID+1 FROM fileUIDS WHERE fileUID < 1000 )
SELECT fileUID,
       TRANSLATE ( CHAR(BIGINT(RAND() * 10000000000 )), 'abcdefgHij', '1234567890' )
FROM fileUIDS;

Answer 1

The WITH syntax is the same as using either a local temp table or inline view. To my knowledge, it's only supported in SQL Server (2005+, called Common Table Expressions) and Oracle (9i+, called Subquery Factoring). The intended use is for creating a basic view that is used (ie: joined to) multiple times in a single query.

Here's a typical example:

WITH example AS (
     SELECT q.question_id,
            t.tag_name
       FROM QUESTIONS q
       JOIN QUESTION_TAG_XREF qtf ON qtf.question_id = t.question_id
       JOIN TAGS t ON t.tag_id = qtf.tag_id)
SELECT t.title,
       e1.tag_name
  FROM QUESTIONS t
  JOIN example e1 ON e1.question_id = t.question_id

...which will return identical results if you use:

SELECT t.title,
       e1.tag_name
  FROM QUESTIONS t
  JOIN (SELECT q.question_id,
               t.tag_name
          FROM QUESTIONS q
          JOIN QUESTION_TAG_XREF qtf ON qtf.question_id = t.question_id
          JOIN TAGS t ON t.tag_id = qtf.tag_id) e1 ON e1.question_id = t.question_id

The example you provided:

WITH fileUIDS(fileUID) AS ( 
     VALUES(1) 
     UNION ALL
     SELECT t.fileUID+1 
       FROM fileUIDS t
      WHERE t.fileUID < 1000 )
INSERT INTO files 
    (fileUID, filename)
SELECT f.fileUID,
       TRANSLATE ( CHAR(BIGINT(RAND() * 10000000000 )), 'abcdefgHij', '1234567890' )
  FROM fileUIDS f;

...is a recursive one. It's starting at 1, generating 999 fileuids in total (it would be 1,000 if it had started at 0).