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 word is used to create a Common Table Expression (CTE). In this case, it's creating an inline table that the "select fileUID, ..." part is pulling data from.

Answer 2

WITH x AS (...)

This will take the output of the ... and treat it as a table named x, temporarily.

WITH x AS (...)
SELECT * FROM x

This statement will essentially give you the exact same thing as the ... outputs but it will instead be referenced as the table x

Answer 3

It is creating CTE (Common Table Expression). This is a basically a table that you don't have to create, drop, or declare in anyway. It will be automatically deleted after the batch has ran.

Check out http://4guysfromrolla.com/webtech/071906-1.shtml for more info.

Answer 4

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).