CROSS JOIN filter

Question

I need to 'manufacture' records for a query, but at the same time, restrict the list of values that are returned from a 'lookup' table.

Is there a way to use 'filter' a CROSS JOIN without having to resort to using an in-line view?

This syntax works as expected (I get the desired results):

SELECT  E.ID,  
        M.VALUE,
        MT.ID, MT.NAME

FROM    ENCOUNTER E
CROSS JOIN (
      SELECT  ID, NAME
      FROM    MEASUREMENT_TYPE
      WHERE   ID IN ('6941','6946')
) MT
LEFT OUTER JOIN MEASURE M ON E.ID=M.ENCOUNTER_ID
        AND MT.ID=M.MEASURE_TYPE_ID

Unfortunately, if I use this approach, I need to use a Command object with Crystal Reports rather than its native 'Visual Linking Expert'. Command objects irritate me.

Adding a filter to the WHERE clause results in an equal join, which is undesirable in this situation.

Answer 1

SELECT
  E.ID,  
  M.VALUE,
  MT.ID,
  MT.NAME
FROM
  ENCOUNTER         AS E
CROSS JOIN
  MEASUREMENT_TYPE  AS MT
LEFT OUTER JOIN
  MEASURE           AS M
    ON  E.ID  = M.ENCOUNTER_ID
    AND MT.ID = M.MEASURE_TYPE_ID
WHERE
  MT.ID IN ('6941','6946')

In this case you can put the filter in the WHERE clause because it's a LEFT JOIN and you're filtering the left side of the query.

If you were filtering the Measure table, the filter would have had to go in the LEFT JOIN's ON clause.

An additional alternative is to INNER JOIN instead of CROSS JOIN and use the filter there. This is because the ON clause doesn't actually need to reference both tables...

  ENCOUNTER         AS E
INNER JOIN
  MEASUREMENT_TYPE  AS MT
    ON MT.ID IN ('6941','6946')

Answer 2

In this case, an inner join would do the same as your cross join. Replace:

CROSS JOIN (
      SELECT  ID, NAME
      FROM    MEASUREMENT_TYPE
      WHERE   ID IN ('6941','6946')
) MT

with

INNER JOIN MEASUREMENT_TYPE MT ON MT.ID IN ('6941','6946')