Crystal Reports: Join tables together with a UNION join

Sometimes, we need to have a record appear in more than one place in our Crystal Report. Examples of this include stock movements in and out of the warehouse, or call centers where calls come in and get resolved.

If you want to display activity or status on a series of days, the same call or stock item needs to appear on several days.

One solution to this problem is to use SQL to create a different view of the data. A UNION join will combine a series of tables into one recordset for your report:

SELECT "In" as CallType, InDate as CallDate, CallID from Calls

UNION ALL

SELECT "Out" as CallType, OutDate as CallDate, CallID from Calls

The UNION ALL includes duplicate records, otherwise a UNION is adequate. You can also combine several of these if you want stock status at different dates. You can’t do this with the SQL inside Crystal Reports but you can use this technique if you can create SQL views or queries in your database. You can also do it in the Crystal SQL designer.

When we use SQL to combine two tables using a UNION statement we usually use "UNION ALL" because that’s what seems to work properly. But why? What does the UNION ALL do that the UNION by itself does not?

UNION will do a reprocessing of the records to remove any duplicates, while UNION ALL will skip this step. For that reason UNION ALL will be a lot faster than a UNION. Some databases discourage UNION and will insist that a UNION ALL is used.

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