Definition of Different Join Types


How do the different SQL joint types affect record selection when using Crystal Reports?

emal1975Asked:
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mlmccConnect With a Mentor Commented:
The joins available to you depend on where you are trying to write them.

An equal join - Includes records from both tables when the joined fields are equal.

A left outer join - Includes all records from the table on the left and records from the right table where the join fields are equal.

A right outer join - Includes all records from the table on the right and records from the left table where the join fields are equal.

Examples

Table1
Rec_Key   LastName     SuperKey
1         Smith          1
2         Jones          2
3         James          4        

Table2
Rec_Key   Supervisor
1         Wilson
2         Johnson
3         Mathews

EQUAL JOIN

Select LastName, Supervisor
from Table1 join table2 on SuperKey = table2.Rec_Key

returns
LastName     Supervisor
Smith        Wilson
Jones        Johnson


LEFT OUTER JOIN
Select LastName, Supervisor
from Table1 left outer join table2 on SuperKey = table2.Rec_Key

returns
LastName     Supervisor
Smith        Wilson
Jones        Johnson
James        <NULL>

RIGHT OUTER JOIN
Select LastName, Supervisor
from Table1 right outer join table2 on SuperKey = table2.Rec_Key

returns
LastName     Supervisor
Smith        Wilson
Jones        Johnson
<NULL>       Mathews

Hope this helps.

If you have questions on a specific example please post it.  Remember the exact SQL statement is database and reporting tool specific.

mlmcc
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emal1975Author Commented:
Thanks so much.
That makes a lot more sense than the included help guide.

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mlmccCommented:
Glad to help

Thanks for the points
mlmcc
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