Not Equal [!=, <>] join

The result set from a Not Equal join includes all records in which the linked field value in the primary table is not equal to the linked field value in the lookup table. This type of join can be used to find possible combinations of items when a table is joined to itself (a self-join). For example, a company can have a table listing all products they sell. When they decide to hold a sale where their customers buy one item and get the second item half price, they may need a list of all possible two item combinations:

SELECT Product1.'Product Name',
    Product2.'Product Name',
FROM 'Product' Product1
    'Product' Product2
WHERE Product1.'Product Name' !=
    Product2.'Product Name'

In this SQL statement, the Product table is opened twice. The first time, it is given the alias name Product1. The second time, it is given the alias name Product2. Then the Product Name field is used to link from the Product1 table to the Product2 table. This is the same table, but since it has been opened twice using different aliases, Crystal Reports considers it two separate tables. A Not Equal join is used to link the tables by the Product Name field. As a result, each product is paired with every other product offered, but is not paired with itself:

Product1 Product2
Product Name Product Name

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Note:    The symbol != is used to represent a Not Equal join, if the ODBC data source driver for the data being accessed supports this symbol. If not, the default symbol <> is used to represent a Not Equal join.



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