use the Key Look up operator to look up data from a table, view, cube, or dimension. For example, use the Key Look up operator when you define a mapping that loads a cube or when you define surrogate keys on the dimension.
The key that you look up can be any unique value. It need not be a primary or unique key, as defined in an RDBMS. The Key Lookup operator reads data from a lookup table using the key input you supply and finds the matching row. This operator returns a row for each input key. You can have multiple Key Lookup operators in the same mapping.
The output of the Key Lookup operator corresponds to the columns in the lookup object. To ensure that only a single lookup row is found for each key input row, use keys in your match condition.
Each output attribute for the key lookup has a property called DEFAULT VALUE. The DEFAULT VALUE property is used instead of NULL in the outgoing row set if no value is found in the lookup table for an input value. The generated code uses the NVL function. The Key Lookup always results in an outer-join statement.
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