Databases Reference
In-Depth Information
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fIGure 13.1
SUM SQL.
The only quantitative value in Table 13.14 is the column named Quantity.
The Fact table in Table 13.14 presents both Entity Keys and Instance Keys.
So, it is possible that a summation operation can leverage either the Entity
Keys or the Instance Keys.
type 1 summary etl
Entity Keys are associated with Type 1 Time Variance. A Summary ETL
application is a Type 1 Summary ETL application when it references Entity
Keys, rather than Instance Keys. Figure 13.2 shows a summation query
that references only Entity Keys. The result set will include only Entity
Keys and will therefore be a Type 1 result set.
type 2 summary etl
Instance Keys are associated with Type 2 T ime Variance. A S ummary
ETL application is a Type 2 Summary ETL application when it references
Instance Keys, rather than Entity Keys. Figure 13.3 shows a summation
query that references only Instance Keys. The result set will include only
Instance Keys and will therefore be a Type 2 result set.
From the perspective of a Summary ETL application, a Type 2 Summary
table and a hybrid Type 1/Type 2 Summary table are extremely similar. A
hybrid Type 1/Type 2 Summary table contains both the Entity Key and the
Instance Key for each dimension. Therefore, the SQL in the Summary ETL
for a hybrid Type 1/Type 2 Summary table, shown in Figure 13.4, includes
both the Entity Key and the Instance Key for every dimension. Because
the Instance Key is the lower granularity of the two keys, the Instance Key,
rather than the Entity Key, determines the cardinality of the summation
operation and the output result set.
