QlikView Reference Manual.pdf - QlikCommunity - QlikView

and an error message will appear. After each reload, QlikView will scan all chart expressions to see if
any ambiguities have occurred as a result of changes in data structures. If ambiguous expressions are
found, a warning dialog will be shown and the expression will be disabled. It will not be possible to
enable the expression until the problem has been corrected. If a log file is enabled, all ambiguous
expressions will be listed in the log.
There is a simple way to overcome these limitations. Load the field an extra time under a new name from the
table where frequency counts should be made. Then use the new field for a list box with frequency, for a statistics
box or for calculations in the charts.
Synthetic Keys
When two or more internal tables have two or more fields in common, this implies a composite key relationship.
QlikView handles this through synthetic keys. These keys are anonymous fields that represent all
occurring combinations of the composite key. When the number of composite keys increases, depending on
data amounts, table structure and other factors, QlikView may or may not handle them gracefully. QlikView
may end up using excessive amount of time and/or memory. Unfortunately the actual limitations are virtually
impossible to predict, which leaves only trial and error as a practical method to determine them.
Therefore we recommend an overall analysis of the intended table structure by the application designer. Typical
tricks include:
l Forming the own non-composite keys, typically using string concatenation inside an AutoNumber
script function.
l Making sure only the necessary fields connect. If for example a date is used as a key, make sure not
to load e.g. year, month or day_of_month from more than one internal table.
Circular References ("Loops")
If there are circular references ("loops") in a data structure, the tables are associated in such a way that there
is more than one path of associations between two fields.
This type of data structure should normally be avoided as much as possible, since it might lead to ambiguities
in the interpretation of data. Unfortunately, circular structures are quite common in the real world. In
some cases they are a result of poor database design, but in certain cases they may be unavoidable.
Three tables with a circular reference
QlikView solves the problem of circular references by breaking the loop with a loosely coupled table. When
QlikView finds circular data structures while executing the load script, a warning dialog will be shown and
one or more tables will be set as loosely coupled. QlikView will typically attempt to loosen the longest
table in the loop, as this is often a transaction table, which normally should be the one to loosen. If you do
not like QlikView's default choice of table to set as loosely coupled, it is possible to declare a specific table
as loosely coupled via a loosen table statement in the script. It is also possible to change the settings for
loosely coupled tables interactively in the Document Properties: Tables page after the script has been
executed. See chapter Logic and Selections (page 107), for more information on loosely coupled tables.
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