Rdb: Ruby-Nachbau der MWC /rdb-Werkzeuge

man/fd.man

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+.TH fd "" "" "/rdb Command"
+.PC "Test for functional dependency of columns"
+\fBfd \fIdeterminecolumn dependcolumn \fB< \fItable\fR
+.PP
+.II "column^functional dependency"
+.II "functional dependency^test"
+.II "test functional dependency"
+The command
+.B fd
+reads
+.I table
+and tests whether its
+.I determinecolumn
+determines
+.IR dependcolumn .
+A column in a table functionally depends upon another column 
+when there is only one value in the second column for each value in
+the first.
+This helps you to determine if a table is normalized.
+(See the section on
+.BR Normalization ,
+below, for details on what this means.)
+.SH Example
+First let's look at our inventory table:
+.DM
+Item    Amount  Cost    Value   Description   
+----    ------  ----    -----   --------------
+   1         3    50      150   rubber gloves
+   2       100     5      500   test tubes
+   3         5    80      400   clamps
+   4        23    19      437   plates
+   5        99    24     2376   cleaning cloth
+   6        89   147    13083   bunsen burners
+   7         5   175      875   scales
+.DE
+.PP
+Note that the first column,
+.B Item,
+is the key column for the table.
+It functionally determines all of the other columns.
+Each value in column
+.B Item
+is unique, so each value determines, or is the key,
+to its row and all of the column values on it.
+.PP
+However, column
+.B Cost
+does not functionally depend upon
+.B Amount
+because there are two `5' values in
+.B Amount
+and each has a different value for
+.B Cost
+associated with it (on the same row or line).
+The common sense (technically called the ``semantics'') of the
+table is that knowing the number of items we have in inventory
+does not tell us how much they cost.
+.PP
+Now let's see if
+.B fd
+can find the dependencies.
+The command
+.DM
+	fd Item Cost < inventory
+.DE
+.PP
+returns the message
+.BR true .
+This is correct, because
+.B Item
+is the key column.
+However, the command
+.DM
+	fd Amount Cost < inventory
+.DE
+.PP
+returns the message
+.BR false .
+This, too, is correct.
+.SH Normalization
+A key column should aways functionally determine all
+of the other columns in the table.
+But nonkey columns should not depend upon each other.
+.PP
+If there are too few data in the table, a
+pseudo-dependency may be discovered.
+Be sure your table has lots of data before you test it.
+.PP
+To normalize your tables, you must check for functional
+dependencies.
+When functional dependency is found with non\-key columns,
+the columns should be projected and uniqued to form a second table.
+Then the functionally dependent column, but not the
+determining column, should be removed from the first table.
+For example:
+.DM
+	column column1 column2 < table | uniq > newtable
+	column `sed 1q table | sed s/column2//` < table > tmp
+	mv tmp table
+.DE
+.PP
+In the second table, the column that determines the other column
+is the key for that table.
+If you need to recreate the original first table, you can
+join the two new tables by using the command
+.BR jointable ,
+as follows:
+.DM
+	jointable -j column1 table newtable > oldtable
+.DE
+.PP
+Normalization, or simplification, is an important process.
+Normalized files are smaller and are easier to maintain.
+.SH "See Also"
+.B
+column,
+jointable,
+paste.rdb
+.R
+.br
+COHERENT Lexicon:
+.B
+cut,
+false,
+mv,
+paste,
+sed,
+true
+.SO fd