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Planning in Intelligent Systems: Aspects, Motivations, and Methods, Edited by Wout van Wezel, Rene Jorna, and Alexander Meystel Copyright # 2006 John Wiley & Sons, Inc.

Imagine you are writing a version of the computer game Minesweeper. Use arrays to create and store a minefield on a 20 x 20 grid. Place ten mines randomly on the grid, then display the grid, using asterisks (*) for the mines and periods (.) for the empty squares. (Hint: To return a random number between 0 and 19 inclusive, use rand( 0, 19 ).)

v(tk1 , tk2 ) ej (n1 1 tk1 +n2 2 tk2 )

The unique and primary key constraints allow you to define uniqueness based on more than one column. For instance, you can store the area code in one column and the local phone number in another. You can then use a composite unique constraint to ensure that the same phone number isn t entered twice while still able to easily query based on the area code. A foreign key constraint is used to enforce referential integrity. Referential integrity means that the relationship of data between tables has integrity. Generally, referential integrity is expected to exist between columns used frequently in joins. A foreign key constraint is enforced between the dept and emp tables that you have seen in the examples. You can t enter just any deptno into the emp table; it must be a deptno that appears in the dept table. Also, you can t delete a row in the dept table if the deptno is in use by rows in the emp table. In referential integrity terms, the dept table is the parent table and the emp table is the child table. A foreign key constraint is defined on a column of the child table (e.g., emp.deptno) and points to a column of the parent table (e.g., dept.deptno). The column on the child table is called the foreign key; the column on the parent table, the referenced key. Table 8.21 lists the foreign key restraints and shows how actions are constrained once a foreign key constraint is in place. Compound foreign key constraints are allowed where both the foreign key and the referenced key are composites of several columns.