Query by Example
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Query by Example (cont.)

Constraints are also used in CCUBE queries to manipulate, as well as express Boolean conditions on, CST objects. To illustrate, assume that the database stores all_desks of type set(DESK*), i.e., a set of references to object of the class DESK. Consider the following CCUBE query which retrieves all desks that would intersect the room area (3 <= u <= 4 && 8 <= v <= 10), if centered at (6,4) and rotated 30 degrees counter-clockwise relatively to to the room's axes. For each retrieved desk, the query also gives the resulting desk's extent in the room's coordinates. 

       SELECT pair(dsk, ((u,v) | dsk_glob) )
       INTO {Bag<CST*>} result
       FROM all_desks AS {DESK*} dsk
       DEFINE dsk_glob AS {CST} dsk->extent && x == 6 && y == 4 &&
                                                         u == x + cos(30) * w + 0.5 * z &&
                                                         v == y + cos(30) * z + 0.5 * w
       WHERE SAT(3 <= u <= 4 && 8 <= v <= 10 && dsk_glob )

In the FROM clause, the query variable dsk iterates over all_desks. Thus, in the DEFINE, WHERE and SELECT clauses the query variable dsk is bound to (a reference to) an object of the class DESK. In the DEFINE statement, dsk->extent is the extent attribute, which is a CST object in free variables w,z. The DEFINE clause in the example defines a CST object dsk_glob as a symbolic constraint expression (in free variables u,v,x,y,w,z), which is then used in the WHERE and the SELECT clauses. 

In the WHERE clause, SAT is a satisfiability test of the area constraint 3 <= u <= 4 && 8 <= v <= 10 conjuncted with dsk_glob, which finds whether dsk in the room's coordinates intersects the area defined by the constraint (3 <= u <= 4 && 8 <= v <= 10). In the SELECT clause, (u,v) | dsk_glob is the projection of dsk_glob on the room's axes u,v.

presentation created by Victor E. Segal