A primary key is a candidate key that is most appropriate to be the main reference key for the table. As its name suggests, it is the primary key of reference for the table and is used throughout the database to help establish relationships with other tables.
1 A concrete driveway with several decorative elements can cost $12 -$18 a square foot, or $3,600 -$5,400 for 12'x25' or $11,500 -$17,500 for 24'x40'.
Costs typically start at $6 per square foot for a basic patio and range up to $15 or more per sq. ft. for a complex design. To break it down, the cost to build a concrete patio depends on the following needs and requests: 1  size and thickness (4 thick is the norm) 2  desired shape.  amount of reinforcement.