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'.
Simple concrete patio costs. A fairly small and simple concrete patio measuring 10 feet by 20 feet costs $1,000 to $2,000 as of 2010, according to the website Cost Helper. Although expensive for such a small area, this price includes professional site preparation, labor and concrete costs.