The relational database is the most popular kind of database for application development today. Although it is less "machine efficient" than its legacy counterparts, it is much easier to work with from a development perspective. A relational database is based on collections of tables, each of which has a （ ） —a field(s) whose value is different for every row of the table. The tables are related to each other by the placement of the primary key from one table into the related table as a （ ）. Most relational database management systems (RDBMSs) support （ ）, the idea of ensuring that values linking the tables together through the primary and foreign keys are valid and correctly synchronized.

Structured Query Language (SQL) is the standard language for accessing the data in the （ ）, and it operates on complete tables, as opposed to the individual records in the tables.

Examples of RDBMS software are Microsoft Access, Oracle, DB2, Sybase, Informix, Microsoft SQL Server, and （ ）。