5 - What Is SQL?

Abstract

This chapter discusses the structured query language (SQL). SQL is the language used to access data from a relational database such as Oracle Database. SQL was originally devised as a computer language with the development of the IBM relational database called System R. The idea of SQL originates from the relational data model. SQL is used to access subsets of data, in the form of rows or tuples from a larger data set. A data set is represented by a related group of entities, in a data model of a set of tables, in an Oracle Database schema. Oracle Database classifies and separates SQL statements into two distinct groups. These two groups are data manipulation language (DML) statements and data definition language (DDL) statements. There are two differences between DML and DDL statements. DML statements manipulate data. A DML statement changes column values in rows, in tables. The INSERT, UPDATE, DELETE, and MERGE statements are by definition DML statements because they change data. The SELECT and TRUNCATE statements are not DML statements. The SELECT statement is not a DML statement because it does not change data. The SELECT statement also does not produce rollback or redo log entries. The TRUNCATE statement, on the other hand, does change data but automatically executes a COMMIT statement. The TRUNCATE statement is therefore a DDL statement.