This chapter explains SQL DML (data manipulation language), which is used for making queries to a database. Queries specify operations on relations that result in desired information. A combination of relational operations is called a derived operation. SQL DDL (data definition language) is used to create a database. A DML query is a SQL statement or command that is a high-level programming language representation for a relational calculus expression. The relational calculus expression is translated by the SQL optimizer (the query planner) into a composition, or a derived operation, of relational algebra operations that specify how the SQL statement is executed.