SQL, a Structured Query Language is now available with most RDBMS (Relational Database Management System) products as the database language. Which can be used both by end-users and system programmers.
Originally, SQL was called SEQUEL( for Structured English QUEry Language).
SQL is now standard language for commercial relational DBMS.
First reason, a query in relational algebra is written as a sequence of operations that, after execution, produce the required result. Hence, the user must specify how –that is, in what order – to execute the query operations.
On the other hand, the SQL provides a high-level declarative language interface, so the user only specifies what the result is to be, leaving the actual optimization and decision on how to execute the query to the DBMS.