SQL Data Access

 There are two broad families of computer languages: declarative languages that are nonprocedural and describe what should be done, and procedural languages such as C++ and Java that describe how things should be done.

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SQL is declarative in the sense that users specify the result that they want, not how to derive it. For example, the following statement queries records for employees whose last name begins with K:

The database performs the work of generating a procedure to navigate the data and retrieve the requested results. The declarative nature of SQL enables you to work with data at the logical level. You need be concerned with implementation details only when you manipulate the data.

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SELECT   last_name, first_name
FROM     hr.employees
WHERE    last_name LIKE 'K%'
ORDER BY last_name, first_name;

The database retrieves all rows satisfying the WHERE condition, also called the predicate, in a single step. The database can pass these rows as a unit to the user, to another SQL statement, or to an application. The application does not need to process the rows one by one, nor does the developer need to know how the rows are physically stored or retrieved.

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All SQL statements use the optimizer, a component of the database that determines the most efficient means of accessing the requested data. Oracle Database also supports techniques that you can use to make the optimizer perform its job better.

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