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What is PLSQL ?

PL/SQL is Oracle's procedural language extension to SQL. It provides a server-side, stored procedural language that is easy-to-use, seamless with SQL, robust, portable, and secure. The PL/SQL compiler and interpreter are embedded in Oracle Database, providing developers with a consistent and leveraged development model on both the client and the server side. In addition, PL/SQL stored subprograms can be called from Oracle clients. PL/SQL enables you to mix SQL statements with procedural constructs.
With PL/SQL, you can create and run PL/SQL program units such as procedures, functions, and packages. PL/SQL program units generally are categorized as anonymous blocks, stored subprograms, and packages.

The basic units (procedures, functions, and anonymous blocks) that make up a PL/SQL program can be nested inside one another.

You can place declarations close to where they are used, such as inside a large subprogram. The declarations are local to the block and cease to exist when the block completes, helping to avoid cluttered namespaces for variables and procedures.

You can nest blocks in the executable and exception-handling parts of a PL/SQL block or subprogram, but not in the declarative part. You can define local subprograms in the declarative part of any block. You can call local subprograms only from the block in which they are defined.

Anonymous block
An anonymous block is a PL/SQL block that appears in your application and is not named or stored in the database. In many applications, PL/SQL blocks can appear wherever SQL statements can appear. A PL/SQL block groups related declarations and statements.

Stored or standalone subprogram
A stored or standalone subprogram is a PL/SQL block that Oracle stores in the database and can be called by name from an application. Subprograms can be procedures or functions; the difference is that functions return a value when executed. When you create a stored subprogram, Oracle parses the subprogram and stores its parsed representation in the database. See Subprograms and Packages: Usage Information.

Package
A package is a group of subprograms and variable definitions that Oracle stores in the database. Subprograms and variables in packages can be called from other packages or subprograms.

--Extract from Oracle documentation

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