Explain the concept of cursors in PL/SQL and their types.

In PL/SQL (Procedural Language/Structured Query Language), a cursor is a database object used to retrieve and manipulate data from result sets returned by SQL statements. Cursors provide a mechanism for sequential processing of query results within a PL/SQL block. Cursors can be classified into two types: implicit cursors and explicit cursors. Let's delve into each type of cursor in-depth:

1. Implicit Cursors:

* Implicit cursors are automatically created by the PL/SQL engine for SQL statements that are not associated with explicit cursor declarations.
* Implicit cursors are used when executing SQL statements that return a single row or a single value, such as SELECT INTO statements or SQL statements within a FOR loop.
* The attributes of an implicit cursor, such as %FOUND, %NOTFOUND, %ROWCOUNT, and %ISOPEN, can be used to check the status of the cursor and perform conditional processing based on the query result.
* Implicit cursors are managed internally by the PL/SQL engine and do not require explicit declaration, opening, or closing.
2. Explicit Cursors:

* Explicit cursors are explicitly declared and used when executing SQL statements that return multiple rows or when more control is needed over the processing of query results.
* Explicit cursors require explicit declaration, opening, fetching, and closing operations within the PL/SQL block.
* The explicit cursor declaration involves defining a cursor variable, specifying the SQL query associated with the cursor, and optionally specifying parameters or variables to be used in the query.
* The cursor is then opened using the OPEN statement, which executes the SQL query and makes the result set available for processing.
* The rows of the result set are fetched one by one using the FETCH statement, and the retrieved data can be processed within a loop or stored in variables for further manipulation.
* The cursor is closed using the CLOSE statement once all the required rows have been fetched or when it is no longer needed.
* Explicit cursors provide greater control over the processing of query results, allowing operations such as bulk fetching, scrolling through result sets, and error handling through exception handling.

Explicit cursors can further be categorized into the following types based on their behavior:

a. Non-Scrollable or Forward-Only Cursors:
- Non-scrollable cursors are the most basic type of explicit cursor.
- They allow fetching rows in a forward-only manner, sequentially from the first row to the last row.
- Once a row is fetched, it cannot be re-fetched or accessed again.
- Non-scrollable cursors are suitable when data is processed in a linear fashion, and there is no need to revisit or randomly access rows.

b. Scrollable Cursors:
- Scrollable cursors provide additional capabilities to move the cursor position back and forth within the result set.
- They allow fetching rows in any order, both forward and backward, and support random access to rows.
- Scrollable cursors provide flexibility when navigating large result sets, implementing pagination, or when the application requires access to rows in a non-sequential manner.

c. Parameterized Cursors:
- Parameterized cursors allow passing parameters to the SQL query associated with the cursor.
- The query can have placeholders for parameters, and the values are passed dynamically at runtime.
- Parameterized cursors enable the reuse of cursor declarations with different parameter values, improving code maintainability and reducing redundancy.

Using cursors in PL/SQL, developers can efficiently retrieve, process, and manipulate data returned by SQL queries. Implicit cursors are automatically created for single-row or single-value queries, while explicit cursors provide more control and flexibility for handling multiple rows or complex result sets. Understanding the different types of cursors and their behavior allows developers to choose