PL / SQL游标

Whenever you try to execute an SQL statement, Oracle processes it in a separate memory zone called as ‘Context Area’. It contains information regarding information retrieved from database tables/records, SQL query, number of rows accessed by it and many more. This Context Area is controlled by a ‘Cursor’. A PL/SQL cursor can hold one or more rows returned by an SQL statement query. The set of rows that a Cursor holds at a single point of time is called as an ‘Active Set’.

A Cursor is basically a pointer to the Context Area. PL/SQL block controls the context area using a cursor. A cursor holds the one or more rows returned by a PL/SQL statement. Cursors and can be named as well as be kept anonymous. Named cursors can help for its reference so that it can be fetched in a program later on and process the rows returned by the SQL statement one by one. 

In Oracle SQL, there are two types of cursors:

1. Implicit cursors
2. Explicit cursors

Implicit Cursors

The Implicit Cursor is the Default Cursor in PL/SQL blocks. These are created when there is no explicit cursor for the statement or when PL/SQL encounters a SELECT statement that returns just one row and also when Data Manipulation Language (DML) statements like DELETE, INSERT and UPDATE are encountered. 

Some of the PL/SQL attributes that are used frequently are mentioned below:

• %ROWCOUNT: It Returns the number of rows influenced by an UPDATE, DELETE or an INSERT statement.
• %NOTFOUND: It Returns TRUE if an UPDATE, DELETE or an INSERT statement influenced zero rows or a SELECT INTO statement returned no rows. Else, it Returns a FALSE.
• %FOUND: It Returns TRUE if an UPDATE, DELETE or an INSERT statement influenced one or more rows or a SELECT INTO statement returned one or more rows. Else, it returns FALSE.
• %ISOPEN: It Returns FALSE for Implicit cursors as Oracle closes the PL/SQL cursor by default after processing its associated PL/SQL statement.

Explicit Cursors

Explicit cursors are used if you need to have better control over the Context Area via Cursor. They should be used when you are executing a SELECT statement query that will return more than one row. Cursors can process one record at a given point of time even though it stores more than one record. An explicit cursor is defined in the declaration section of the PL/SQL Block. Explicit Cursor and Implicit Cursor both have the same output, but accessing is different.

Syntax For An Explicit Cursor

Processing an Explicit Cursor involves the following steps:

1. Initialize the Cursor: Initializing the Cursor also means the Declaration of the cursor in the Declaration Section of a PL/SQL Block. The name of the cursor requires to be defined along with the SELECT Statement.

Example

CURSOR admin_no IS SELECT ano, aname from  Admin;

2. Open the Cursor: Opening the Cursor also means allocating the memory for the Cursor in the Context Area which thereby makes it sufficient to fetch and store records in it.

Example

3. Fetch the Cursor: Fetching the Cursor involves retrieval of data using the Fetch statement. It is used to help the Cursor process and access records or rows at a time. 

Example

FETCH a_admin 
INTO
a_ano, a_aname;

4. Close the Cursor: Once the work associated for a Cursor to be completed is accomplished, it is necessary to release the Allocated Memory of the Cursor to let other tasks occupy memory with any memory down-time.

Example

Program to write a Cursor to list all the Administrators in the Database Table named as Admin

declare
CURSOR c1 IS
SELECT aname from Admin;
rec c1%rowtype;
begin
for rec in c1
loop
dbms_output.put_line(rec.aname);
end loop;
end;
/

Whenever you try to execute an SQL statement, Oracle processes it in a separate memory zone called as ‘Context Area’. It contains information regarding information retrieved from database tables/records, SQL query, number of rows accessed by it and many more. This Context Area is controlled by a ‘Cursor’. A PL/SQL cursor can hold one or more rows returned by an SQL statement query. The set of rows that a Cursor holds at a single point of time is called as an ‘Active Set’.

A Cursor is basically a pointer to the Context Area. PL/SQL block controls the context area using a cursor. A cursor holds the one or more rows returned by a PL/SQL statement. Cursors and can be named as well as be kept anonymous. Named cursors can help for its reference so that it can be fetched in a program later on and process the rows returned by the SQL statement one by one. 

In Oracle SQL, there are two types of cursors:

1. Implicit cursors
2. Explicit cursors

Implicit Cursors

The Implicit Cursor is the Default Cursor in PL/SQL blocks. These are created when there is no explicit cursor for the statement or when PL/SQL encounters a SELECT statement that returns just one row and also when Data Manipulation Language (DML) statements like DELETE, INSERT and UPDATE are encountered. 

Some of the PL/SQL attributes that are used frequently are mentioned below:

• %ROWCOUNT: It Returns the number of rows influenced by an UPDATE, DELETE or an INSERT statement.
• %NOTFOUND: It Returns TRUE if an UPDATE, DELETE or an INSERT statement influenced zero rows or a SELECT INTO statement returned no rows. Else, it Returns a FALSE.
• %FOUND: It Returns TRUE if an UPDATE, DELETE or an INSERT statement influenced one or more rows or a SELECT INTO statement returned one or more rows. Else, it returns FALSE.
• %ISOPEN: It Returns FALSE for Implicit cursors as Oracle closes the PL/SQL cursor by default after processing its associated PL/SQL statement.

Explicit Cursors

Explicit cursors are used if you need to have better control over the Context Area via Cursor. They should be used when you are executing a SELECT statement query that will return more than one row. Cursors can process one record at a given point of time even though it stores more than one record. An explicit cursor is defined in the declaration section of the PL/SQL Block. Explicit Cursor and Implicit Cursor both have the same output, but accessing is different.

Syntax For An Explicit Cursor

Processing an Explicit Cursor involves the following steps:

1. Initialize the Cursor: Initializing the Cursor also means the Declaration of the cursor in the Declaration Section of a PL/SQL Block. The name of the cursor requires to be defined along with the SELECT Statement.

Example

CURSOR admin_no IS SELECT ano, aname from  Admin;

2. Open the Cursor: Opening the Cursor also means allocating the memory for the Cursor in the Context Area which thereby makes it sufficient to fetch and store records in it.

Example

3. Fetch the Cursor: Fetching the Cursor involves retrieval of data using the Fetch statement. It is used to help the Cursor process and access records or rows at a time. 

Example

FETCH a_admin 
 INTO
 a_ano, a_aname;

4. Close the Cursor: Once the work associated for a Cursor to be completed is accomplished, it is necessary to release the Allocated Memory of the Cursor to let other tasks occupy memory with any memory down-time.

Example

Program to write a Cursor to list all the Administrators in the Database Table named as Admin

declare
 CURSOR c1 IS
 SELECT aname from Admin;
 rec c1%rowtype;
 begin
 for rec in c1
 loop
 dbms_output.put_line(rec.aname);
 end loop;
 end;
 /

翻译自: https://www.thecrazyprogrammer.com/2015/07/plsql-cursors.html