Tài liệu Oracle PLSQL Language- P6 doc

than a reference or pointer. A constant is nothing more than a value, whereas a variable points to its
value. Similarly, a static cursor acts as a constant, whereas a cursor variable points to a cursor object.
These distinctions are shown in Figure 6.3. Notice that two different cursor variables in different
programs both refer to the same cursor object.
Figure 6.3: The referencing character of cursor variables
Declaration of a cursor variable does not create a cursor object. To do that, you must instead use the
OPEN FOR syntax to create a new cursor object and assign it to the variable.
6.12.4 Opening Cursor Variables
You assign a value (the cursor object) to a cursor when you OPEN the cursor. So the syntax for the
OPEN statement is now modified in PL/SQL Release 2.3 to accept a SELECT statement after the
FOR clause, as shown below:
OPEN cursor_name FOR select_statement;
where cursor_name is the name of a cursor or cursor variable and select_statement is a SQL SELECT
statement.
For strong REF CURSOR type cursor variables, the structure of the SELECT statement (the number
and datatypes of the columns) must match or be compatible with the structure specified in the
RETURN clause of the type statement.
Figure 6.4 offers an example of the kind of compatibility
required.
Figure 6.4" contains the full set of compatibility rules.
Figure 6.4: Compatible REF CURSOR rowtype and SELECT list
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If cursor_name is a cursor variable defined with a weak REF CURSOR type, you can OPEN it for
any query, with any structure. In the following example, I open (assign a value to) the cursor variable
twice, with two different queries:
DECLARE
TYPE emp_curtype IS REF CURSOR;
emp_curvar emp_curtype;
BEGIN
OPEN emp_curvar FOR SELECT * FROM emp;
OPEN emp_curvar FOR SELECT employee_id FROM emp;
OPEN emp_curvar FOR SELECT company_id, name FROM
company;
END;
That last open didn't even have anything to do with the employee table!
If the cursor variable has not yet been assigned to any cursor object, the OPEN FOR statement
implicitly creates an object for the variable.
If at the time of the OPEN the cursor variable already is pointing to a cursor object, then OPEN FOR
does not create a new object. Instead, it reuses the existing object and attaches a new query to that
object. The cursor object is maintained separately from the cursor or query itself.
6.12.5 Fetching from Cursor Variables
As mentioned earlier, the syntax for a FETCH statement using a cursor variable is the same as that
for static cursors:
FETCH <cursor variable name> INTO <record name>;
FETCH <cursor variable name> INTO <variable name>,
<variable name> ;
When the cursor variable was declared with a strong REF CURSOR type, the PL/SQL compiler
makes sure that the data structure(s) listed after the INTO keyword are compatible with the structure
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of the query associated with cursor variable.
6.12.5.1 Strong and weak REF CURSOR types
If the cursor variable is of the weak REF CURSOR type, the PL/SQL compiler cannot perform the
same kind of check. Such a cursor variable can FETCH into any data structures, because the REF
CURSOR type it is not identified with a rowtype at the time of declaration. At compile time, there is
no way to know which cursor object (and associated SQL statement) will be assigned to that variable.
Consequently, the check for compatibility must happen at run time, when the FETCH is about to be
executed. At this point, if the query and the INTO clause do not structurally match (and PL/SQL will
use implicit conversions if necessary and possible), then the PL/SQL runtime engine will raise the
predefined ROWTYPE_MISMATCH exception.
6.12.5.2 Handling the ROWTYPE_MISMATCH exception
Before PL/SQL actually performs its FETCH, it checks for compatibility. As a result, you can trap
the ROWTYPE_MISMATCH exception and attempt to FETCH from the cursor variable using a
different INTO clause and you will not have skipped any rows in the result set.
Even though you are executing a second FETCH statement in your program, you will still retrieve the
first row in the result set of the cursor object's query. This functionality comes in especially handy for
weak REF CURSOR types.
In the following example, a centralized real estate database stores information about properties in a
variety of tables, one for homes, another for commercial properties, etc. There is also a single, central
table which stores an address and a building type (home, commercial, etc.). I use a single procedure
to open a weak REF CURSOR variable for the appropriate table, based on the street address. Each
individual real estate office can then call that procedure to scan through the matching properties:
1. Define my weak REF CURSOR type:
TYPE building_curtype IS REF CURSOR;
2. Create the procedure. Notice that the mode of the cursor variable parameter is IN OUT:
PROCEDURE open_site_list
(address_in IN VARCHAR2,
site_cur_inout IN OUT building_curtype)
IS
home_type CONSTANT INTEGER := 1;
commercial_type CONSTANT INTEGER := 2;
/* A static cursor to get building type. */
CURSOR site_type_cur IS
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SELECT site_type FROM property_master
WHERE address = address_in;
site_type_rec site_type_cur%ROWTYPE;
BEGIN
/* Get the building type for this address. */
OPEN site_type_cur;
FETCH site_type_cur INTO site_type_rec;
CLOSE site_type_cur;
/* Now use the site type to select from the right
table.*/
IF site_type_rec.site_type = home_type
THEN
/* Use the home properties table. */
OPEN site_cur_inout FOR
SELECT * FROM home_properties
WHERE address LIKE '%' || address_in || '%';
ELSIF site_type_rec.site_type = commercial_type
THEN
/* Use the commercial properties table. */
OPEN site_cur_inout FOR
SELECT * FROM commercial_properties
WHERE address LIKE '%' || address_in || '%';
END IF;
END open_site_list;
3. Now that I have my open procedure, I can use it to scan properties.
In the following example, I pass in the address and then try to fetch from the cursor, assuming a home
property. If the address actually identifies a commercial property, PL/SQL will raise the
ROWTYPE_MISMATCH exception (incompatible record structures). The exception section then
fetches again, this time into a commercial building record, and the scan is complete.[
2]
[2] The "prompt" and "show" programs referenced in the example interact with users
and are not documented here.
DECLARE
/* Declare a cursor variable. */
building_curvar building_curtype;
/* Define record structures for two different tables.
*/
home_rec home_properties%ROWTYPE;
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commercial_rec commercial_properties%ROWTYPE;
BEGIN
/* Get the address from the user. */
prompt_for_address (address_string);
/* Assign a query to the cursor variable based on the
address. */
open_site_list (address_string, building_curvar);
/* Give it a try! Fetch a row into the home record. */
FETCH building_curvar INTO home_rec;
/* If I got here, the site was a home, so display it.
*/
show_home_site (home_rec);
EXCEPTION
/* If the first record was not a home */
WHEN ROWTYPE_MISMATCH
THEN
/* Fetch that same 1st row into the commercial
record. */
FETCH building_curvar INTO commercial_rec;
/* Show the commercial site info. */
show_commercial_site (commercial_rec);
END;
6.12.6 Rules for Cursor Variables
This section examines in more detail the rules and issues regarding the use of cursor variables in your
programs. This includes rowtype matching rules, cursor variable aliases, and scoping issues.
Remember that the cursor variable is a reference to a cursor object or query in the database. It is not
the object itself. A cursor variable is said to "refer to a given query" if either of the following is true:
● An OPEN statement FOR that query was executed with the cursor variable.
● A cursor variable was assigned a value from another cursor variable that refers to that query.
You can perform assignment operations with cursor variables and also pass these variables as
arguments to procedures and functions. In order to perform such actions between cursor variables
(and to bind a cursor variable to a parameter), the different cursor variables must follow a set of
compile-time and runtime rowtype matching rules.
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6.12.6.1 Compile-time rowtype matching rules
These are the rules that PL/SQL follows at compile-time:
● Two cursor variables (including procedure parameters) are compatible for assignments and
argument passing if any of the following are true:
❍ Both variables (or parameters) are of a strong REF CURSOR type with the same
<rowtype_name>.
❍ Both variables (or parameters) are of some weak REF CURSOR type, regardless of the
<rowtype_name>.
❍ One variable (parameter) is of any strong REF CURSOR type, and the other is of any
weak REF CURSOR type.
● A cursor variable (parameter) of a strong REF CURSOR type may be OPEN FOR a query that
returns a rowtype which is structurally equal to the <rowtype_name> in the original type
declaration.
● A cursor variable (parameter) of a weak REF CURSOR type may be OPEN FOR any query.
The FETCH from such a variable is allowed INTO any list of variables or record structure.
In other words, if either of the cursor variables are of the weak REF CURSOR type, then the PL/SQL
compiler cannot really validate whether the two different cursor variables will be compatible. That
will happen at runtime; the rules are covered in the next section.
6.12.6.2 Run-time rowtype matching rules
These are the rules that PL/SQL follows at run time:
● A cursor variable (parameter) of a weak REF CURSOR type may be made to refer to a query
of any rowtype regardless of the query or cursor object to which it may have referred earlier.
● A cursor variable (parameter) of a strong REF CURSOR type may be made to refer only to a
query which matches structurally the <rowtype_name> of the RETURN clause of the REF
CURSOR type declaration.
● Two records (or lists of variables) are considered structurally matching with implicit
conversions if both of the following are true:
❍ The number of fields is the same in both records (lists).
❍ For each field in one record (or variable on one list), a corresponding field in the
second list (or variable in second list) has the same PL/SQL datatype, or one which can
be converted implicitly by PL/SQL to match the first.
● For a cursor variable (parameter) used in a FETCH statement, the query associated with the
cursor variable must structurally match with implicit conversions the record or list of variables
of the INTO clause of the FETCH statement. This is, by the way, the same rule used for static
cursors.
6.12.6.3 Cursor variable aliases
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If you assign one cursor variable to another cursor variable, those two cursor variables become
aliases for the same cursor object. They share the reference to the cursor object (result set of the
cursor's query). An action taken against the cursor object through one variable is also available to and
reflected in the other variable.
The following anonymous block illustrates the way cursor aliases work:
1 DECLARE
2 TYPE curvar_type IS REF CURSOR;
3 curvar1 curvar_type;
4 curvar2 curvar_type;
5 story fairy_tales%ROWTYPE;
6 BEGIN
7 /* Assign cursor object to curvar1. */
8 OPEN curvar1 FOR SELECT * FROM fairy_tales;
9
10 /* Assign same cursor object to curvar2. */
11 curvar2 := curvar1;
12
13 /* Fetch first record from curvar1. */
14 FETCH curvar1 INTO story;
15
16 /* Fetch second record from curvar2. */
17 FETCH curvar2 INTO story;
18
19 /* Close the cursor object by referencing curvar2.
*/
20 CLOSE curvar2;
21
22 /* This statement raises INVALID_CURSOR exception!
*/
23 FETCH curvar1 INTO story;
24 END;
The following table is an explanation of cursor variable actions.
Lines Action
1-5 Declare my weak REF CURSOR type and cursor variable through line 5.
8 Creates a cursor object and assigns it to curvar1.
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11 Assigns that same cursor object to the second cursor variable, curvar2.
14 Fetches the first record using the curvar1 variable.
17 Fetches the second record using the curvar2 variable. (Notice that it doesn't matter which of
the two variables you use. The pointer to the current record resides with the cursor object,
not any particular variable.)
20 Closes the cursor object referencing curvar2.
23 Raises the INVALID_CURSOR exception when I try to fetch again from the cursor object.
(When I closed the cursor through curvar2, it also closed it as far as curvar1 was
concerned.)
Any change of state in a cursor object will be seen through any cursor variable which is an alias to
that cursor object.
6.12.6.4 Scope of cursor object
The scope of a cursor variable is the same as that of a static cursor: the PL/SQL block in which the
variable is declared (unless declared in a package, which makes the variable globally accessible). The
scope of the cursor object to which a cursor variable is assigned, however, is a different matter.
Once an OPEN FOR creates a cursor object, that cursor object remains accessible as long as at least
one active cursor variable refers to that cursor object. This means that you can create a cursor object
in one scope (PL/SQL block) and assign it to a cursor variable. Then, by assigning that cursor
variable to another cursor variable with a different scope, the cursor object remains accessible even if
the original cursor variable has gone out of scope.
In the following example I use nested blocks to demonstrate how the cursor object can persist outside
of the scope in which it was originally created:
DECLARE
/* Define weak REF CURSOR type, cursor variable
and local variable */
TYPE curvar_type IS REF CURSOR;
curvar1 curvar_type;
do_you_get_it VARCHAR2(100);
BEGIN
/*
|| Nested block which creates the cursor object and
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|| assigns it to the curvar1 cursor variable.
*/
DECLARE
curvar2 curvar_type;
BEGIN
OPEN curvar2 FOR SELECT punch_line FROM jokes;
curvar1 := curvar2;
END;
/*
|| The curvar2 cursor variable is no longer active,
|| but "the baton" has been passed to curvar1, which
|| does exist in the enclosing block. I can therefore
|| fetch from the cursor object, through this other
|| cursor variable.
*/
FETCH curvar1 INTO do_you_get_it;
END;
6.12.7 Passing Cursor Variables as Arguments
You can pass a cursor variable as an argument in a call to a procedure or function. When you use a
cursor variable in the parameter list of a program, you need to specify the mode of the parameter and
the datatype (the REF CURSOR type).
6.12.7.1 Identifying the REF CURSOR type
In your program header, you must identify the REF CURSOR type of your cursor variable parameter.
To do this, that cursor type must already be defined.
If you are creating a local module within another program (see
Chapter 15 for more information
about local modules), then you can also define the cursor type in the same program. It will then be
available for the parameter. This approach is shown below:
DECLARE
/* Define the REF CURSOR type. */
TYPE curvar_type IS REF CURSOR RETURN company%ROWTYPE;
/* Reference it in the parameter list. */
PROCEDURE open_query (curvar_out OUT curvar_type)
IS
local_cur curvar_type;
BEGIN
OPEN local_cur FOR SELECT * FROM company;
curvar_out := local_cur;
END;
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BEGIN

END;
If you are creating a standalone procedure or function, then the only way you can reference a pre-
existing REF CURSOR type is by placing that type statement in a package. All variables declared in
the specification of a package act as globals within your session, so you can then reference this cursor
type using the dot notation as shown below:
1. Create the package with a REF CURSOR type declaration:
PACKAGE company
IS
/* Define the REF CURSOR type. */
TYPE curvar_type IS REF CURSOR RETURN company%
ROWTYPE;
END package;
2. In a standalone procedure, reference the REF CURSOR type by prefacing the name of the
cursor type with the name of the package:
PROCEDURE open_company (curvar_out OUT company.
curvar_type) IS
BEGIN

END;
See
Chapter 16 for more information on this feature.
6.12.7.2 Setting the parameter mode
Just like other parameters, a cursor variable argument can have one of the following three modes:
IN
Can only be read by program
OUT
Can only be written to by program
IN OUT
Read/write in program
Remember that the value of a cursor variable is the reference to the cursor object and not the state of
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