CURSOR Expressions

CURSOR Expressions

A CURSOR expression returns a nested cursor. This form of expression is equivalent to the PL/SQL REF CURSOR and can be passed as a REF CURSOR argument to a function.

CRUSOR表达式返回一个嵌套的CUROS。这种格式的表达式相当于PL/SQL中的REF CURSOR，并且可以作为 一个REF CURSOR类型的参数传给函数。



A nested cursor is implicitly opened when the cursor expression is evaluated. For example, if the cursor expression appears in a select list, a nested cursor will be opened for each row fetched by the query. The nested cursor is closed only when:

当对cursor表达式求值时，嵌套cursor会被隐式的打开。比如，cursor表达式出现在select语句中，查询语句获取的每一行数据都会打开一个嵌套的cursor。嵌套cursor在以下情况下会被关闭：

The nested cursor is explicitly closed by the user

嵌套cursor被用户显示的关闭

The parent cursor is reexecuted

父cursor被重新执行

The parent cursor is closed

父cursor被关闭

The parent cursor is cancelled

父cursor被取消

An error arises during fetch on one of its parent cursors (it is closed as part of the clean-up)

获取父cursor时出错（嵌套cursor被闭是清理的一部分）

Restrictions on CURSOR
Expressions

Cursor表达式的限制

The following restrictions apply to CURSOR expressions:

Cursor表达式有以下限制：

If the enclosing statement is not a SELECT statement, then nested cursors can appear only as REF CURSOR arguments of a procedure.

如果外层语句不是select语句，嵌套cursor只能作为存储过程的REF CURSOR类型的参数被使用（以处应该也包含function）。

If the enclosing statement is a SELECT statement, then nested cursors can also appear in the outermost select list of the query specification or in the outermost select list of another nested cursor.

如果外层语句是select语句，嵌套cursor可以出现在查询语句的最外层，或是另一个嵌套cursor中的查询语句的最外层。

Nested cursors cannot appear in views.

嵌套cursor不能出现在视图中。

You cannot perform BIND and EXECUTE operations on nested cursors.

不能在在嵌套cursor上执行BIND和EXECUTE。

Examples The
following example shows the use of a CURSOR expression in the select list of a query:

以下事例演示了cursor表达式在select语句的用法：

SELECT department_name, CURSOR(SELECT salary, commission_pct
FROM employees e
WHERE e.department_id = d.department_id)
FROM departments d
ORDER BY department_name;

The next example shows the use of a CURSOR expression as a function argument. The example begins by creating a function in the sample OE schema that can accept theREF CURSOR argument. (The PL/SQL function body is shown in italics.)

接下来的事例演示了cursor作为函数参数的用法。此事例以在OE用户下创建一个可以接收REF CURSOR类型参数的函数开始（PL/SQL函数体以斜体展示）

CREATE FUNCTION f(cur SYS_REFCURSOR, mgr_hiredate DATE)
RETURN NUMBER IS
emp_hiredate DATE;
before number :=0;
after number:=0;
begin
loop
fetch cur into emp_hiredate;
exit when cur%NOTFOUND;
if emp_hiredate > mgr_hiredate then
after:=after+1;
else
before:=before+1;
end if;
end loop;
close cur;
if before > after then
return 1;
else
return 0;
end if;
end;
/

The function accepts a cursor and a date. The function expects the cursor to be a query returning a set of dates. The following query uses the function to find those managers in the sample employees table, most of whose employees were hired before the manager.

此函数有两个参数，一个cursor类型，一个日期类型。函数要求cursor返回一组日期。以下的查询使用上面的函数在employees表中查找大部分下属入取时间比自己早的经理的名字。

SELECT e1.last_name FROM employees e1
WHERE f(
CURSOR(SELECT e2.hire_date FROM employees e2
WHERE e1.employee_id = e2.manager_id),
e1.hire_date) = 1
ORDER BY last_name;

LAST_NAME
-------------------------
Cambrault
Higgins
Hunold
Kochhar
Mourgos
Zlotkey