Oracle显式游标Explicit Cursor
在Oracle数据库中，游标（Cursor）是一个数据库查询的效果集的引用。它答应你逐行地处理查询效果，而不是一次性地处理整个效果集。游标分为隐式游标和显式游标两种。显式游标（Explicit Cursor）
显式游标是你在PL/SQL步调中明确声明和操作的游标。当你需要对查询效果进行更复杂的处理时，好比逐行处理查询效果或者需要获取查询的某些状态信息（如是否找到行），就需要使用显式游标。
显式游标的声明
显式游标在PL/SQL中通过CURSOR关键字进行声明，并通常指定一个游标名称和一个查询语句。例如：
DECLARE
CURSOR employee_cur IS
SELECT employee_id, last_name FROM employees;
BEGIN
-- 游标的使用将在这里进行
END;
显式游标的打开、获取和关闭
[*] 打开游标：使用OPEN语句打开游标，并执行其定义的查询。
[*] 获取游标数据：通常使用FETCH INTO语句从游标中获取数据行，并将其放入PL/SQL变量中。这可以在循环中完成，以逐行处理查询效果。
[*] 关闭游标：使用CLOSE语句关闭游标。关闭游标后，你将不能再从它那边获取数据。
示例1 LOOP … EXIT WHEN … END LOOP
下面是一个使用显式游标逐行处理employees表中数据的例子：
DECLARE
CURSOR employee_cur IS
SELECT employee_id, last_name FROM employees;
v_employee_id employees.employee_id%TYPE;
v_last_name employees.last_name%TYPE;
BEGIN
OPEN employee_cur;
LOOP
FETCH employee_cur INTO v_employee_id, v_last_name;
EXIT WHEN employee_cur%NOTFOUND;
DBMS_OUTPUT.PUT_LINE('Employee ID: ' || v_employee_id || ', Last Name: ' || v_last_name);
END LOOP;
CLOSE employee_cur;
END;
在这个例子中，employee_cur是一个显式游标，它遍历employees表中的全部行。对于表中的每一行，我们都将employee_id和last_name列的值提取到变量中，并使用DBMS_OUTPUT.PUT_LINE输出这些值。当游标没有更多行可以提取时（即employee_cur%NOTFOUND为TRUE），循环竣事，并且游标被关闭。
示例2 Implicit Cursor FOR LOOP Statement
In this example, an implicit cursor FOR LOOP statement prints the last name and job ID of every clerk whose manager has an ID greater than 120.
BEGIN
FOR item IN (
SELECT last_name, job_id
FROM employees
WHERE job_id LIKE '%CLERK%'
AND manager_id > 120
ORDER BY last_name
)
LOOP
DBMS_OUTPUT.PUT_LINE
('Name = ' || item.last_name || ', Job = ' || item.job_id);
END LOOP;
END;
/
-- OR
DECLARE
CURSOR c1 IS
SELECT last_name, job_id FROM employees
WHERE job_id LIKE '%CLERK%' AND manager_id > 120
ORDER BY last_name;
BEGIN
FOR item IN c1
LOOP
DBMS_OUTPUT.PUT_LINE
('Name = ' || item.last_name || ', Job = ' || item.job_id);
END LOOP;
END;
/
Result:
Name = Atkinson, Job = ST_CLERK
Name = Bell, Job = SH_CLERK
Name = Bissot, Job = ST_CLERK
...
Name = Walsh, Job = SH_CLERK
示例3 Passing Parameters to Explicit Cursor FOR LOOP Statement
This example declares and defines an explicit cursor that accepts two parameters, and then uses it in an explicit cursor FOR LOOP statement to display the wages paid to employees who earn more than a specified wage in a specified department.
DECLARE
CURSOR c1 (job VARCHAR2, max_wage NUMBER) IS
SELECT * FROM employees
WHERE job_id = job
AND salary > max_wage;
BEGIN
FOR person IN c1('ST_CLERK', 3000)
LOOP
-- process data record
DBMS_OUTPUT.PUT_LINE (
'Name = ' || person.last_name || ', salary = ' ||
person.salary || ', Job Id = ' || person.job_id
);
END LOOP;
END;
/
Result:
Name = Nayer, salary = 3200, Job Id = ST_CLERK
Name = Bissot, salary = 3300, Job Id = ST_CLERK
Name = Mallin, salary = 3300, Job Id = ST_CLERK
Name = Ladwig, salary = 3600, Job Id = ST_CLERK
Name = Stiles, salary = 3200, Job Id = ST_CLERK
Name = Rajs, salary = 3500, Job Id = ST_CLERK
Name = Davies, salary = 3100, Job Id = ST_CLERK
示例4 Cursor FOR Loop References Virtual Columns
In this example, the implicit cursor FOR LOOP references virtual columns by their aliases, full_name and dream_salary.
BEGIN
FOR item IN (
SELECT first_name || ' ' || last_name AS full_name,
salary * 10 AS dream_salary
FROM employees
WHERE ROWNUM <= 5
ORDER BY dream_salary DESC, last_name ASC
) LOOP
DBMS_OUTPUT.PUT_LINE
(item.full_name || ' dreams of making ' || item.dream_salary);
END LOOP;
END;
/
Result:
Stephen King dreams of making 240000
Lex Garcia dreams of making 170000
Neena Yang dreams of making 170000
Alexander James dreams of making 90000
Bruce Miller dreams of making 60000
示例5 Subquery in FROM Clause of Parent Query
This example defines explicit cursor c1 with a query whose FROM clause contains a subquery.
DECLARE
CURSOR c1 IS
SELECT t1.department_id, department_name, staff
FROM departments t1,
( SELECT department_id, COUNT(*) AS staff
FROM employees
GROUP BY department_id
) t2
WHERE (t1.department_id = t2.department_id) AND staff >= 5
ORDER BY staff;
BEGIN
FOR dept IN c1
LOOP
DBMS_OUTPUT.PUT_LINE ('Department = '
|| dept.department_name || ', staff = ' || dept.staff);
END LOOP;
END;
/
Result:
Department = IT, staff = 5
Department = Finance, staff = 6
Department = Purchasing, staff = 6
Department = Sales, staff = 34
Department = Shipping, staff = 45
示例6 FETCH Statements Inside LOOP Statements
DECLARE
CURSOR c1 IS
SELECT last_name, job_id FROM employees
WHERE REGEXP_LIKE (job_id, 'S_CLERK')
ORDER BY last_name;
v_lastnameemployees.last_name%TYPE;-- variable for last_name
v_jobid employees.job_id%TYPE; -- variable for job_id
CURSOR c2 IS
SELECT * FROM employees
WHERE REGEXP_LIKE (job_id, '_M')
ORDER BY job_id;
v_employees employees%ROWTYPE;-- record variable for row of table
BEGIN
OPEN c1;
LOOP-- Fetches 2 columns into variables
FETCH c1 INTO v_lastname, v_jobid;
EXIT WHEN c1%NOTFOUND;
DBMS_OUTPUT.PUT_LINE( RPAD(v_lastname, 25, ' ') || v_jobid );
END LOOP;
CLOSE c1;
DBMS_OUTPUT.PUT_LINE( '-------------------------------------' );
OPEN c2;
LOOP-- Fetches entire row into the v_employees record
FETCH c2 INTO v_employees;
EXIT WHEN c2%NOTFOUND;
DBMS_OUTPUT.PUT_LINE( RPAD(v_employees.last_name, 25, ' ') ||
v_employees.job_id );
END LOOP;
CLOSE c2;
END;
/
Result:
Atkinson ST_CLERK
Bell SH_CLERK
Bissot ST_CLERK
...
Walsh SH_CLERK
-------------------------------------
Higgins AC_MGR
Gruenberg FI_MGR
Martinez MK_MAN
...
Errazuriz SA_MAN
示例7 Fetching Data with Cursor Variables
This example uses one cursor variable to do what Example 6 does with two explicit cursors. The first OPEN FOR statement includes the query itself. The second OPEN FOR statement references a variable whose value is a query.
DECLARE
cv SYS_REFCURSOR;-- cursor variable
v_lastnameemployees.last_name%TYPE;-- variable for last_name
v_jobid employees.job_id%TYPE; -- variable for job_id
query_2 VARCHAR2(200) :=
'SELECT * FROM employees
WHERE REGEXP_LIKE (job_id, ''_M'')
ORDER BY job_id';
v_employees employees%ROWTYPE;-- record variable row of table
BEGIN
OPEN cv FOR
SELECT last_name, job_id FROM employees
WHERE REGEXP_LIKE (job_id, 'S_CLERK')
ORDER BY last_name;
LOOP-- Fetches 2 columns into variables
FETCH cv INTO v_lastname, v_jobid;
EXIT WHEN cv%NOTFOUND;
DBMS_OUTPUT.PUT_LINE( RPAD(v_lastname, 25, ' ') || v_jobid );
END LOOP;
DBMS_OUTPUT.PUT_LINE( '-------------------------------------' );
OPEN cv FOR query_2;
LOOP-- Fetches entire row into the v_employees record
FETCH cv INTO v_employees;
EXIT WHEN cv%NOTFOUND;
DBMS_OUTPUT.PUT_LINE( RPAD(v_employees.last_name, 25, ' ') ||
v_employees.job_id );
END LOOP;
CLOSE cv;
END;
/
Result:
Atkinson ST_CLERK
Bell SH_CLERK
Bissot ST_CLERK
...
Walsh SH_CLERK
-------------------------------------
Higgins AC_MGR
Gruenberg FI_MGR
Martinez MK_MAN
...
Errazuriz SA_MAN
示例8 Fetching from Cursor Variable into Collections
This example fetches from a cursor variable into two collections (nested tables), using the BULK COLLECT clause of the FETCH statement.
DECLARE
TYPE empcurtyp IS REF CURSOR;
TYPE namelist IS TABLE OF employees.last_name%TYPE;
TYPE sallist IS TABLE OF employees.salary%TYPE;
emp_cvempcurtyp;
names namelist;
sals sallist;
BEGIN
OPEN emp_cv FOR
SELECT last_name, salary FROM employees
WHERE job_id = 'SA_REP'
ORDER BY salary DESC;
FETCH emp_cv BULK COLLECT INTO names, sals;
CLOSE emp_cv;
-- loop through the names and sals collections
FOR i IN names.FIRST .. names.LAST
LOOP
DBMS_OUTPUT.PUT_LINE
('Name = ' || names(i) || ', salary = ' || sals(i));
END LOOP;
END;
/
Result:
Name = Ozer, salary = 11500
Name = Abel, salary = 11000
Name = Vishney, salary = 10500
...
Name = Kumar, salary = 6100
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