PLSQL——04、游标

dreamhua 2020-01-31

隐式游标

隐式游标的属性:

  • SQL%ROWCOUNT 成功操作的行的数量
  • SQL%FOUND 发现复合条件的行返回TRUE
  • SQL%NOTFOUND 没有发现复合条件的行回TRUE
  • SQL%ISOPEN 游标打开状态(boolean)

演示:打印隐式游标属性

declare
  v_count number;
begin
  select count(*) into v_count from scott.emp;
  dbms_output.put_line(chr(10)||‘select return ‘||sql%rowcount||‘ rows!‘);
end;
/

begin
  delete emp;
  dbms_output.put_line(chr(10)||sql%rowcount||‘ rows deleted!‘); 
  rollback; 
end;
/

显式游标

显式游标使用流程:

  1. 声明 declare:定义cursor c1 is select ename,sal from emp;
  2. 打开 open:打开后才会执行上述查询语句
  3. 获取 fetch:由内存中逐行获取,获取一行少一行
  4. 关闭 close:没有关闭会一直占着内存

练习 1: 基本loop循环+显示游标的使用

DECLARE
  v_empno employees.employee_id%TYPE;
  v_ename employees.last_name%TYPE;
  CURSOR emp_cursor IS SELECT employee_id, last_name FROM employees; --声明
BEGIN
  OPEN emp_cursor; --打开
  LOOP
    FETCH emp_cursor INTO v_empno, v_ename; --获取
    exit when emp_cursor%rowcount>20;
    DBMS_OUTPUT.PUT_LINE (TO_CHAR(v_empno)||‘ ‘|| v_ename);
  END LOOP;
  CLOSE emp_cursor; --关闭
END ;
/

练习 2: for循环+显示游标的使用

DECLARE
  v_empno employees.employee_id%TYPE;
  v_ename employees.last_name%TYPE;
  CURSOR emp_cursor IS SELECT employee_id, last_name FROM employees;
BEGIN
  OPEN emp_cursor;
  FOR i IN 1..10 LOOP        --此时i为数字类型
    FETCH emp_cursor INTO v_empno, v_ename;
    DBMS_OUTPUT.PUT_LINE (TO_CHAR(v_empno)||‘ ‘|| v_ename);
  END LOOP;
  CLOSE emp_cursor;
END ;
/

游标for循环: 可以省略打开,获取和关闭操作。

declare 
  cursor emp_cursor is select rownum,employee_id,last_name from employees; --声明
begin
  for emp_record in emp_cursor loop --隐式打开隐式获取        此时i为record类型
    exit when emp_cursor%rowcount>23;
      dbms_output.put_line(emp_record.rownum||‘ ‘||
                           emp_record.employee_id||‘ ‘||
                           emp_record.last_name);
  end loop; --隐式关闭
end;
/

省略游标定义

可以省略游标的定义,在for....in中使用查询。但不能使用游标控制,如%rowcount等,因为没有定义游标名字。所以要使用游标的属性,就不要缺少定义。

begin
  for r in (select last_name from employees) loop
      dbms_output.put_line(r.last_name);
  end loop;
end;
/

高级显式游标(带参数的游标)

练习 1:通过传入不同的参数使打开游标时取到不同的结果集

declare
  cursor c1 (p_deptno number,p_job varchar2)
  is
  select empno,ename
  from emp
  where deptno=p_deptno
  and job=p_job;
begin
  Dbms_output.put_line(‘first fetch cursor!‘);
  for r_c1 in c1(10,‘MANAGER‘) loop   --open cursor时传入不同的实际参数得到不同的游标上下文!
    Dbms_output.put_line(r_c1.empno||‘ ‘||r_c1.ename);
  end loop;

  Dbms_output.put_line(‘second fetch cursor!‘);
  for r_c1 in c1(20,‘MANAGER‘) loop
    Dbms_output.put_line(r_c1.empno||‘ ‘||r_c1.ename);
  end loop;

  Dbms_output.put_line(‘third fetch cursor!‘);
  for r_c1 in c1(30,‘MANAGER‘) loop
    Dbms_output.put_line(r_c1.empno||‘ ‘||r_c1.ename);
  end loop;
end;
/

练习:获取每个部门前两个雇员的信息
获取10部门前两个人的信息

declare
  cursor c1 is select * from scott.emp 
                      where deptno=10;
begin
  for r1 in c1 loop
    exit when c1%rowcount=3 or c1%notfound;
    dbms_output.put_line(r1.ename||‘ ‘||r1.deptno);
  end loop;
end;
/

使用替代变量取指定部门的前两个人的信息

declare
  cursor c1 is select * from scott.emp 
                      where deptno=&p_deptno;
begin
  for r1 in c1 loop
    exit when c1%rowcount=3 or c1%notfound;
    dbms_output.put_line(r1.ename||‘ ‘||r1.deptno);
  end loop;
end;
/

使用高级游标代替替代变量

declare
  cursor c1(p_deptno number) is select * from scott.emp 
                      where deptno=p_deptno;
begin
  for r1 in c1(10) loop
    exit when c1%rowcount=3 or c1%notfound;
    dbms_output.put_line(r1.ename||‘ ‘||r1.deptno);
  end loop;
  for r1 in c1(20) loop
    exit when c1%rowcount=3 or c1%notfound;
    dbms_output.put_line(r1.ename||‘ ‘||r1.deptno);
  end loop;
  for r1 in c1(30) loop
    exit when c1%rowcount=3 or c1%notfound;
    dbms_output.put_line(r1.ename||‘ ‘||r1.deptno);
  end loop;
end;
/

使用循环嵌套简化上面的代码

declare
  cursor c2 is select distinct deptno from scott.emp;
  cursor c1(p_deptno number) is 
                     select * from scott.emp 
                      where deptno=p_deptno;
begin
  for r2 in c2 loop
    for r1 in c1(r2.deptno) loop
      exit when c1%rowcount=3 or c1%notfound;
      dbms_output.put_line(r1.ename||‘ ‘||r1.deptno);
    end loop;
  end loop;
end;
/

练习 2:将每个部门工资小于2000的职员工资涨10%

declare
  cursor c1 is select deptno from scott.dept;
  cursor c2 (p_deptno number,p_job varchar2)
  is
  select empno,ename,sal
  from emp
  where deptno=p_deptno
  and job=p_job
  for update of sal;    --指明锁哪张表(其中of sal指明所emp表,不加的话会锁两张表,当然多张表才有意义)
begin
  for r_c1 in c1 loop 
    dbms_output.put_line(‘第‘||c1%rowcount||‘次获取游标c1‘ || ‘修改‘||r_c1.deptno||‘部门职员的工资‘);
    for r_c2 in c2(r_c1.deptno,‘CLERK‘) loop  /* 参数游标的好处的就是可以使锁定行更少 更有利于并发 */
      if r_c2.sal<2000 then
              update scott.emp set sal=sal*1.1
              where current of c2;  /* 只锁C2游标所涉及到的行 */
           end if;
    end loop;
  end loop;
end;
/

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