openGauss每日一练第17天|定义游标

学习目标

学习openGauss定义游标

为了处理SQL语句，存储过程进程分配一段内存区域来保存上下文联系，游标是指向上下文区域的句柄或指针。借助游标，存储过程可以控制上下文区域的变化。

课程学习

连接数据库

#第一次进入等待15秒
#数据库启动中...
su - omm
gsql -r

1.准备数据

create schema tpcds;
CREATE TABLE tpcds.reason
(
r_reason_sk integer,
r_reason_id character(16),
r_reason_desc character(100)
);
INSERT INTO tpcds.reason values(3,'AAAAAAAABAAAAAAA','reason 1'),
(10,'AAAAAAAABAAAAAAA','reason 2'),(4,'AAAAAAAABAAAAAAA','reason 3'),
(10,'AAAAAAAABAAAAAAA','reason 4'),(10,'AAAAAAAABAAAAAAA','reason 5'),
(20,'AAAAAAAACAAAAAAA','reason 6'),(30,'AAAAAAAACAAAAAAA','reason 7');

create table company(name varchar(100), loc varchar(100), no integer);
insert into company values ('macrosoft',    'usa',          001);
insert into company values ('oracle',       'usa',          002);
insert into company values ('backberry',    'canada',       003);

2. SELECT语句，用一个游标读取一个表

–开始一个事务

start transaction;

–建立一个名为cursor1的游标。

CURSOR cursor1 FOR SELECT * FROM tpcds.reason ORDER BY 1;

–在系统视图pg_cursors中查看可用游标

select * from pg_cursors;

–抓取头3行到游标cursor1里

FETCH FORWARD 3 FROM cursor1;

–从当前关联位置开始，抓取前面的1行

FETCH BACKWARD 1 FROM cursor1;

–关闭游标并提交事务

CLOSE cursor1;
select * from pg_cursors;
end;

3. VALUES子句，用一个游标读取VALUES子句中的内容

–建立一个名为cursor2的游标

start transaction;
CURSOR cursor2 FOR VALUES(1,2),(0,3) ORDER BY 1;
FETCH FORWARD 2 FROM cursor2;
CLOSE cursor2;
end;

4. WITH HOLD游标的使用

–声明该游标在创建它的事务结束后仍可继续使用

DECLARE cursor1 CURSOR WITH HOLD FOR SELECT * FROM tpcds.reason ORDER BY 1;

–抓取接下来的3行

FETCH FORWARD 3 FROM cursor1;
CLOSE cursor1;

5.移动游标

START TRANSACTION;
CURSOR cursor1 FOR SELECT * FROM tpcds.reason ORDER BY 1;

–忽略游标cursor1的前3行

MOVE FORWARD 3 FROM cursor1;

–抓取游标cursor1的前4行

FETCH 4 FROM cursor1;
CLOSE cursor1;
end;

6.存储过程中使用游标

create or replace procedure test_cursor_1
as
    company_name    varchar(100);
    company_loc varchar(100);
    company_no  integer;

    cursor c1_all is --cursor without args
        select name, loc, no from company order by 1, 2, 3;
begin
    if not c1_all%isopen then
        open c1_all;
    end if;
    loop
        fetch c1_all into company_name, company_loc, company_no;
		RAISE INFO 'company_name: %' ,company_name;
        exit when c1_all%notfound;
    end loop;
    if c1_all%isopen then
        close c1_all;
    end if;
end;
/
call test_cursor_1();
drop procedure test_cursor_1;

7.清理数据

drop schema tpcds cascade;
drop table company;

课程作业

1.创建游标，且使用select子句指定游标返回的行，分别使用FETCH抓取数据，MOVE重定位游标

create table test(id integer,name character(10));
insert into test values(1,'AAA'),(2,'BBB'),(3,'CCC'),(4,'DDD');
start transaction;
CURSOR cursor1 FOR SELECT * FROM test ORDER BY 1;
FETCH FORWARD 3 FROM cursor1;
FETCH BACKWARD 1 FROM cursor1;
MOVE FORWARD 3 FROM cursor1;
close cursor1;
end;

2.在系统视图pg_cursors中查看游标

DECLARE cursor1 CURSOR WITH HOLD FOR SELECT * FROM test ORDER BY 1;
select * from pg_cursors;

3.创建一个使用游标的存储过程

create or replace procedure procedure1
as
    c_id    integer;
    c_name varchar(10);

    cursor c1_all is --cursor without args
        select id,name from test order by 1, 2;
begin
    if not c1_all%isopen then
        open c1_all;
    end if;
    loop
        fetch c1_all into c_id, c_name;
		RAISE INFO 'id: %,name: %' ,c_id,c_name;
        exit when c1_all%notfound;
    end loop;
    if c1_all%isopen then
        close c1_all;
    end if;
end;
/
call procedure1();
drop procedure procedure1;

4.清理数据

drop table test;