T-SQL游标的使用

news2024/10/7 3:20:19

一.建表

INSERT INTO cloud VALUES( '你' )
INSERT INTO cloud VALUES( '一会看我' )
INSERT INTO cloud VALUES( '一会看云' )
INSERT INTO cloud VALUES( '我觉得' )
INSERT INTO cloud VALUES( '你看我时很远' )
INSERT INTO cloud VALUES( '你看云时很近' )

二.建立游标

1.游标的一般格式

DECLARE 游标名称 CURSOR 
FOR 
SELECT 字段1,字段2,字段3,... FROM 表名 WHERE ...
OPEN 游标名称
FETCH NEXT FROM 游标名称 INTO 变量名1,变量名2,变量名3,...
WHILE @@FETCH_STATUS=0
   BEGIN
      SQL语句
      FETCH NEXT FROM 游标名称 INTO 变量名1,变量名2,变量名3,...
    END
CLOSE 游标名称
DEALLOCATE 游标名称 (释放游标)

cursor：表示在光标当前位置执行操作

2.实例：

DECLARE
@v_line varchar(50)                     ---------声明一个装载语句的字符串
DECLARE cursor_cloud CURSOR
FOR
SELECT linetext from cloud;
BEGIN 
OPEN cursor_cloud;                       -------打开游标
FETCH NEXT FROM cursor_cloud INTO @v_line-------将游标移向下一行，把获取的内容放入@V_line
WHILE @@FETCH_STATUS = 0                 -------检测是否成功获取数据
BEGIN
PRINT @v_line                            -------进行SQL语句操作
FETCH NEXT FROM cursor_cloud INTO @v_line-------向下移行
END
CLOSE cursor_cloud;                      -------关闭游标
DEALLOCATE cursor_cloud;                 -------释放游标
END;

结果得：

三.使用游标修改数据（update）

DECLARE
 @v_line varchar(50),
 @i int
 DECLARE cursor_cloud CURSOR
 FOR
 SELECT linetext from cloud;
BEGIN
 SET @i = 0;
 OPEN cursor_cloud;
 FETCH NEXT FROM cursor_cloud INTO @v_line
 WHILE @@FETCH_STATUS = 0
 BEGIN
 SET @i = @i + 1;
 UPDATE cloud SET linetext = CAST(@i as varchar(5)) + ' ' + @v_line WHERE linetext =
@v_line;
 FETCH NEXT FROM cursor_cloud INTO @v_line 
 END
 CLOSE cursor_cloud;
 DEALLOCATE cursor_cloud;
END;

结果得：

四.可灵活移动的游标

DECLARE
 @v_line varchar(50)
 DECLARE cursor_cloud CURSOR SCROLL
 FOR
 SELECT linetext from cloud;
BEGIN
 OPEN cursor_cloud;
 FETCH FIRST FROM cursor_cloud INTO @v_line
 PRINT '第一行 ' + @v_line
 FETCH NEXT FROM cursor_cloud INTO @v_line
 PRINT '第一行的下一行 ' + @v_line
 FETCH LAST FROM cursor_cloud INTO @v_line
 PRINT '最后一行 ' + @v_line
 FETCH PRIOR FROM cursor_cloud INTO @v_line
 PRINT '最后一行的前一行 ' + @v_line
 CLOSE cursor_cloud;
 DEALLOCATE cursor_cloud;
END;

结果得

五.可更新的游标

DECLARE
 @v_line varchar(50),
 @i int
 DECLARE cursor_cloud CURSOR
 FOR
 SELECT linetext from cloud for update;
BEGIN
 DECLARE @v_new_line varchar(50);
 SET @i = 0;
 OPEN cursor_cloud;
 FETCH NEXT FROM cursor_cloud INTO @v_line
 WHILE @@FETCH_STATUS = 0
 BEGIN
 set @v_new_line = SUBSTRING( @v_line, 2,100 );
 UPDATE cloud SET linetext = @v_new_line WHERE CURRENT OF cursor_cloud;
 FETCH NEXT FROM cursor_cloud INTO @v_line 
 END
 CLOSE cursor_cloud;
 DEALLOCATE cursor_cloud;
END;

通过update...........where current of进行游标的更新

 UPDATE cloud SET linetext = @v_new_line WHERE CURRENT OF cursor_cloud;

表示更新最后一个fetch命令从cursor_cloud游标获得的行

where current of不仅能结合update操作，也能结合delete进行操作

delete from cloud
WHERE CURRENT OF cursor_clould;

表示删除最后一个fetch命令从cursor_cloud游标获得的行

注：

嵌入式SQL UPDATE或DELETE可以使用WHERE子句(不带游标)或WHERE CURRENT OF(带声明游标)，但不能同时使用两者。
如果指定的UPDATE或DELETE既不带WHERE也不带WHERE CURRENT OF，则会更新或删除表中的所有记录。

更新的限制
当使用WHERE CURRENT OF子句时，不能使用当前字段值更新字段以生成更新的值。
例如，SET Salary=Salary+200或SET Name=UPPER(Name)：将字段的字母全部变成大写。
尝试这样做会导致

SQLCODE -69错误:SET <field> = <value expression> not allowed with WHERE CURRENT OF <cursor>。
————————————————

原文链接：https://blog.csdn.net/yaoxin521123/article/details/121602006

练习：

插入表如下


insert into words2 values(1,'你最可爱')
insert into words2 values(2,'我说时来不及思索')
insert into words2 values(3,'而思索过后')
insert into words2 values(4,'还是会这么说')

（1）

declare
 @num int,@linetext varchar(100)
 declare cursor_words cursor scroll
 for
 select linetext from words2
 set @num=1
 open cursor_words
 fetch next from  cursor_words into @linetext
 while @@FETCH_STATUS=0
 begin
 print '普希金说的第'+cast(@num as varchar(10))+'句话：'+@linetext
 fetch next from cursor_words into @linetext
 set @num = @num+1
 end
 print ' '
 print'他一共说了'+cast(@num-1 as varchar(10))+'句话'
 close cursor_words
 deallocate cursor_words

（2）在奇数行后面加逗号，偶数行后面加句号

 declare 
 @i int,@linetext varchar(100)
 declare  cursor_words cursor 
 for
 select linetext from words2
 begin
 set @i=0
 open cursor_words;
 fetch next from cursor_words into @linetext 
 while @@FETCH_STATUS=0
 begin 
 set @i=@i+1
 if(@i%2=1)
 update words2
 set linetext=@linetext+',' where linenum=@i
 else
 update words2  
 set linetext=@linetext+'。' where linenum=@i
 fetch next from cursor_words into @linetext
 end
 close cursor_words
 deallocate cursor_words
 end

再用

 select * from words2

得到

（3）将奇数行和偶数行合并在一起，同时调整相应的行号

重点：where current of

 declare @num int,@linetext varchar(50),@newline varchar(100)
 declare cursor_words2 cursor
 for
 select linetext from words2 for update;-----手工加锁语句
 set @num = 1
 set @newline = ''
 open cursor_words2
 fetch next from cursor_words2 into @linetext
 while @@FETCH_STATUS=0
 begin
set @newline=@newline+@linetext
if @num%2=1
delete from words2 WHERE CURRENT OF cursor_words2----------删除奇数行
else
begin
update words2 set linetext=@newline,linenum=@num/2 WHERE CURRENT OF cursor_words2-----------修改行号
set @newline=''
end
fetch next from cursor_words2 into @linetext
set @num = @num +1
end
close cursor_words2
deallocate cursor_words2

再用