Cursors 游标
游标实现功能摘要：
DECLARE cursor-name CURSOR FOR SELECT ...;
OPEN cursor-name;
FETCH cursor-name INTO variable [, variable];
CLOSE cursor-name;
现在我们开始着眼游标了。虽然我们的存储过程中的游标语法还并没有完整的实现，但是已经可以完成基本的事务如声明游标，打开游标，从游标里读取，关闭游标。
1. Cursor Example
CREATE PROCEDURE p25 (OUT return_val INT)
BEGIN
DECLARE a,b INT;
DECLARE cur_1 CURSOR FOR SELECT s1 FROM t;
DECLARE CONTINUE HANDLER FOR NOT FOUND
SET b = 1;
OPEN cur_1;
REPEAT
FETCH cur_1 INTO a;
UNTIL b = 1
END REPEAT;
CLOSE cur_1;
SET return_val = a;
END;//
我们看一下包含游标的存储过程的新例子。
2. Cursor Example
CREATE PROCEDURE p25 (OUT return_val INT)
BEGIN
DECLARE a,b INT; <--
DECLARE cur_1 CURSOR FOR SELECT s1 FROM t;
DECLARE CONTINUE HANDLER FOR NOT FOUND
SET b = 1;
OPEN cur_1;
REPEAT
FETCH cur_1 INTO a;
UNTIL b = 1
END REPEAT;
CLOSE cur_1;
SET return_val = a;
END;//
这个过程开始声明了三个变量。附带说一下，顺序是十分重要的。首先要进行变量声明，然后声明条件，随后声明游标，再后面才是声明错误处理器。如果你没有按顺序声明，系统会提示错误信息。
3. Cursor Example
CREATE PROCEDURE p25 (OUT return_val INT)
BEGIN
DECLARE a,b INT;
DECLARE cur_1 CURSOR FOR SELECT s1 FROM t; <--
DECLARE CONTINUE HANDLER FOR NOT FOUND
SET b = 1;
OPEN cur_1;
REPEAT
FETCH cur_1 INTO a;
UNTIL b = 1
END REPEAT;
CLOSE cur_1;
SET return_val = a;
END;//
程序第二步声明了游标cur_1，如果你使用过嵌入式SQL的话，就知道这和嵌入式SQL差不多。
4. Cursor Example
CREATE PROCEDURE p25 (OUT return_val INT)
BEGIN
DECLARE a,b INT;
DECLARE cur_1 CURSOR FOR SELECT s1 FROM t;
DECLARE CONTINUE HANDLER FOR NOT FOUND <--
SET b = 1; <--
OPEN cur_1;
REPEAT
FETCH cur_1 INTO a;
UNTIL b = 1
END REPEAT;
CLOSE cur_1;
SET return_val = a;
END;//
最后进行的是错误处理器的声明。这个CONTINUE处理没有引用SQL错误代码和SQLSTATE值。它使用的是NOT FOUND系统返回值，这和SQLSTATE 02000是一样的。
5. Cursor Example
CREATE PROCEDURE p25 (OUT return_val INT)
BEGIN
DECLARE a,b INT;
DECLARE cur_1 CURSOR FOR SELECT s1 FROM t;
DECLARE CONTINUE HANDLER FOR NOT FOUND
SET b = 1;
OPEN cur_1; <--
REPEAT
FETCH cur_1 INTO a;
UNTIL b = 1
END REPEAT;
CLOSE cur_1;
SET return_val = a;
END;//
过程第一个可执行的语句是OPEN cur_1，它与SELECT s1 FROM t语句是关联的，过程将执行SELECT s1 FROM t，返回一个结果集。
6. Cursor Example
CREATE PROCEDURE p25 (OUT return_val INT)
BEGIN
DECLARE a,b INT;
DECLARE cur_1 CURSOR FOR SELECT s1 FROM t;
DECLARE CONTINUE HANDLER FOR NOT FOUND
SET b = 1;
OPEN cur_1;
REPEAT
FETCH cur_1 INTO a; <--
UNTIL b = 1
END REPEAT;
CLOSE cur_1;
SET return_val = a;
END;//
这里第一个FETCH语句会获得一行从SELECT产生的结果集中检索出来的值，然而表t中有多行，因此这个语句会被执行多次，当然这是因为语句在循环块内。
7. Cursor Example
CREATE PROCEDURE p25 (OUT return_val INT)
BEGIN
DECLARE a,b INT;
DECLARE cur_1 CURSOR FOR SELECT s1 FROM t;
DECLARE CONTINUE HANDLER FOR NOT FOUND
SET b = 1; <--
OPEN cur_1;
REPEAT
FETCH cur_1 INTO a;
UNTIL b = 1
END REPEAT;
CLOSE cur_1;
SET return_val = a;
END;// 8. Cursor Example
CREATE PROCEDURE p25 (OUT return_val INT)
BEGIN
DECLARE a,b INT;
DECLARE cur_1 CURSOR FOR SELECT s1 FROM t;
DECLARE CONTINUE HANDLER FOR NOT FOUND
SET b = 1;
OPEN cur_1;
REPEAT
FETCH cur_1 INTO a;
UNTIL b = 1
END REPEAT;
CLOSE cur_1; <--
SET return_val = a;
END;//
到了这一步UNTIL b=1条件就为真，循环结束。在这里我们可以自己编写代码关闭游标，也可以由系统执行，系统会在复合语句结束时自动关闭游标，但是最好不要太依赖系统的自动关闭行为（译注：这可能跟Java的Gc一样，不可信）。
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