在PL/SQL中，有三种循环结构：（1）简单循环；（2）while循环；（3）for循环
1.简单循环
LOOP
sequence_of_statement;
exit [when condition]; --exit很重要，它是用来终止循环的，如果没有会陷入死循环。
END LOOP;
for example：
declare
v_counter BINARY_INTERGER :=1;
BEGIN
LOOP
INSERT INTO user values(v_Counter,'Loop index' );
v_counter = v_counter + 1;
EXIT WHEN v_counter>50;
END LOOP;
END;
2.while循环
WHILE condition LOOP
sequence_of_statement;
END LOOP;
for example:
declare
v_counter BINARY_INTERGER :=1;
BEGIN
WHILE v_counter<=50 LOOP
INSERT INTO user values(v_Counter,'Loop index' );
v_counter = v_counter + 1;
END LOOP;
ENDL;
3.for循环
FOR loop-counter in （reverse） low_bound..high_bound LOOP
sequence_of_statement
END LOOP;
for example：
BEGIN
FOR v_counter IN 1..50 LOOP
INSERT INTO user values(v_counter,'LOOP_INDEX');
END LOOP;
END;
