CASE 用法：
两种用法：
IF 用法：
循环语句：
ITERATE Syntax
ITERATE 仅用于Loop，repeat,while 中；ITERATE,指下一次循环开始。
14.6.5.4 LEAVE Syntax
LEAVE label，退出控制流。
LEAVE can be used within BEGIN ... END or loop constructs (LOOP, REPEAT, WHILE).
Loop,
REPEAT,
delimiter //
create procedure dorepeat(p1 int)
begin
set @x=0;
repeat
set @x=@x+1;
until @x>p1
end repeat;
select @x;
end //
call dorepeat(1000);
WHILE,
CREATE PROCEDURE dowhile()
BEGIN
DECLARE v1 INT DEFAULT 5; WHILE v1 > 0 DO
...
SET v1 = v1 - 1;
END WHILE;
END;