LOOP（循环）
语法结构
[begin_label:] LOOP
statement_list
END LOOP [end_label]
描述说明
LOOP 实现了循环控制，使能够重复执行statement list部分。statement list部分 有一条或者是多条语句，每个语句都以(;) 结束。loop循环会一直持续，直到遇到LEAVE 退出。
LOOP 不能以labeled. end_label结束，除非loop以begin_label 开始。如果开始，结束部分都不曾省略，那loop名一定要相同。
有关(;)的用法更多可参考 Delimiters in the mysql client。
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