The SIGNAL statement rolls back the changes that were attempted by the triggering statement (that is, our update statement).
SIGNAL语句回滚由触发语句（也就是我们的update 语句）尝试的更改。
The SIGNAL statement rolls back the changes that were attempted by the triggering statement (that is, our update statement).
SIGNAL语句回滚由触发语句（也就是我们的update 语句）尝试的更改。
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