你的ALTER TABLE table ADD CONSTRAINT foreign_key FOREIGN KEY是一样的，所以它们不是错误的原因.

Your ALTER TABLE table ADD CONSTRAINT foreign_key FOREIGN KEY are the same, so they cannot be the reason of the error.

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无法解释这种奇怪的行为.可以建议一个解决方法:

Cannot explain that strange behavior. Can suggest a workaround:

SET FOREIGN_KEY_CHECKS = 0;
DELETE FROM A WHERE ID=1;
SET FOREIGN_KEY_CHECKS = 1;