清除所有外键：set foreign_key_checks=0;
逐个清除外键：alter table tbname drop foreign key foreign_key_name

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SELECT
CONCAT(' ALTER TABLE ',TABLE_CONSTRAINTS.CONSTRAINT_SCHEMA,'.',TABLE_CONSTRAINTS.TABLE_NAME,'? DROP FOREIGN KEY? ',TABLE_CONSTRAINTS.CONSTRAINT_NAME,';')
FROM
TABLE_CONSTRAINTS WHERE CONSTRAINT_TYPE = 'FOREIGN KEY'
AND CONSTRAINT_SCHEMA IN ('ecshop')

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MySQL官方文档 http://dev.mysql.com/doc/refman/5.1/zh/index.html