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Foreign keys definitions are subject to the following conditions:
Both tables must be InnoDB tables and they must not be TEMPORARY tables.
Corresponding columns in the foreign key and the referenced key must have similar internal data types inside InnoDB so that they can be compared without a type conversion. The size and sign of integer types must be the same. The length of string types need not be the same. For non-binary (character) string columns, the character set and collation must be the same.
In the referencing table, there must be an index where the foreign key columns are listed as the first columns in the same order. Such an index is created on the referencing table automatically if it does not exist.
In the referenced table, there must be an index where the referenced columns are listed as the first columns in the same order.
Index prefixes on foreign key columns are not supported. One consequence of this is that BLOB and TEXT columns cannot be included in a foreign key, because indexes on those columns must always include a prefix length.
If the CONSTRAINT symbol clause is given, the symbol value must be unique in the database. If the clause is not given, InnoDB creates the name automatically.
参照 mysql 中的说明，建外键时一定要注意 2 个表字段之间的关系，一定要一致，mysql 中的数据类型相对要比 oracle 的多，我遇到了 int，unsigned 的问题。1 个表的 id 字段是 int，令 1 个表的 id 字段是 int，unsigned。结果建了很多次外键，都没成功，找了半天才看到。看来还是太粗心。。
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