文档: Modifying columns http://db.apache.org/derby/docs/10.5/ref/rrefsqlj81859.html
The column-alteration allows you to alter the named column in the following ways:
To increase the width of a column of these types, specify the data type and new size after the column name.
你不能减少列宽也不能改变数据类型 You are not allowed to decrease the width or to change the data type. You are not allowed to increase the width of a column that is part of a primary or unique key referenced by a foreign key constraint or that is part of a foreign key constraint.
To set an interval between consecutive values of the identity column, specify the integer-constant. You must previously define the column with the IDENTITY attribute (SQLSTATE 42837). If there are existing rows in the table, the values in the column for which the SET INCREMENT default was added do not change.
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