【单选题】【消耗次数：1】
现有一个“教师”表，其中一个字段是教师的住址（字符型，20位长），如果不希望此字段包含空值，即某位教师现没有住址，则希望此字段自动填入“还没有”，应该（ ）。
①
为此列创建一个check约束
②
为此列创建一个foreign key约束
③
为此列创建一个default约束
④
为此列创建一个primary key约束
参考答案：
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已知SN是一个字符型字段，下列SQL查询语句SELECT SN FROM S WHERE SN LIKE ‘AB％’；的执行结果为( )。
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