You can assign a default value to a column of a table so that when a new row is
inserted and a value for the column is omitted or keyword DEFAULT is supplied, a
default value is supplied automatically. Default column values work as though an
INSERT statement actually specifies the default value.
The datatype of the default literal or expression must match or be convertible to the
column datatype.
If a default value is not explicitly defined for a column, then the default for the column
is implicitly set to NULL.
列默认值
1. 默认值或默认表达式的数据类型应与相应列的数据类型相同,或能够进行数据转换
2. 如果没有为某列显示地定义默认值 ,那么此列的默认值被隐式地定义为 NULL.
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