Null and default value Compression (V8 GA) : Compression for null, zero length data in variable length columns and system default value.
null值和缺省值压缩(V 8 GA)：对变长列中的零长度空数据和系统缺省值进行压缩。
2
In lines 22 through 34 you select an operating system that is not NULL from the list obtained in previous step.
从第22行到第34行从上一步中获得的数组中选择一个不为空的操作系统。
3
You can also use the system catalog views to retrieve information about NOT NULL, unique, primary key, foreign key, and table check constraints. For example.