null 值处理

处理NULL值的方法有很多，但任何一种方式都不是很好。大致而言，必须根据对数据的认知对症下药

Surrogate key

A surrogate key is a unique identifier derived from the data itself.

• It often takes the form of a hashed value of multiple columns that will create a uniqueness constraint for each row.
• You will need to define a surrogate key for every table that doesn't have a natural primary key.

  • It's often an integer value that is automatically generated by the DBMS when a new row is inserted into the table.

• It's common for a dimension table to include

alternate key

aka. business key, natural key

• identifies a specific instance of an entity in the transactional source system
• It works similarly but is not primary key
• It has business meaning
• e.g. product code, customer ID or employee number