db碎碎念

1. db中一个table的foreign key必须是另一个table的primary key么？
2. 如何使得两个table中主码互为对方的外码
   参考了https://social.msdn.microsoft.com/Forums/sqlserver/en-US/431c8ca9-5c4e-402b-8af0-cd58b71c2429/2-tables-referencing-each-other-using-foreign-keyis-it-possible?forum=transactsql
   
   1. DDL (data define language)
      provides commands for defining relation schemas, deleting relations and modifying relation schemas
      
      • integrity
      • view definition
      • authorization
        DML(data manipulation language)
        add, delete, modify and check
3. union vs union all