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BI (EBI, Data, 定义, 数据可视化, 数据分析), relational database, HRMS-Employee Schema…

- - - - joins is time consuming
- - - - nchar(10) is a fixed-length Unicode string of length 10. nvarchar(10) is a variable-length Unicode string with a maximum length of 10. Typically, you would use the former if all data values are 10 characters and the latter if the lengths vary.
      - char vs nchar
      - email: predefined
    - - 举例，sign a contract； post on Twitter
  - - - not necessary?
        
        auto increment？？？
    - - The default constraint is used to set the value of the attributes in case no value is passed while insertion is taking place in the database.
        This ensures that ambiguity does not arise in the stored data and the stored data remains meaningful. By the use of default constraints, we save the costs of cleaning and processing data before analysis.
    - - Check constraint is explicitly put by the user while creating the table or is introduced when the table is being altered.
        It can be applied to one single attribute or can be applied to the whole table. It helps us ensure that the value we are inserting in our database satisfies the required condition.
  - - - control
        
        revoke, grant
    - - definition
        
        create, alter, drop, truncate, rename, commit
        
        比较delte和truncatre
        
        record level and structure level
        
        rollback, commit(transaction)
        
        delete usually per some conditions while truncate usually conditionless
    - - maniputlation
        
        select， delte， update, insert, merge
    - - ALTER Command is used to add, delete, modify the attributes of the relations (tables) in the database. UPDATE Command is used to update existing records in a database.
- - - - database look like
    - - normalized
  - - - database look like
    - - denomalized
- - - - sorted(<iterable>, <key is something assigned by a lambda function))
  - - - 'index' and location is related thru a hash func
    - - add
        
        list can't, not hashable
        
        tuple, immutable, hence hashable, hence can add
      - update
        
        for >1 elements to add, accept list, string, tuples, sets
      - discard
    - - union
        
        return union
  - - - **kwargs(Keyword arguments)
    - - *args(Non-Keyword arguments)