Coursera: Introduction to Databases Part II

    Continuing from my previous article this post is a collection of my notes, and my study of SQL databasing.

Two selection operators in a row can always be replaced by a single selection operator whose condition is the "and" of the two selection conditions. If there are two projection operators in a row, the attribute list of the second (outer) projection must be a subset of the attribute list of the first (inner) projection. Thus, the first projection can be removed without changing the result of the expression.

  • Join
    • Inner Join on Condition
    • Natural Join
    • Inner Join Using (attrs)
Aggregate Functions (Transact-SQL)
Aggregate function - Wikipedia, the free encyclopedia
SQL Functions

Null Values in relational database are undefined or unknown, and used to define these to things.
What happens to null values in the database and run queries? In a query they run as blanks and in conditions where the values must meet, null will not be included unless null is specified just like in linq. Distinct includes null values!

Data Modifications Statement Types
  1. Insert
  2. Delete
    • Example: "Delete From Table where Condition"
  3. Update
    • Example: "Update Table Set Attr = Expression Where Condition"
    • We can update multiple of tuples!

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