vault backup: 2024-01-21 00:27:23

Semester 2/Database Systems/Week 1/Week 1 Database Systems.md

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-### [Relational Algebra - Recap](obsidian://open?vault=Computer%20Science%20Year%201&file=Semester%201%2FDatabase%20Systems%2FWeek%2010%2FWeek%2010)
+### [Relational Algebra - Recap](obsidian://open?vault=Computer%20Science%20Year%201&file=Semester%201%2FDatabase%20Systems%2FWeek%2010%2FWeek%2010%20Database%20Systems)
 
 - High level & procedural
 - Defines how to build relations
+- SELECT represented by pi ( projection )
+- FROM represented by sigma ( selection )
+- WHERE represented by x ( cartesian product )
 
 #### Data Manipulation Language
 
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 - Operations work on one or more relations to define another relation without changing the original
 - Both operands and results are relations, output from one operation can become the input for another operation.
 - This allows nested expressions.
+
+#### Theta Join
+
+R theta(f) S defines a relation that contains tuples satisfying F from the Cartesian product of two relations R and S.
+Takes the Cartesian product of two relations r and s than selects the rows which satisfy the condition F.
+R theta(f) S is the same as sigma(f) (R x S ).
+F may contain comparison operators to form a condition.
+
+#### Query Processing
+The activities involved in parsing, validating, optimising and executing a query.
+Purpose is to transform a query written in a high level language into correct and efficient strategy in low level language. Also to execute strategy to retrieve data.
+
+#### Common Optimisation Rules
+- Begin with initial query tree for SQL
+- Move SELECT operations down the tree
+- Apply more restrictive SELECT operations first ( eg. equalities before range queries )
+- Replace Cartesian products followed by selection with theta joins ( eg. *sigma(f) ( RxS )* -> *R theta(f) S* )
+- Move PROJECT operations down the query tree ( add project operations as inputs to theta joins )