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 * order_id
 * product_id
 * quantity
+
+* Data structure: data are organized in the form of tables, with rows and columns.
+* Data manipulation: SQL language used to manipulate data
+* Data integrity: mechanism used to specify business rules that maintain data integrity.
+
+# Relation: two dimensional table of data.
+
+Employee1(id, name) # shorthand notation for describing a relation.
+
+## Relational Keys
+
+* primary key: attribute or combination of attributes that uniquely identifies each row in a relation.
+* composite key: primary key that consists of more than one attribute.
+* foreign key: attribute in a relation that serves as the primary key of another relation in the same db.
+
+Not all tables are relations.
+
+1. each relation has a unique name in the db.
+2. each cell (intersection of row and column) is atomic (single valued). multi-valued attribute not allowed.
+3. each row is unique. no two rows in a relation can be identical.
+4. each attribute (column) within the table has a unique name.
+5. sequence of columns is insignificant (left to right). order of columns can change without affecting meaning.
+6. sequence of rows is insignificant. (top to bottom)
+
+Suggestions:
+
+It's good to create an instance of your relational schema with sample data because...
+
+1. the sample data allow you to test your assumptions regarding the design.
+2. the sample data provide a convenient way to check the accuracy of your design.
+3. the sample data help improve communications with users in discussing your design.
+4. you can use the sample data to develop prototype applications and to test queries.
+
+## Integrity Constraints
+
+Rules limiting acceptable values and actions. Purpose is to facilitate maintaining the accuracy
+and integrity of data in the db.
+
+The major types of integrity constraints are domain constraints, entity integrity and referential integrity.
+
+### Domain Constraints
+
+All the values that appear in a column of a relation must be from the same domain. huh?
+
+> Domain: the set of values that may be assigned to an attribute.
+
+Domain definition consists of the following components:
+
+* domain name
+* meaning
+* data type
+* size (length)
+* allowable values or allowable range
+
+#### Entity Integrity
+
+Ensures that every relation has a primary key and that the data values for the primary key are all valid.
+
+* Null: a value that may be assigned to an attribute when no other value applies or when the applicable value is unknown.
+
+* Entity integrity rule: No primary key attribute (or component of primary key attribute) may be null.
+
+#### Referential Integrity
+
+Associations between tables are defined through foreign keys.
+
+* Referential integrity constraint: each foreign key value must match a primary key value in another relation or the foreign key value must be null.
+
+
+## Well structured relations
+
+What makes up a well structured relation?
+
+* minimal redundancy
+* crud operations can occur without errors or inconsistencies.
+
+Anomaly: an error or inconsistency that may result when a user attempts to update a table that
+contains redundant data. The three types of anomalies are insertion, deletion and modification anomalies.
+
+* insertion anomaly
+  * insert into employees also requires course data.
+  * Should be able to insert employee data without supplying course data.
+* deletion anomaly:
+* modification anomaly: a change to a single employee requires updating multiple records in the same relation.
+
+1. Regular entity: has independent existence.
+2. Weak entity: cannot exist without relationship to owner.
+3. Associative Entity: many-to-many relationships. join tables.
+
+> Surrogate primary key: serial # or other system assigned primary key for a relation.
+
+# Normalization
+
+Is a formal process for deciding which attributes should be grouped together in a relation
+so that all anomalies are removed.
+
+> Normalization: the process of decomposing relations with anomalies to produce smaller, well-structured relations.
+
+1. minimize redundancy.
+2. simplify enforcement of referential integrity constraints.
+3. Make CUD operations easy.
+4. Future growth.
+
+## Normal form
+
+What is normal form?
+
+A normal form is a state of a relation that requires that certain rules regarding relationships between attributes are satisfied.
+
+> Normal form: A state of a relation that requires that certain rules regarding relationships between attributes are satisfied.
+
+* 1st normal form: multivalued attributes have been removed. each cell has a single value.
+* 2nd normal form: partial functional dependencies have been removed. (?)
+* 3rd normal form: transitive dependencies have been removed. (?)
+* Boyce-Codd normal form: remaining anomalies from functional dependencies have been removed.
+* 4th normal form: multivalued dependencies have been removed.
+* 5th normal form: remaining anomalies have been removed.