Adds data integrity section.

database/notes/02-integrity-er-diagram.md

@@ -15,10 +15,8 @@
 > In a relational database, the schema defines the tables, fields, relationships, views, indexes, packages, procedures, functions, queues, triggers, types, sequences, materialized views, synonyms, database links, directories, XML schemas, and other elements
 
 ### Data Integrity
-### Entity Integrity
+> is the overall accuracy, completeness, and consistency of data
-### Referential integrity
+> the maintenance of, and the assurance of, data accuracy and consistency over its entire life-cycle
-### Domain integrity
-### User-defined integrity
 ### ER Diagrams
 
 ## Schema
@@ -43,8 +41,42 @@ Following is a schema diagram for the above database. Note that the primary key
 
 ---
 ## Data Integrity
+Remember when we stored our data in [files](01-database-fundamentals.md#brute-force---files)? That was the simplest way to store data, but it had a lot of issues. One of the major issues was that we can never be sure that our data is consistent with the way we want it to be.
+
+Some issues that we can encounter are
+* Duplicate rows - multiple rows with the same data
+* A string in a field where a number is expected, or vice versa.
+* Updating or deleting a mentor that has batches assigned to it
+* Entering 20 digits as a phone number instead of a 10-digit phone number
+
+This is where DBMS excels. Data integrity is normally enforced in a database system by a series of integrity constraints or rules.
+Data integrity (or actually logical integrity) can be divided into four main categories:
+### Entity integrity
+Each row to be unique within its table. No two rows can be the same. To achieve this, a **primary key** can be defined.
+
+The primary key field contains a unique identifier – no two rows can contain the same unique identifier.
+
+> Every table must have a primary key and that the column or columns chosen to be the primary key should be unique and not null
+
+### Referential integrity
+> Referential integrity is a property of data stating that all its references are valid.
+
+
+When a foreign key value is used it must reference a valid, existing primary key in the parent table.
+For referential integrity to hold in a relational database, any column in a base table that is declared a foreign key can only contain either **null values or values from a parent table's primary key or a candidate key**.
+
+>  For instance, deleting a record that contains a value referred to by a foreign key in another table would break referential integrity.
+
+### Domain integrity
+Domain integrity concerns the validity of entries for a given column. Selecting the appropriate data type for a column is the first step in maintaining domain integrity. Other steps could include, setting up appropriate constraints and rules to define the data format and/or restricting the range of possible values.
+
+### User-defined integrity
+
+User-defined integrity allows the user to apply business rules to the database that aren’t covered by any of the other three data integrity types.
 
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+
+
 ## ER Diagrams
 ---
 ## References