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+# Introducing Joins
+
+From the previous exercise/challenge.
+
+![Chart](Files/image1.png)
+
+We'll talk about the __has planet relationship__
+
+## Joining tables
+```SQL
+SELECT Lead.name,
+    Rhythm.name, Bass.name,
+    Drums.name
+FROM Lean, Rhythm, Bass, Drums;
+```
+This is called a **Cross Join**
+
+Let's say we have
+Lead guitarists: 7
+Rhythm Guitarists: 12
+Bass guitarists: 8
+Drummers: 15
+
+Cross Join: 7 x 12 x 8 x 15 = 10,080 results in the query.
+
+A cross join gets big easily.
+
+```SQL
+SELECT Planet.Name,
+    Moon.Name, HasLiquidWater
+FROM Planet, Moon
+WHERE Planet.Name = Moon.hasPlanet
+AND DayLength < 11;
+```
+
+This is an **Inner Join**. The DBMS will typically detect it's an inner join, instead of a different join, and treat it as such, but it can be explicitly stated as:
+
+```SQL
+SELECT Planet.Name,
+    Moon.Name, HasLiquidWater
+FROM Planet INNER JOIN Moon
+ON Planet.Name = Moon.hasPlanet
+AND DayLength < 11;
+```
+
+Planets: 8
+Moons: 173
+
+
+Cross join: 8 x 173 = 1,384
+
+Inner join: 1 x 173 = 173
+
+**Outer Join**
+
+```SQL
+SELECT Planet.Name,
+    Moon.Name, HasLiquidWater
+FROM Planet LEFT JOIN Moon
+ON Planet.Name = Moon.hasPlanet;
+```
+
+Planets: 8
+Moons: 173
+
+Cross join: 8 x 173 = 1,384
+Inner join: 1 x 173 = 173
+Outer join: 1 x 173 + 2 (null records for planets which don't  have moons, **no match**)
+
+# Drawing a database II - More about joins
+
+## Cardinality
+**1:n**
+One row in table x joins with zero, one or more rows in table y
+How many rows in one table matches with how many rows in another table.
+
+Cardinality is expressed as a ratio:
+* 1:1
+* 1:n
+* m:n
+
+1:1 cardinality is rare
+
+m:n many to many
+
+Cardinality affects implementation
+
+1:n or n:1
+
+![ER](Files/image2.png)
+
+Use a Foreign Key - Place the Primary Key of the tabl with 1 value into the table with n values.
+
+```SQL
+CREATE TABLE Moons (
+    MoonName CHAR(20),
+    PlanetName CHAR(10),
+    Diameter INT,
+    PRIMARY KEY (MoonName)
+);
+```
+OR add it
+```SQL
+CREATE TABLE Moons (
+    MoonName CHAR(20),
+    PlanetName CHAR(10),
+    Diameter INT,
+    PRIMARY KEY (MoonName),
+    FOREIGN KEY
+        (Planetname)
+        REFERENCE
+        Planets(PlanetName)
+);
+```
+
+1:1
+![ER](Files/image3.png)
+
+This could be understood as an entity with a property. However, there are reasons not to do it too, implementing it as a 1:n and putting the PK of one of the tables into the other table
+
+```SQL
+CREATE TABLE Projects (
+    Student VARCHAR(100),
+    Title VARCHAR(100),
+    Mark INT,
+    PRIMARY KEY (Student)
+);
+```
+
+**m:n**
+
+It's impossible in a relational model unless we change it a little bit.
+
+![ER](Files/image3.png)
+
+We add a new entity
+
+![ER](Files/image4.png)
+
+A tutor can be inmultiple tutor roles.
+
+A student can be in multiple tutor roles, as a student.
+
+```SQL
+CREATE TABLE TutorRole (
+    Student VARCHAR(100),
+    Tutor VARCHAR(100),
+    PRIMARY KEY (Student, Tutor)
+)
+```
+
+So we need to modify the ER diagram before implementing the DB to accommodate the restrictions of the Relational Model.
+
+# Database integrity and the role of keys
+## What could go wrong?
+
+```SQL
+MoonName: Deimos
+PlanetName: MERS NULL
+Area: 495 / -495
+```
+
+Knowing what values are valid is important
+
+# Avoiding errors
+## Integrity constraints
+
+When we mistype a field, Mers instead of Mars for example.
+Join fields must match
+
+Use FOREIGN KEY
+INSERT with wrong value will fail
+
+```SQL
+FOREIGN KEY
+    (PlanetName)
+    REFERENCES
+    Planets(PlanetName)
+```
+
+If we implement this, inserting the wrong PlanetName will make the insertion fail.
+
+## Only some values of a field are valid
+For example for positive values
+```SQL
+Area INT
+    CHECK (Area > 0)
+```
+
+## Table values should not be inconsistent
+We avoid repeating information in a database. We use **PRIMARY KEY** to guarantee uniqueness.
+
+Avoid repeating information in a database.
+
+Avoid storing calculated values.
+
+## Changes should not cause inconsistency
+Use foreign key rules to enforce the correct behavior.
+
+```SQL
+FOREGN KEY
+    (PlanetName)
+    REFERENCES
+    Planets(PlanetName)
+    ONE DELETE CASCADE
+```
+
+When parent entry (in planets) is deleted, remove related moons.
+
+## Table values should not be inconsistent
+Remove functional dependencies.
+