DB - Joins

Signed-off-by: Serhii Horodilov <sgorodil@gmail.com>

src/rdbms/relations.txt

@@ -128,3 +128,237 @@ a time.
  erDiagram
  employee }o--|{ employee_role : execute
  role }o--|{ employee_role : assign
+
+Joining data
+============
+
+``JOIN`` is used to *join* data fetched from multiple related tables.
+To build a valid query you are to set the tables to join and parameter to use
+for this operation.
+
+Common syntax is:
+
+::
+
+ T1 join_type T2 [ join_condition ]
+
+There are various join types available.
+Here are is some dummy data to demonstrate their purpose.
+
+.. table:: poem
+
+ +----+------------------------+-----------+
+ | id | title | author_id |
+ +====+========================+===========+
+ | 1 | On the Field of Battle | 1 |
+ +----+------------------------+-----------+
+ | 2 | Moses | 2 |
+ +----+------------------------+-----------+
+ | 3 | The Forest Song | 1 |
+ +----+------------------------+-----------+
+ | 4 | In the Catacombs | null |
+ +----+------------------------+-----------+
+ | 5 | The Singing Stones | 1 |
+ +----+------------------------+-----------+
+ | 6 | Ancient Fairy Tales | null |
+ +----+------------------------+-----------+
+ | 7 | My Thoughts | 2 |
+ +----+------------------------+-----------+
+ | 8 | To Shevchenko | null |
+ +----+------------------------+-----------+
+
+
+.. table:: author
+
+ +----+---------------+
+ | id | name |
+ +====+===============+
+ | 1 | Larysa Kosach |
+ +----+---------------+
+ | 2 | Ivan Franko |
+ +----+---------------+
+ | 3 | John Doe |
+ +----+---------------+
+
+.. code-block:: sql
+
+ CREATE TABLE author
+ (
+ id SERIAL PRIMARY KEY,
+ name VARCHAR(32) NOT NULL UNIQUE
+ );
+ CREATE TABLE poem
+ (
+ id SERIAL PRIMARY KEY,
+ title VARCHAR(32) NOT NULL UNIQUE,
+ author_id INT REFERENCES author (id)
+ );
+ INSERT INTO author(name)
+ VALUES ('Larysa Kosach'),
+ ('Ivan Franko'),
+ ('John Doe');
+ INSERT INTO poem(title, author_id)
+ VALUES ('On the Field of Battle', 1),
+ ('Moses', 2),
+ ('The Forest Song', 1),
+ ('In the Catacombs', null),
+ ('The Singing Stones', 1),
+ ('Ancient Fairy Tales', null),
+ ('My Thoughts', 2),
+ ('To Shevchenko', null)
+ ;
+
+Join types
+----------
+
+:Cross join:
+
+ .. code-block::
+
+ T1 CROSS JOIN T2
+
+ For every possible combination of rows from T1 and T2
+ (i.e., a Cartesian product), the joined table will contain a row consisting
+ of all columns in T1 followed by all columns in T2. If the tables have N
+ and M rows respectively, the joined table will have N * M rows.
+
+:Qualified joins:
+
+ .. code-block::
+
+ T1 { [INNER] | { LEFT | RIGHT | FULL } [OUTER] } JOIN T2 ON condition
+
+ The words ``INNER`` and ``OUTER`` are optional for all forms.
+ ``INNER`` is the default; ``LEFT``, ``RIGHT`` and ``FULL`` imply ``OUTER``
+ join.
+
+INNER JOIN
+----------
+
+For each row R1 of T1, the joined table has a row for each row in T2 that
+satisfies the join condition with R1.
+
+.. code-block:: sql
+ :caption: Inner join
+
+ SELECT p.title, a.name
+ FROM poem p
+ JOIN author a on p.author_id = a.id;
+
++------------------------+---------------+
+| title | name |
++========================+===============+
+| On the Field of Battle | Larysa Kosach |
++------------------------+---------------+
+| Moses | Ivan Franko |
++------------------------+---------------+
+| The Forest Song | Larysa Kosach |
++------------------------+---------------+
+| The Singing Stones | Larysa Kosach |
++------------------------+---------------+
+| My Thoughts | Ivan Franko |
++------------------------+---------------+
+
+LEFT OUTER JOIN
+---------------
+
+First, an inner join is performed. Then, for each row in T1 that does not
+satisfy the join condition with any row in T2, a joined row is added with null
+values in columns of T2. Thus, the joined table always has at least one row for
+each row in T1.
+
+.. code-block:: sql
+ :caption: Left join
+
+ SELECT p.title, a.name
+ FROM poem p
+ LEFT JOIN author a on p.author_id = a.id;
+
++------------------------+---------------+
+| title | name |
++========================+===============+
+| On the Field of Battle | Larysa Kosach |
++------------------------+---------------+
+| Moses | Ivan Franko |
++------------------------+---------------+
+| The Forest Song | Larysa Kosach |
++------------------------+---------------+
+| In the Catacombs | null |
++------------------------+---------------+
+| The Singing Stones | Larysa Kosach |
++------------------------+---------------+
+| Ancient Fairy Tales | null |
++------------------------+---------------+
+| My Thoughts | Ivan Franko |
++------------------------+---------------+
+| To Shevchenko | null |
++------------------------+---------------+
+
+RIGHT OUTER JOIN
+----------------
+
+First, an inner join is performed. Then, for each row in T2 that does not
+satisfy the join condition with any row in T1, a joined row is added with
+null values in columns of T1. This is the converse of a left join: the result
+table will always have a row for each row in T2.
+
+.. code-block:: sql
+ :caption: Right join
+
+ SELECT p.title, a.name
+ FROM poem p
+ RIGHT JOIN author a on p.author_id = a.id;
+
++------------------------+---------------+
+| title | name |
++========================+===============+
+| On the Field of Battle | Larysa Kosach |
++------------------------+---------------+
+| Moses | Ivan Franko |
++------------------------+---------------+
+| The Forest Song | Larysa Kosach |
++------------------------+---------------+
+| The Singing Stones | Larysa Kosach |
++------------------------+---------------+
+| My Thoughts | Ivan Franko |
++------------------------+---------------+
+| null | John Doe |
++------------------------+---------------+
+
+FULL OUTER JOIN
+---------------
+
+First, an inner join is performed. Then, for each row in T1 that does not
+satisfy the join condition with any row in T2, a joined row is added with
+null values in columns of T2. Also, for each row of T2 that does not satisfy
+the join condition with any row in T1, a joined row with null values in
+the columns of T1 is added.
+
+.. code-block:: sql
+ :caption: Full join
+
+ SELECT p.title, a.name
+ FROM poem p
+ FULL JOIN author a on p.author_id = a.id;
+
++------------------------+---------------+
+| title | name |
++========================+===============+
+| On the Field of Battle | Larysa Kosach |
++------------------------+---------------+
+| Moses | Ivan Franko |
++------------------------+---------------+
+| The Forest Song | Larysa Kosach |
++------------------------+---------------+
+| In the Catacombs | null |
++------------------------+---------------+
+| The Singing Stones | Larysa Kosach |
++------------------------+---------------+
+| Ancient Fairy Tales | null |
++------------------------+---------------+
+| My Thoughts | Ivan Franko |
++------------------------+---------------+
+| To Shevchenko | null |
++------------------------+---------------+
+| null | John Doe |
++------------------------+---------------+