What is Join?

JOIN 

JOIN is the action performed on multiple tables that returns a result as a table. It's what makes a database 'relational'.

There are several types of joins. Let's look at LEFT JOIN (OUTER JOIN) and RIGHT JOIN

Let's first check out the contents of the tables we're going to use

mysql> SELECT * FROM names;
+------------+-----------+----------+------------+
| contact_id | FirstName | LastName | BirthDate |
+------------+-----------+----------+------------+
| 3 | Tia | Carrera | 1975-09-18 |
| 2 | Nikki | Taylor | 1972-03-04 |
| 1 | Yamila | Diaz | 1974-10-13 |
+------------+-----------+----------+------------+
3 rows in set (0.00 sec)

mysql> SELECT * FROM email;
+------------+-------------------+
| contact_id | Email |
+------------+-------------------+
| 1 | yamila@yamila.com |
| 2 | nikki@nikki.com |
| 3 | tia@tia.com |
+------------+-------------------+
3 rows in set (0.00 sec)

A LEFT JOIN First:

mysql> SELECT * FROM names LEFT JOIN email USING (contact_id);
+------------+-----------+----------+------------+------------+------------------+
| contact_id | FirstName | LastName | BirthDate | contact_id | Email|
+------------+-----------+----------+------------+------------+-------------------+
| 3 | Tia | Carrera | 1975-09-18 | 3 | tia@tia.com |
| 2 | Nikki | Taylor | 1972-03-04 | 2 | nikki@nikki.com |
| 1 | Yamila | Diaz | 1974-10-13 | 1 | yamila@yamila.com |
+------------+-----------+----------+------------+------------+-------------------+
3 rows in set (0.16 sec)

To find the people who have a home phone number.

mysql> SELECT names.FirstName FROM names LEFT JOIN telephones ON names.contact_id = telephones.contact_id WHERE TelephoneHome IS NOT NULL;
+-----------+
| FirstName |
+-----------+
| Tia |
| Nikki |
| Yamila |
+-----------+
3 rows in set (0.02 sec)

These same query leaving out 'names' (from names.FirstName) is still the same and will generate the same result.

mysql> SELECT FirstName FROM names LEFT JOIN telephones ON names.contact_id = telephones.contact_id WHERE TelephoneHome IS NOT NULL;
+-----------+
| FirstName |
+-----------+
| Tia |
| Nikki |
| Yamila |
+-----------+
3 rows in set (0.00 sec)

And now a RIGHT JOIN:

mysql> SELECT * FROM names RIGHT JOIN email USING(contact_id);
+------------+-----------+----------+------------+------------+----------------- --+
| contact_id | FirstName | LastName | BirthDate | contact_id | Email |
+------------+-----------+----------+------------+------------+-------------------+
| 1 | Yamila | Diaz | 1974-10-13 | 1 | yamila@yamila.com |
| 2 | Nikki | Taylor | 1972-03-04 | 2 | nikki@nikki.com
|
| 3 | Tia | Carrera | 1975-09-18 | 3 | tia@tia.com
|
+------------+-----------+----------+------------+------------+-----------------
--+
3 rows in set (0.03 sec)

---

BETWEEN

This conditional statement is used to select data where a certain related contraint falls between a certain range of values. The following example illustrates it's use.

mysql> SELECT * FROM names;
+------------+-----------+----------+------------+
| contact_id | FirstName | LastName | BirthDate |
+------------+-----------+----------+------------+
| 3 | Tia | Carrera | 1975-09-18 |
| 2 | Nikki | Taylor | 1972-03-04 |
| 1 | Yamila | Diaz | 1974-10-13 |
+------------+-----------+----------+------------+
3 rows in set (0.06 sec)

mysql> SELECT FirstName, LastName FROM names WHERE contact_id BETWEEN 2 AND 3;
+-----------+----------+
| FirstName | LastName |
+-----------+----------+
| Tia | Carrera |
| Nikki | Taylor |
+-----------+----------+
2 rows in set (0.00 sec)

---

ALTER

The ALTER statement is used to add a new column to an existing table or to make changes to it.

mysql> ALTER TABLE names ADD Age SMALLINT;
Query OK, 3 rows affected (0.11 sec)
Records: 3 Duplicates: 0 Warnings: 0

Now let's take a look at the 'ALTER'ed Table.

mysql> SHOW COLUMNS FROM names;
+------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+----------------+
| contact_id | smallint(6) | | PRI | 0 | auto_increment |
| FirstName | char(20) | YES | | NULL | |
| LastName | char(20) | YES | | NULL | |
| BirthDate | date | YES | | NULL | |
| Age | smallint(6) | YES | | NULL | |
+------------+-------------+------+-----+---------+----------------+
5 rows in set (0.06 sec)

But we don't require Age to be a SMALLINT type when a TINYINT would suffice. So we use another ALTER statement.

mysql> ALTER TABLE names CHANGE COLUMN Age Age TINYINT;
Query OK, 3 rows affected (0.02 sec)
Records: 3 Duplicates: 0 Warnings: 0

mysql> SHOW COLUMNS FROM names;
+------------+-------------+------+-----+---------+--------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+--------+----------------+
| contact_id | smallint(6) | | PRI | NULL |
| FirstName | char(20) | YES | | NULL | |
| LastName | char(20) | YES | | NULL | |
| BirthDate | date | YES | | NULL | |
| Age | tinyint(4) | YES | | NULL | |
+------------+-------------+------+-----+---------+--------+----------------+
5 rows in set (0.00 sec)

MODIFY

You can also use the MODIFY statement to change column data types.

mysql> ALTER TABLE names MODIFY COLUMN Age SMALLINT;
Query OK, 3 rows affected (0.03 sec)
Records: 3 Duplicates: 0 Warnings: 0

mysql> SHOW COLUMNS FROM names;
+------------+-------------+------+-----+---------+----------------+---------------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+----------------+---------------+
| contact_id | smallint(6) | | PRI | NULL | auto_increment |
| FirstName | char(20) | YES | | NULL | |
| LastName | char(20) | YES | | NULL | |
| BirthDate | date | YES | | NULL | |
| Age | smallint(6) | YES | | NULL | |
+------------+-------------+------+-----+---------+----------------+---------------+
5 rows in set (0.00 sec)

To Rename a Table:

mysql> ALTER TABLE names RENAME AS mynames;
Query OK, 0 rows affected (0.00 sec)

mysql> SHOW TABLES;
+--------------------+
| Tables_in_contacts |
+--------------------+
| address |
| company_details |
| email |
| mynames |
| telephones |
+--------------------+
5 rows in set (0.00 sec)

We rename it back to the original name.

mysql> ALTER TABLE mynames RENAME AS names;
Query OK, 0 rows affected (0.01 sec)

---

UPDATE

The UPDATE command is used to add a value to a field in a table.

mysql> UPDATE names SET Age ='23' WHERE FirstName='Tia';
Query OK, 1 row affected (0.06 sec)
Rows matched: 1 Changed: 1 Warnings: 0

The Original Table:

mysql> SELECT * FROM names;
+------------+-----------+----------+------------+------+
| contact_id | FirstName | LastName | BirthDate | Age |
+------------+-----------+----------+------------+------+
| 3 | Tia | Carrera | 1975-09-18 | 23 |
| 2 | Nikki | Taylor | 1972-03-04 | NULL |
| 1 | Yamila | Diaz | 1974-10-13 | NULL |
+------------+-----------+----------+------------+------+
3 rows in set (0.05 sec)

The Modified Table:

mysql> SELECT * FROM names;
+------------+-----------+----------+------------+------+
| contact_id | FirstName | LastName | BirthDate | Age |
+------------+-----------+----------+------------+------+
| 3 | Tia | Carrera | 1975-09-18 | 24 |
| 2 | Nikki | Taylor | 1972-03-04 | NULL |
| 1 | Yamila | Diaz | 1974-10-13 | NULL |
+------------+-----------+----------+------------+------+
3 rows in set (0.00 sec)

---

DELETE

mysql> DELETE FROM names WHERE Age=23;
Query OK, 1 row affected (0.00 sec)

mysql> SELECT * FROM names;
+------------+-----------+----------+------------+------+
| contact_id | FirstName | LastName | BirthDate | Age |
+------------+-----------+----------+------------+------+
| 2 | Nikki | Taylor | 1972-03-04 | NULL |
| 1 | Yamila | Diaz | 1974-10-13 | NULL |
+------------+-----------+----------+------------+------+
2 rows in set (0.00 sec)

A DEADLY MISTAKE...

mysql> DELETE FROM names;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT * FROM names;
Empty set (0.00 sec)

One more destructive tool...

DROP TABLE

mysql> DROP TABLE names;
Query OK, 0 rows affected (0.00 sec)

mysql> SHOW TABLES;
+--------------------+
| Tables in contacts |
+--------------------+
| address |
| company_details |
| email |
| telephones |
+--------------------+
4 rows in set (0.05 sec)

mysql> DROP TABLE address ,company_details, telephones;
Query OK, 0 rows affected (0.06 sec)

mysql> SHOW TABLES;
Empty set (0.00 sec)

As you can see, the table 'names' no longer exists. MySQL does not give a warning so be careful.

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FULL TEXT INDEXING and Searching

Since version 3.23.23, Full Text Indexing and Searching has been introduced into MySQL. FULLTEXT indexes can be created from VARCHAR and TEXT columns. FULLTEXT searches are performed with the MATCH function. The MATCH function matches a natural language query on a text collection and from each row in a table it returns relevance.The resultant rows are organized in order of relevance.

Full Text searches are a very powerful way to search through text. But is not ideal for small tables of text and may produce inconsistent results. Ideally it works with large collections of textual data.

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Optimizing your Database

Well, databases do tend to get large at some or the other. And here arises the issue of database optimization. Queries are going to take longer and longer as the database grows and certain things can be done to speed things up.

Clustering

The easiest method is that of 'clustering'. Suppose you do a certain kind of query often, it would be faster if the database contents were arranged in a in the same way data was requested. To keep the tables in a sorted order you need a clustering index. Some databases keep stuff sorted automatically.

Ordered Indices

These are a kind of 'lookup' tables of sorts. For each column that may be of interest to you, you can create an ordered index.It needs to be noted that again these kinds of optimization techniques produce a system load in terms of creating a new index each time the data is re-arranged.

There are additional method such as B-Trees, Hashing which you may like to read up about but will not be discussed here.

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Replication

Replication is the term given to the process where databases synchronize with each other. In this process one database updates it's own data with respect to another or with reference to certain criteria for updates specified by the programmer. Replication can be used under various circumstances. Examples may be : safety and backup, to provide a closer location to the database for certain users.

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