What is KEY?

KEYS:

The relationships between columns located in different tables are usually described through the use of keys.

As you can see we have a PRIMARY KEY in each table. The Primary key serves as a mechanism to refer to other fields within the same row. In this case, the Primary key is used to identify a relationship between a row under consideration and the person whose name is located inside the 'names' table. We use the AUTO_INCREMENT statement only for the 'names' table as we need to use the generated contact_id number in all the other tables for identification of the rows.

This type of table design where one table establishes a relationship with several other tables is known as a 'one to many'relationship.
In a 'many to many' relationship we could have several Auto Incremented Primary Keys in various tables with several inter-relationships.

Foreign Key:

A foreign key is a field in a table which is also the Primary Key in another table. This is known commonly as 'referential integrity'.

Execute the following commands to see the newly created tables and their contents.

To see the tables inside the database:

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

To see the columns within a particular table:

mysql>SHOW COLUMNS FROM address;
+---------------+-------------+------+-----+---------+-------+---------------------------------+
| Field | Type | Null | Key | Default | Extra | Privileges
|
+---------------+-------------+------+-----+---------+-------+---------------------------------+
| contact_id | smallint(6) | | PRI | 0 | | select,insert,update,references |
| StreetAddress | char(50) | YES | | NULL | | select,insert,update,references |
| City | char(20) | YES | | NULL | | select,insert,update,references |
| State | char(20) | YES | | NULL | | select,insert,update,references |
| Zip | char(10) | YES | | NULL | | select,insert,update,references |
| Country | char(20) | YES | | NULL | | select,insert,update,references |
+---------------+-------------+------+-----+---------+-------+------------------ ---------------+
6 rows in set (0.00 sec)

So we have the tables created and ready. Now we put in some data.Let's start with the 'names' table as it uses a unique AUTO_INCREMENT field which in turn is used in the other tables.

Inserting data, one row at a time:

mysql> INSERT INTO names (FirstName, LastName, BirthDate) VALUES ('Yamila','Diaz ','1974-10-13');
Query OK, 1 row affected (0.00 sec)

Inserting multiple rows at a time:

mysql> INSERT INTO names (FirstName, LastName, BirthDate) VALUES ('Nikki','Taylor','1972-03-04'),('Tia','Carrera','1975-09-18');
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0

Let's see what the data looks like inside the table. We use the SELECT command for this.

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)

Try another handy command called 'DESCRIBE'.

mysql> DESCRIBE names;
+------------+-------------+------+-----+---------+----------------+---------------------------------+
| Field | Type | Null | Key | Default | Extra | Privileges
|
+------------+-------------+------+-----+---------+----------------+---------------------------------+
| contact_id | smallint(6) | | PRI | NULL | auto_increment | select,insert,update,references |
| FirstName | char(20) | YES | | NULL | | select,insert,update,references |
| LastName | char(20) | YES | | NULL | | select,insert,update,references |
| BirthDate | date | YES | | NULL | | select,insert,update,references |
+------------+-------------+------+-----+---------+----------------+---------------------------------+
4 rows in set (0.00 sec)

Now lets populate the other tables. Observer the syntax used.

mysql> INSERT INTO address(contact_id, StreetAddress, City, State, Zip, Country) VALUES ('1', '300 Yamila Ave.', 'Los Angeles', 'CA', '300012', 'USA'),('2','4000 Nikki St.','Boca Raton','FL','500034','USA'),('3','404 Tia Blvd.','New York','NY','10011','USA');
Query OK, 3 rows affected (0.05 sec)
Records: 3 Duplicates: 0 Warnings: 0

mysql> SELECT * FROM address;
+------------+-----------------+-------------+-------+--------+---------+
| contact_id | StreetAddress | City | State | Zip | Country |
+------------+-----------------+-------------+-------+--------+---------+
| 1 | 300 Yamila Ave. | Los Angeles | CA | 300012 | USA |
| 2 | 4000 Nikki St. | Boca Raton | FL | 500034 | USA |
| 3 | 404 Tia Blvd. | New York | NY | 10011 | USA |
+------------+-----------------+-------------+-------+--------+---------+
3 rows in set (0.00 sec)

mysql> INSERT INTO company_details (contact_id, CompanyName, Designation) VALUES ('1','Xerox','New Business Manager'), ('2','Cabletron','Customer Support Eng'),('3','Apple','Sales Manager');
Query OK, 3 rows affected (0.05 sec)
Records: 3 Duplicates: 0 Warnings: 0

mysql> SELECT * FROM company_details;
+------------+-------------+----------------------+
| contact_id | CompanyName | Designation |
+------------+-------------+----------------------+
| 1 | Xerox | New Business Manager |
| 2 | Cabletron | Customer Support Eng |
| 3 | Apple | Sales Manager |
+------------+-------------+----------------------+
3 rows in set (0.06 sec)

mysql> INSERT INTO email (contact_id, Email) VALUES ('1', 'yamila@yamila.com'),( '2', 'nikki@nikki.com'),('3','tia@tia.com');
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0

mysql> SELECT * FROM email;
+------------+-------------------+
| contact_id | Email |
+------------+-------------------+
| 1 | yamila@yamila.com |
| 2 | nikki@nikki.com |
| 3 | tia@tia.com |
+------------+-------------------+
3 rows in set (0.06 sec)

mysql> INSERT INTO telephones (contact_id, TelephoneHome, TelephoneWork) VALUES ('1','333-50000','333-60000'),('2','444-70000','444-80000'),('3','555-30000','55 5-40000');
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0

mysql> SELECT * FROM telephones;
+------------+---------------+---------------+
| contact_id | TelephoneHome | TelephoneWork |
+------------+---------------+---------------+
| 1 | 333-50000 | 333-60000 |
| 2 | 444-70000 | 444-80000 |
| 3 | 555-30000 | 555-40000 |
+------------+---------------+---------------+
3 rows in set (0.00 sec)

Okay, so we now have all our data ready for experimentation.

---

Before we start experimenting with manipulating the data let's look at how MySQL stores the Data.

To do this execute the following command from the shell prompt.

mysqldump contacts > contacts.sql

Note: The reverse operation for this command is:

mysql contacts < contacts.sql

The file generated is a text file that contains all the data and SQL instruction needed to recreate the same database. As you can see, the SQL here is slightly different than what was typed in. Don't worry about this. It's all good ! It would also be obvious that this is a good way to backup your stuff.

# MySQL dump 8.2
#
# Host: localhost Database: contacts
#--------------------------------------------------------
# Server version 3.22.34-shareware-debug

#
# Table structure for table 'address'
#

CREATE TABLE address (
contact_id smallint(6) DEFAULT '0' NOT NULL,
StreetAddress char(50),
City char(20),
State char(20),
Zip char(10),
Country char(20),
PRIMARY KEY (contact_id)
);

#
# Dumping data for table 'address'
#

INSERT INTO address VALUES (1,'300 Yamila Ave.','Los Angeles','CA','300012','USA');
INSERT INTO address VALUES (2,'4000 Nikki St.','Boca Raton','FL','500034','USA');
INSERT INTO address VALUES (3,'404 Tia Blvd.','New York','NY','10011','USA');

#
# Table structure for table 'company_details'
#

CREATE TABLE company_details (
contact_id smallint(6) DEFAULT '0' NOT NULL,
CompanyName char(25),
Designation char(20),
PRIMARY KEY (contact_id)
);

#
# Dumping data for table 'company_details'
#

INSERT INTO company_details VALUES (1,'Xerox','New Business Manager');
INSERT INTO company_details VALUES (2,'Cabletron','Customer Support Eng');
INSERT INTO company_details VALUES (3,'Apple','Sales Manager');

#
# Table structure for table 'email'
#

CREATE TABLE email (
contact_id smallint(6) DEFAULT '0' NOT NULL,
Email char(20),
PRIMARY KEY (contact_id)
);

#
# Dumping data for table 'email'
#

INSERT INTO email VALUES (1,'yamila@yamila.com');
INSERT INTO email VALUES (2,'nikki@nikki.com');
INSERT INTO email VALUES (3,'tia@tia.com');

#
# Table structure for table 'names'
#

CREATE TABLE names (
contact_id smallint(6) DEFAULT '0' NOT NULL auto_increment,
FirstName char(20),
LastName char(20),
BirthDate date,
PRIMARY KEY (contact_id)
);

#
# Dumping data for table 'names'
#

INSERT INTO names VALUES (3,'Tia','Carrera','1975-09-18');
INSERT INTO names VALUES (2,'Nikki','Taylor','1972-03-04');
INSERT INTO names VALUES (1,'Yamila','Diaz','1974-10-13');

#
# Table structure for table 'telephones'
#

CREATE TABLE telephones (
contact_id smallint(6) DEFAULT '0' NOT NULL,
TelephoneHome char(20),
TelephoneWork char(20),
PRIMARY KEY (contact_id)
);

#
# Dumping data for table 'telephones'
#

INSERT INTO telephones VALUES (1,'333-50000','333-60000');
INSERT INTO telephones VALUES (2,'444-70000','444-80000');
INSERT INTO telephones VALUES (3,'555-30000','555-40000');

---

Let's try some SELECT statement variations:

To select all names whose corresponding contact_id is greater than 1.

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

As a condition we can also use NOT NULL. This statement will return all names where there exists a contact_id.

mysql> SELECT * FROM names WHERE contact_id IS NOT NULL;
+------------+-----------+----------+------------+
| 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)

Result's can be arranged in a particular way using the statement ORDER BY.

mysql> SELECT * FROM names WHERE contact_id IS NOT NULL ORDER BY LastName;
+------------+-----------+----------+------------+
| contact_id | FirstName | LastName | BirthDate |
+------------+-----------+----------+------------+
| 3 | Tia | Carrera | 1975-09-18 |
| 1 | Yamila | Diaz | 1974-10-13 |
| 2 | Nikki | Taylor | 1972-03-04 |
+------------+-----------+----------+------------+
3 rows in set (0.06 sec)

'asc' and 'desc' stand for ascending and descending respectively and can be used to arrange the results.

mysql> SELECT * FROM names WHERE contact_id IS NOT NULL ORDER BY LastName desc;
+------------+-----------+----------+------------+
| contact_id | FirstName | LastName | BirthDate |
+------------+-----------+----------+------------+
| 2 | Nikki | Taylor | 1972-03-04 |
| 1 | Yamila | Diaz | 1974-10-13 |
| 3 | Tia | Carrera | 1975-09-18 |
+------------+-----------+----------+------------+
3 rows in set (0.04 sec)

You can also place date types into conditional statements.

mysql> SELECT * FROM names WHERE BirthDate > '1973-03-06';
+------------+-----------+----------+------------+
| contact_id | FirstName | LastName | BirthDate |
+------------+-----------+----------+------------+
| 3 | Tia | Carrera | 1975-09-18 |
| 1 | Yamila | Diaz | 1974-10-13 |
+------------+-----------+----------+------------+

2 rows in set (0.00 sec)

LIKE is a statement to match field values using wildcards. The % sign is used for denoting wildcards and can represent multiple characters.

mysql> SELECT FirstName, LastName FROM names WHERE LastName LIKE 'C%';
+-----------+----------+
| FirstName | LastName |
+-----------+----------+
| Tia | Carrera |
+-----------+----------+
1 row in set (0.06 sec)

'_' is used to represent a single wildcard.

mysql> SELECT FirstName, LastName FROM names WHERE LastName LIKE '_iaz';
+-----------+----------+
| FirstName | LastName |
+-----------+----------+
| Yamila | Diaz |
+-----------+----------+
1 row in set (0.00 sec)

SQL Logical Operations (operates from Left to Right)

1.NOT or !

2. AND or &&

3. OR or ||

4. = : Equal

5. <> or != : Not Equal

6. <=

7. >=

8 <,>

Here are some more variations with Logical Operators and using the 'IN' statement.

mysql> SELECT FirstName FROM names WHERE contact_id < 3 AND LastName LIKE 'D%';
+-----------+
| FirstName |
+-----------+
| Yamila |
+-----------+
1 row in set (0.00 sec)

mysql> SELECT contact_id FROM names WHERE LastName IN ('Diaz','Carrera');
+------------+
| contact_id |
+------------+
| 3 |
| 1 |
+------------+
2 rows in set (0.02 sec)

To return the number of rows in a table

mysql> SELECT count(*) FROM names;
+----------+
| count(*) |
+----------+
| 3 |
+----------+
1 row in set (0.02 sec)

mysql> SELECT count(FirstName) FROM names;
+------------------+
| count(FirstName) |
+------------------+
| 3 |
+------------------+
1 row in set (0.00 sec)

To do some basic arithmetic aggregate functions.

mysql> SELECT SUM(contact_id) FROM names;
+-----------------+
| SUM(contact_id) |
+-----------------+
| 6 |
+-----------------+
1 row in set (0.00 sec)

To select a largest value from a row. Substitute 'MIN' and see what happens next.

mysql> SELECT MAX(contact_id) FROM names;
+-----------------+
| MAX(contact_id) |
+-----------------+
| 3 |
+-----------------+
1 row in set (0.00 sec)

HAVING

Take a look at the first query using the statement WHERE and the second statement using the statement HAVING.

mysql> SELECT * FROM names WHERE contact_id >=1;
+------------+-----------+----------+------------+
| contact_id | FirstName | LastName | BirthDate |
+------------+-----------+----------+------------+
| 1 | Yamila | Diaz | 1974-10-13 |
| 2 | Nikki | Taylor | 1972-03-04 |
| 3 | Tia | Carrera | 1975-09-18 |
+------------+-----------+----------+------------+
3 rows in set (0.03 sec)

mysql> SELECT * FROM names HAVING contact_id >=1;
+------------+-----------+----------+------------+
| 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)

---

Now lets work with multiple tables and see how information can be pulled out of the data.

mysql> SELECT names.contact_id, FirstName, LastName, Email FROM names, email WHERE names.contact_id = email.contact_id;
+------------+-----------+----------+-------------------+
| contact_id | FirstName | LastName | Email |
+------------+-----------+----------+-------------------+
| 1 | Yamila | Diaz | yamila@yamila.com |
| 2 | Nikki | Taylor | nikki@nikki.com |
| 3 | Tia | Carrera | tia@tia.com |
+------------+-----------+----------+-------------------+
3 rows in set (0.11 sec)

mysql> SELECT DISTINCT names.contact_id, FirstName, Email, TelephoneWork FROM names, email, telephones WHERE names.contact_id=email.contact_id=telephones.contact_id;
+------------+-----------+-------------------+---------------+
| contact_id | FirstName | Email | TelephoneWork |
+------------+-----------+-------------------+---------------+
| 1 | Yamila | yamila@yamila.com | 333-60000 |
| 2 | Nikki | nikki@nikki.com | 333-60000 |
| 3 | Tia | tia@tia.com | 333-60000 |
+------------+-----------+-------------------+---------------+
3 rows in set (0.05 sec)

---