Database Management Systems and Databases

Models in the MVA pattern often interact with a database so we are going to have a whirlwind tour of databases to learn how to do this.

For our purposes, a database management systems (DBMS) is used to manage a collection of databases.
It manages how the data is stored in each database, and who can access it when.
A given database consists of a collection of tables.
Each table has a type for each of its columns and then has rows of data where the data values match the values of the column types.
For example, car(make VARCHAR(10), model VARCHAR(10), year DATE) is an example table schema (specification). A possible row of car is:
```
('Toyota', 'Corolla', '1970-01-01')
```

MySQL Databases

Let's learn how to write to databases now, so we have a good infrastructure to write models.
To manage MySQL databases you need to know how to:
- Create and manage users
- Create new databases and tables
- Do lots of other stuff I won't talk about
- Many of these operations can be done using phpMyAdmin.
- Alternatively, you can use the command line.

MySQL Command Line

I will assume you have set your path so that you can directly run mysqladmin and mysql.
To set the root password you can go to the command prompt and type:
```
mysqladmin -uroot password what_password_is
```
You can get a mysql shell by then typing:
```
mysql -uroot -pwhat_password_is
```

MySQL Command Line

To see what databases are available you can use the command:
```
show databases;
```
To use a particular database type:
```
use dbname;
```
To see what tables the currently use database has type:
```
show tables;
```

To add a new user you can type at the prompt:

GRANT SELECT, INSERT, UPDATE, DELETE, LOCK TABLES 
  on dbname.* TO username@127.0.0.1 IDENTIFIED BY 'password';

To quit mysql type \q or quit. For help type \h or help.
To login using this new user type at the command prompt:
```
mysql -uusername -ppassword
```

Quiz

Which of the following statements is true?

Once a file is locked, unless flock is called with the LOCK_UN flag, no process can access that file.
Cookies are read and written using a non-HTTP protocol.
One goal of the POST-REDIRECT-GET pattern is to ensure that POST'd forms are not accidentally resubmitted.

Basic SQL

SQL (Structured Query Language) is the most common language for issuing commands to a database system.
These commands can usually be given either from the mysql prompt or from within a PHP transaction that is connecting to the database.

Database, Table Creation Deletion

To create a database one uses the CREATE DATABASE command:
```
CREATE DATABASE my_nifty_db;
```
Then one can type use my_nifty_db; to use this database.

To create a table in the currently used database one can use a command like:

CREATE TABLE customer (
    cust_id int(5) NOT NULL,
    name VARCHAR(50),
    title CHAR(3),
    PRIMARY KEY(cust_id)
);

To delete a table type:
```
DROP TABLE customer;
```
To delete a database
```
DROP DATABASE my_nifty_db;
```

SQL Query

A basic SQL query has the format:

select col1, col2, col3
from table1, table2, table3
where condition_holds;
// you may also have additional clauses for grouping and ordering results.

The above query return rows consisting of the given columns of tables such that the given condition holds.

Let's look at some examples:

/* To Get First Names and Last Names of 
   Employees with Salary's bigger than $10000
 */
SELECT FNAME, LNAME
FROM EMPLOYEE
WHERE SALARY > 10000;

/*
   To get triples (FNAME, LNAME, PNAME) of Employees
   who work on projects with names containing
   the string "SECRET", sorting result in descending
   alphabetical order, showing results 10 through 60.
 */
SELECT E.LNAME AS LNAME,
   E.FNAME AS FNAME,    
   P.PROJECT_NAME AS PNAME
FROM EMPLOYEE E, PROJECT P, WORKS_ON W
WHERE E.EID=W.EID AND P.PID=W.PID
   AND P.PROJECT_NAME LIKE '%SECRET%'
ORDER BY E.LNAME DESC, E.FNAME DESC, 
   P.PROJECT_NAME DESC
LIMIT 10,50; /* start at 10 plus next 50 */

/* Number of classes each student is taking
 */
SELECT STUDENT.NAME, COUNT(*) /* AVG, COUNT, SUM are aggregate functions*/
FROM STUDENT, COURSE
WHERE STUDENT.ID=COURSE.SID
GROUP BY STUDENT.NAME;

The general form of a select statement looks like:
```
 
SELECT T1.FIELD_NAME_1 AS NAME1, ..., T1.FIELD_NAME_N AS NAME_N,
 ..., TM.FIELD_NAME_N AS SOME_NAME
FROM T1, T2, ..., TM
WHERE T1.FIELD_NAME_j = some value [OR|AND] ... T1.FIELD_NAME_k = TJ.FIELD_NAME_w
[GROUP BY ...]
[HAVING ...]
[ORDER BY ...]
[LIMIT ...]
```
The above are called SELECT statements and consist of three main lines (plus optional ones). The first line lists the columns we want, the second line lists the tables to use, the last line lists the conditions that must be met by the returned row (join conditions).
We have tried to format the SQL nicely but there is no requirement to do so.

SQL Insert, Delete, Update

To insert rows into a table you can use a command like:

INSERT INTO table_name VALUES ('col1_value', 'col2_value', 'col3_value');

or

INSERT INTO table_name VALUES ('col1_value_1', 'col2_value_1', 'col3_value_1'),
    ('col1_value_2', 'col2_value_2', 'col3_value_2'), ...
    ('col1_value_n', 'col2_value_n', 'col3_value_n');

or

INSERT INTO table_name 
    SELECT col_1, ..., col_n
    FROM table_1, ..., table_m
    WHERE condition;

To delete a set of rows from a table you can use a command like:
```
DELETE FROM table_name WHERE cond;
```
To update a set of rows you can use a command like:
```
UPDATE table_name SET column='value' WHERE cond;
```

PHP Database APIs

PHP can connect with many different database systems: DB2, Oracle, MariaDB, Mysql, Postgres, SQLite, etc.
To start we are going to look at how to do this for Mysql/MariaDB because this is what comes with MAMP, WAMP, XAMPP.
As we'll see there is usually more than one API to connect to the same DBMS in PHP: the "native" API, and the "generic" API using PDO.
The former tends to expose more DBMS specific abilities, the later tends to help your code portability if you need to switch which DBMS you are using.
For some DBMS's, there are actually two "native" APIs rather than one. For example, for mysql there are functions mysql_ and mysqli_, the former being the very common original way to connect to a Mysql database from PHP and the latter, the more modern way.
For new code, you should use mysqli_ as the mysql_ functions have been removed in PHP 7.
mysql_ is also a function only interface; whereas, mysqli_ can be used in a function (almost the same call as mysql_ but using mysqli_ ) or object-oriented approach (where create an object and use its methods).
Another situation of this type occur with the Sqlite and Sqlite3 APIs.

Connecting to a database from PHP

To start a connection to a MySQL database one can issue the command:
```
$db = mysqli_connect();
```
This function actually can take up to six parameters: the host, the username, the password, a database, a port, and a socket.
These default to localhost, the default mysql user and password, blank, default port and socket. A slightly more advanced, connection call might be:
```
$db = mysqli_connect($host, $uname, $pword, $db_name); 	
```
Depending on how mysql is configured, the first example above might work and saves some typing.
This function returns false if a connection is not made.
To close a database connection, one can call mysqli_close();

Selecting a Database and queries

To select a database after a connection has been made (sometimes useful) one calls:
```
mysqli_select_db($db, "vehicles");
```

One can then do a query with code like:

$query ="SELECT * FROM CAR";
$result = mysqli_query($db, $query);
$num_rows = mysqli_num_rows($result);
$num_fields = mysqli_num_fields($result);
echo $num_fields;
for($j = 1; $j <= $num_rows; $j++)
{
	$row = mysqli_fetch_array($result);
	print_r($row);
}

mysqli_query can also be used to do inserts, etc.

Database

Outline

Database Management Systems and Databases

MySQL Databases

MySQL Command Line

MySQL Command Line

Quiz

Basic SQL

Database, Table Creation Deletion

SQL Query

SQL Insert, Delete, Update

PHP Database APIs

Connecting to a database from PHP

Selecting a Database and queries