SQL Query

On Monday, we began talking about dataases and went over simple SQL to create a database, create a table, and do very simple inserts.
Today we begin by looking at how to see what's in a database table.

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 above code is slightly older SQL syntax, one could have written the above as:

SELECT STUDENT.NAME, COUNT(*) /* AVG, COUNT, SUM are aggregate functions*/
FROM STUDENT
JOIN COURSE
ON STUDENT.ID=COURSE.SID //the ON clause is use to say how tables are connected
                         // one can also have a where clause for condition not related two tables
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 // might use JOINs
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;
```

In-Class Exercise

Give the SQL needed to do the following:
1. Create a Blog Database. Create a Entry table (come up with columns which you think would be suitable for storing a blog entry).
2. Insert three cheese related entries into your blog.
3. Change the title of the blog entry with "I like cheese" to "Jaime fromage".
4. Get all entries from the blog posted in the last week.
Please post your solution to the Oct 5 In-Class Exercise Thread

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.

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.

SQL and Mysql

Outline

SQL Query

SQL Insert, Delete, Update

In-Class Exercise

PHP Database APIs

Connecting to a database from PHP

Selecting a Database and queries

Selecting a Database and queries