ITEC325 mysqli-basics -- Interacting with a Database

mysqli-basics
Interacting with a Database
Ullman chapter 12

youtube (8m34s):

This chapter will focus on accessing MySql database tables from PHP programs. In DB1, you worked with an Oracle database. For this course, we'll show examples using MySql, for two shallow reasons:

it hardly makes any difference, since both accept the standard SQL you learned about in DB1. (There are, however, non-standard variants that each of these accept that the other won’t. We try to steer clear of such differences.)

You can easily install MySQL on your own machine. (If you already install XAMP, note that the "M" stands for "MySQL"!)

The only two differences, if you wanted to connect to a different database engine than the one shown here: a slightly different connection string, and (unfortunately) changing the names of all functions “mysqli_something” to “oci_something”.
We'll mention (our appalling lack of) using ODBC below.

Setting up a MySQL database on your RU account:

Go to php.radford.edu » MySQL hosting page.
Follow the instructions for Request/create a MySQL database.
Once you have set up your MySQL account, you can write php programs that save/retrieve info from your database.
If you want to interact with your database via MySQL Admin Tool, you can follow links to php.radford.edu » MySQL hosting page » RU Mysql Admin Tool.

If you want to connect to the department Oracle server you used in itec340, instead of a MySQL database, here is an example using Oracle: db-oracle-connect.php.

SQL queries from PHP

no video: We talked about, in class, the basics of using php to create a string holding your DB query.

PHP's mysqli_query function is the most-used command. It sends an SQL command to MySQL, and returns the result from the DB:
$result = mysqli_query(connection, SQL statement);
However, before we do this, we need to get a connection to the database: create the connection (with username and password), and then select which schema you want to use (in this class, you'll use the one personal schema that RU gives you).

RU's connection to a MySQL database —

$conn = mysqli_connect(hostname, username, password, schemaToUse );
// @param hostname  For php.radford.edu, use 'localhost'
// @param username  the database account, which is the same as your RU userid
// @param password  the *database* password.
// @param schemaToUse your RU userid (again!) -- it's a DB account *and* a DB-schema.

Eventually, when you are done working with the database, be sure to close the connection!
mysqli_close(connection);
Although the database and/or php will eventually reclaim old, unused database connections, it's always better to do so yourself.
(Inadvertent denial-of-resource attack: students create connections w/o closing them; after 10 students each running their program 10 times, the DB server gets clogged up, and new connections start taking a long time!)

How to handle a failed-connection:

If the database connection fails, you want to throw an exception:
throw new Exception("Error: surroundingFunction: database connection failed." ).

A common expediency is to crash — by calling die. This is lousy practice — exceptions are explicitly created for this purpose instead! (and it's not any more difficult to do). Some notes:

If you really need to, you can do some contortions to catch a die.)

Beware being sneaky, using ||: A common snippet seen in web pages is:

$connect = mysqli_connect(...) || die('connect failed.');
echo "The connection ", ($connect ? "" : "NOT "), "established.\n";

// If I use '... = connect || die' like this, then the following fails(!):
$info = mysqli_query( $connect, "SELECT * FROM Students" );

Note that the green and red text don't mingle: The boolean operator || always returns a boolean (not merely boolean-ish), which not the same as whatever the result of mysqli_connect was. So when we try mysqli_querythe first argument is true, and not a connection-resource.

Beware being sneaky, using or: The “or” operator is just like the || operator, except not:
- || works with true-ish values, but always returns a real boolean, while or returns the true-ish value that triggered the short-circuit (or, the last value).
- || has medium-high precedence, while or has very-low precedence, lower even that =.
This means that writing $connect = mysqli_connect(...) or die('connect failed.'); is the same as writing ($connect = mysqli_connect(...) ) or die('connect failed.'); — which does what we want, but is relying on some subtle corner-cases that other programmers may not expect/understand.
(Note that you'll see this construct because an older, deprecated function mysql_query had a convenient feature that the connection was optional, and defaulted to the most-recently-opened-connection. Nowadays, the preferred solution is to use a connection-object, and call it's method query.)

Using odbc_* as opposed to mysqli_*:

Alas, some important function have no generic odbc counterpart: e.g. mysqli_real_escape_string.
There are some minor differences: e.g. there is no odbc_select_db because that functionality is subsumed by odbc_connect's more general first argument (which is a connection-string including many details).
odbc_connect is not installed on php.radford.edu (I'm not sure why — isn't it a provided, standard php function?)

Example To connect to an oracle database (instead of mysql), the function names are prefixed with “oci”.
See: db-oracle-connect.php
(be sure to close the connection after making it)

Security note: Your php code contains your database password. Be extra sure your .php file is not public-readable (that is, readable by any other user logged in to rucs)!

Common strategy: Put your 'connect' statements into a separate, included file. (a) This helps because you'll be re-using this code many times; (b) even if you show_source of the main file, your password is in the other file.

One kludgy way of doing this is to have the include-file store the connection it creates in a special variable — perhaps “theConnection” — and then all your files using the database will all have to remember ot use that one variable (and not to use that variable themselves).

Better, I suggest having that file define a function named something like DB_connect_as_ibarland(), and it returns the database connection. That lets each file use whatever variable it wants to store the result (though yes, each file still needs to not trample on the name of that function.)

It's also possible to simply have the external file define a couple variables (like $dbUserName = … and $dbUserPasswd = …), and keep that file non-public.

Common SQL Commands

SELECT - retrieves records from a table
INSERT - adds a record to the table
UPDATE - updates records in a table
DELETE - deletes individual records from a table
CREATE - creates a table
DROP - deletes a table

INSERT -


$var = 'Amy Johnson';
$qu = "INSERT INTO $tablename (numItems,product,custName) VALUES (17,'abc','$var')";
$allRows = mysqli_query($connect, $qu);

SELECT -

video via distance section: We talked about this material in class, week of apr.04

$qu = "SELECT * FROM $tablename WHERE (product = 'abc')";

$allRows = mysqli_query($connect, $qu);

After execution of this query, $allRows will presumably be a table — “bunch” of rows (PHP/Mysql calls it a “resource”). You'll then want to process each row, one-by-one:

while ($oneRow = mysqli_fetch_assoc($allRows)) {  # Note use of `=` for assignment *and* return value
    echo $oneRow['custName'], " bought ", $oneRow['product'];
    }

aside: There are a bunch of concepts raised, by the above code.

What is the abstract type of a database table? What type does php use?

What's the difference between a set and (say) a list?

What is the abstract type of a table-row? What type does php use?

An assignment statement, inside a loop-condition?!

Is that even allowed? (assignments as expressions)

Is it good practice? If not, why is it even allowed?

How does Java's Scanner avoid this, when reading input?

See also: mysqli_fetch_all: it gets the entire table into a single array. What are the efficiency implications of this? (And, do we care?)

You might want to check for an error — something like the database complaining that it wasn't a valid SQL statement, or that the database connection had been lost (after being previously created).

$allRows = mysqli_query($connect, $qry);         // A table object (possibly w/ 0 rows); NULL on query-failure.
if (!$allRows)  echo "query failed -- lost connection?";

Note that getting back a table with 0 rows is not an error!

If you know your query should always return 1 row, so you won't have a loop. Other times, you just want to know whether the query returned 0 or 1 rows (e.g., you queried something like studentID='1234' and libraryFine = 0). In that case, you might have:

$allRows = mysqli_query($connect, $qry);         // A table object (possibly w/ 0 rows); NULL on query-failure.
if (!$allRows)  echo "query failed -- lost connection?";

$topRow = mysqli_fetch_array($allRows);

if (!$topRow)  {
    echo "table had 0 rows"
    }
else {
    echo "table had (at least) 1 row".
    }

Even if your query should return 0 or 1 rows, because of SQL injection (below) be wary about assuming "there was at least 1 row" with "there was exactly 1 row"; you might want to check:

$allRows = mysqli_query($connect, $qry);         // A table object (possibly w/ 0 rows); NULL on query-failure.
if (!$allRows)  echo "query failed -- lost connection?";

$numRows = mysqli_num_rows($allRows);
if ($numRows == 0) {
    echo "table had 0 rows"
    }
else if ($numRows == 1) {
    echo "table had exactly 1 row".
    $topRow = mysqli_fetch_array($allRows);
    }
else {
    echo <<<END_OF_MSG
    Table had more than 1 row.
    If I think my query shouldn't every yield > 1 row,
      then I might just have built my query incorrectly.

    OR, if I incorporated user-input when building my query,
    then my code may have just succumbed to a SQL-injection attack! 
    Read below, about using  on user-input,
    before splicing it into a SQL query.
    END_OF_MSG;
    }

If your loop sometimes finishes early, w/o looping over every row of the table, then you should tell the server it doesn't need to keep holding the other results: mysqli_free_result( $allRows );. (Or, if you've been quick and the DB is still processing the (end results of) the query, this will even get the DB server to cancel its work.) Even better, if you know in advance that you only want 1 (or 7, or 29) rows, some SQL variants allow you to specify that in the SELECT statement via a LIMIT or ROWNUM < n clause.

DELETE -
$qu = "DELETE FROM $tablename WHERE (field1 = 'abc')";
To see if a DELETE query worked, you can use the mysqli_affected_rows($connect)
this function returns the number of rows affected by an INSERT, DELETE, or UPDATE
UPDATE -
$qu = "UPDATE $tablename SET $field_name1 = 'abc' , $field_name2 = '$newval' WHERE ($field_name1 = '$var')"; $allRows = mysqli_query($connect, $qu);
Remember that usually, when using SQL's UPDATE and DELETE, we'll provide a WHERE clause involving the table's primary key, so that the update/delete targets just one row. (You can certainly do mass updates/deletes, but they're not as common.)
Example: mysqli-example.php.
Also, you can look at object-oriented version: instead of someMethod($someObj, $otherArgs …) you just ¹ write $someObj->someMethod($otherArgs …).

Foreign Key Constraints in mysql

Warning:
In order to get a Foreign Key constraint, you must create both tables involved with the Storage Engine option as "InnoDB". (Sadly, on php.radford.edu, this isn't installed, and you have to settle for the MyISAM engine which doesn't allow Foreign Key constraints.)

You can either include the FK constraint with the CREATE TABLE command, or you can create the table (perhaps using the GUI view) and then go back and add the FK constraint one of two ways:
Via a raw SQL command: In yourTable » SQL, type in the exact SQL command, such as:
-- To create a FK:
-- make column `actor` in table `Movies` has to refer to some entry in table `Actors` column `name`:
alter table Movies
add constraint Movies_actor_FK
Foreign key (actor)
references Actors(name)

-- To delete:
alter table Goo
drop foreign key Goo_ij_FK
  -- it seems odd to me, to say 'drop foreign key ...' rather than 'drop constraint ...'.
Via the GUI: In yourTable » Structure » Relation View, in the column to the right of internal, you can set the foreign key. (That column won’t be there if not using InnoDB.) You can't use the GUI approach if your key involves a composite key.

Warning: MySQL ignores check constraints — it lets you specify them, but ignores them w/o letting you know(!).

Rumor: I saw one reasonable-sounding post that claimed that you also have to build an index on both the source and destination key-columns.

tip: If googling problems, stackoverflow tends to have better-curated answers than most sites. (e.g. this one)

Q: What is the problem with the following?:

$theItem = "coffee"; $theQuery = "SELECT * FROM Inventory WHERE (product = $theItem)"; $results = mysqli_query($connect, $theQuery );
A1: in the string “SELECT * FROM Inventory WHERE (product = coffee)”, there are no SQL-quotes around the value “coffee”, so that the contents of $theItem is interpreted as a SQL expression (not a string constant): “SELECT * FROM Inventory WHERE (product = '$theItem')”.
This is a very common sort of error, and it can be detected/debugged by printing $theQuery, and running that query on the database by hand to check whether it really is legal SQL.
What if we change the first line to $theItem = htmlspecialchars($_POST['itemSelection']);? (Presuming we came from a form with a text-entry named itemSelection).
A2: If somebody had entered the four-character string “M&Ms” we would not want want to look in our database for something including the characters “amp”. (Our database should include the correct name of the product; we'll turn “M&Ms” into “M&Ms” only when we display that on a web page — but not in emails, internal reports, etc..)
Okay, so we shouldn't use htmlspecialchars since we're not converting the raw-data to HTML-presentation-data.
So it seems our first line should be $theItem = $_POST['itemSelection']; to use info from a form, right?
A3: What if the user had entered a name that contains a single-quote? Then the string-you-made-and-though-was-valid-SQL suddenly might not be valid SQL any more. See the next lecture, re SQL Injection!

¹ Well, in this case, there are two other minor differences: to create the connection call “new mysqli(…)” instead of “mysqli_connect(…)”; and to check for errors look at the (string) field “$connect->connect_error” instead of checking for $connect being null. ↩

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(incl. typos, broken links)
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mysqli-basics Interacting with a DatabaseUllman chapter 12

SQL queries from PHP

mysqli-basics
Interacting with a Database
Ullman chapter 12