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+<html>
+<head>
+ <title>Tips on Writing Portable SQL for Multiple Databases for PHP</title>
+</head>
+
+<body bgcolor=white>
+<table width=100% border=0><tr><td><h2>Tips on Writing Portable SQL </h2></td><td>
+ <div align=right><img src="cute_icons_for_site/adodb.gif"></div></td></tr></table>
+ <p>Updated 18 Sep 2003. Added Portable Native SQL section.
+<p>
+
+ If you are writing an application that is used in multiple environments and
+ operating systems, you need to plan to support multiple databases. This article
+ is based on my experiences with multiple database systems, stretching from 4th
+ Dimension in my Mac days, to the databases I currently use, which are: Oracle,
+ FoxPro, Access, MS SQL Server and MySQL. Although most of the advice here applies
+ to using SQL with Perl, Python and other programming languages, I will focus on PHP and how
+ the <a href="http://adodb.sourceforge.net/">ADOdb</a> database abstraction library
+ offers some solutions.<p></p>
+<p>Most database vendors practice product lock-in. The best or fastest way to
+ do things is often implemented using proprietary extensions to SQL. This makes
+ it extremely hard to write portable SQL code that performs well under all conditions.
+ When the first ANSI committee got together in 1984 to standardize SQL, the database
+ vendors had such different implementations that they could only agree on the
+ core functionality of SQL. Many important application specific requirements
+ were not standardized, and after so many years since the ANSI effort began,
+ it looks as if much useful database functionality will never be standardized.
+ Even though ANSI-92 SQL has codified much more, we still have to implement portability
+ at the application level.</p>
+<h3><b>Selects</b></h3>
+<p>The SELECT statement has been standardized to a great degree. Nearly every
+ database supports the following:</p>
+<p>SELECT [cols] FROM [tables]<br>
+ [WHERE conditions]<br>
+ [GROUP BY cols]<br>
+ [HAVING conditions] <br>
+ [ORDER BY cols]</p>
+<p>But so many useful techniques can only be implemented by using proprietary
+ extensions. For example, when writing SQL to retrieve the first 10 rows for
+ paging, you could write...</p>
+<table width="80%" border="1" cellspacing="0" cellpadding="0" align="center">
+ <tr>
+ <td><b>Database</b></td>
+ <td><b>SQL Syntax</b></td>
+ </tr>
+ <tr>
+ <td>DB2</td>
+ <td>select * from table fetch first 10 rows only</td>
+ </tr>
+ <tr>
+ <td>Informix</td>
+ <td>select first 10 * from table</td>
+ </tr>
+ <tr>
+ <td>Microsoft SQL Server and Access</td>
+ <td>select top 10 * from table</td>
+ </tr>
+ <tr>
+ <td>MySQL and PostgreSQL</td>
+ <td>select * from table limit 10</td>
+ </tr>
+ <tr>
+ <td>Oracle 8i</td>
+ <td>select * from (select * from table) where rownum <= 10</td>
+ </tr>
+</table>
+<p>This feature of getting a subset of data is so useful that in the PHP class
+ library ADOdb, we have a SelectLimit( ) function that allows you to hide the
+ implementation details within a function that will rewrite your SQL for you:</p>
+<pre>$connection->SelectLimit('select * from table', 10);
+</pre>
+<p><b>Selects: Fetch Modes</b></p>
+<p>PHP allows you to retrieve database records as arrays. You can choose to have
+ the arrays indexed by field name or number. However different low-level PHP
+ database drivers are inconsistent in their indexing efforts. ADOdb allows you
+ to determine your prefered mode. You set this by setting the variable $ADODB_FETCH_MODE
+ to either of the constants ADODB_FETCH_NUM (for numeric indexes) or ADODB_FETCH_ASSOC
+ (using field names as an associative index).</p>
+<p>The default behaviour of ADOdb varies depending on the database you are using.
+ For consistency, set the fetch mode to either ADODB_FETCH_NUM (for speed) or
+ ADODB_FETCH_ASSOC (for convenience) at the beginning of your code. </p>
+<p><b>Selects: Counting Records</b></p>
+<p>Another problem with SELECTs is that some databases do not return the number
+ of rows retrieved from a select statement. This is because the highest performance
+ databases will return records to you even before the last record has been found.
+</p>
+<p>In ADOdb, RecordCount( ) returns the number of rows returned, or will emulate
+ it by buffering the rows and returning the count after all rows have been returned.
+ This can be disabled for performance reasons when retrieving large recordsets
+ by setting the global variable $ADODB_COUNTRECS = false. This variable is checked
+ every time a query is executed, so you can selectively choose which recordsets
+ to count.</p>
+<p>If you prefer to set $ADODB_COUNTRECS = false, ADOdb still has the PO_RecordCount(
+ ) function. This will return the number of rows, or if it is not found, it will
+ return an estimate using SELECT COUNT(*):</p>
+<pre>$rs = $db->Execute("select * from table where state=$state");
+$numrows = $rs->PO_RecordCount('table', "state=$state");</pre>
+<p><b>Selects: Locking</b> </p>
+<p>SELECT statements are commonly used to implement row-level locking of tables.
+ Other databases such as Oracle, Interbase, PostgreSQL and MySQL with InnoDB
+ do not require row-level locking because they use versioning to display data
+ consistent with a specific point in time.</p>
+<p>Currently, I recommend encapsulating the row-level locking in a separate function,
+ such as RowLock($table, $where):</p>
+<pre>$connection->BeginTrans( );
+$connection->RowLock($table, $where); </pre>
+<pre><font color=green># some operation</font></pre>
+<pre>if ($ok) $connection->CommitTrans( );
+else $connection->RollbackTrans( );
+</pre>
+<p><b>Selects: Outer Joins</b></p>
+<p>Not all databases support outer joins. Furthermore the syntax for outer joins
+ differs dramatically between database vendors. One portable (and possibly slower)
+ method of implementing outer joins is using UNION.</p>
+<p>For example, an ANSI-92 left outer join between two tables t1 and t2 could
+ look like:</p>
+<pre>SELECT t1.col1, t1.col2, t2.cola <br> FROM t1 <i>LEFT JOIN</i> t2 ON t1.col = t2.col</pre>
+<p>This can be emulated using:</p>
+<pre>SELECT t1.col1, t1.col2, t2.cola FROM t1, t2 <br> WHERE t1.col = t2.col
+ UNION ALL
+SELECT col1, col2, null FROM t1 <br> WHERE t1.col not in (select distinct col from t2)
+</pre>
+<p>Since ADOdb 2.13, we provide some hints in the connection object as to legal
+ join variations. This is still incomplete and sometimes depends on the database
+ version you are using, but is useful as a general guideline:</p>
+<p><font face="Courier New, Courier, mono">$conn->leftOuter</font>: holds the
+ operator used for left outer joins (eg. '*='), or false if not known or not
+ available.<br>
+ <font face="Courier New, Courier, mono">$conn->rightOuter</font>: holds the
+ operator used for right outer joins (eg '=*'), or false if not known or not
+ available.<br>
+ <font face="Courier New, Courier, mono">$conn->ansiOuter</font>: boolean
+ that if true means that ANSI-92 style outer joins are supported, or false if
+ not known.</p>
+<h3><b>Inserts</b> </h3>
+<p>When you create records, you need to generate unique id's for each record.
+ There are two common techniques: (1) auto-incrementing columns and (2) sequences.
+</p>
+<p>Auto-incrementing columns are supported by MySQL, Sybase and Microsoft Access
+ and SQL Server. However most other databases do not support this feature. So
+ for portability, you have little choice but to use sequences. Sequences are
+ special functions that return a unique incrementing number every time you call
+ it, suitable to be used as database keys. In ADOdb, we use the GenID( ) function.
+ It has takes a parameter, the sequence name. Different tables can have different
+ sequences. </p>
+<pre>$id = $connection->GenID('sequence_name');<br>$connection->Execute("insert into table (id, firstname, lastname) <br> values ($id, $firstname, $lastname)");</pre>
+<p>For databases that do not support sequences natively, ADOdb emulates sequences
+ by creating a table for every sequence.</p>
+<h3><b>Binding</b></h3>
+<p>Binding variables in an SQL statement is another tricky feature. Binding is
+ useful because it allows pre-compilation of SQL. When inserting multiple records
+ into a database in a loop, binding can offer a 50% (or greater) speedup. However
+ many databases such as Access and MySQL do not support binding natively and
+ there is some overhead in emulating binding. Furthermore, different databases
+ (specificly Oracle!) implement binding differently. My recommendation is to
+ use binding if your database queries are too slow, but make sure you are using
+ a database that supports it like Oracle. </p>
+<p>ADOdb supports portable Prepare/Execute with:</p>
+<pre>$stmt = $db->Prepare('select * from customers where custid=? and state=?');
+$rs = $db->Execute($stmt, array($id,'New York'));</pre>
+<p>Oracle uses named bind placeholders, not "?", so to support portable binding, we have Param() that generates
+the correct placeholder (available since ADOdb 3.92):
+<pre><font color="#000000">$sql = <font color="#993300">'insert into table (col1,col2) values ('</font>.$DB->Param('a').<font color="#993300">','</font>.$DB->Param('b').<font color="#993300">')'</font>;
+<font color="#006600"># generates 'insert into table (col1,col2) values (?,?)'
+# or 'insert into table (col1,col2) values (:a,:b)</font>'
+$stmt = $DB->Prepare($sql);
+$stmt = $DB->Execute($stmt,array('one','two'));
+</font></pre>
+<a name="native"></a>
+<h2>Portable Native SQL</h2>
+<p>ADOdb provides the following functions for portably generating SQL functions
+ as strings to be merged into your SQL statements (some are only available since
+ ADOdb 3.92): </p>
+<table width="75%" border="1" align=center>
+ <tr>
+ <td width=30%><b>Function</b></td>
+ <td><b>Description</b></td>
+ </tr>
+ <tr>
+ <td>DBDate($date)</td>
+ <td>Pass in a UNIX timestamp or ISO date and it will convert it to a date
+ string formatted for INSERT/UPDATE</td>
+ </tr>
+ <tr>
+ <td>DBTimeStamp($date)</td>
+ <td>Pass in a UNIX timestamp or ISO date and it will convert it to a timestamp
+ string formatted for INSERT/UPDATE</td>
+ </tr>
+ <tr>
+ <td>SQLDate($date, $fmt)</td>
+ <td>Portably generate a date formatted using $fmt mask, for use in SELECT
+ statements.</td>
+ </tr>
+ <tr>
+ <td>OffsetDate($date, $ndays)</td>
+ <td>Portably generate a $date offset by $ndays.</td>
+ </tr>
+ <tr>
+ <td>Concat($s1, $s2, ...)</td>
+ <td>Portably concatenate strings. Alternatively, for mssql use mssqlpo driver,
+ which allows || operator.</td>
+ </tr>
+ <tr>
+ <td>IfNull($fld, $replaceNull)</td>
+ <td>Returns a string that is the equivalent of MySQL IFNULL or Oracle NVL.</td>
+ </tr>
+ <tr>
+ <td>Param($name)</td>
+ <td>Generates bind placeholders, using ? or named conventions as appropriate.</td>
+ </tr>
+ <tr><td>$db->sysDate</td><td>Property that holds the SQL function that returns today's date</td>
+</tr>
+<tr><td>$db->sysTimeStamp</td><td>Property that holds the SQL function that returns the current
+timestamp (date+time).
+</td>
+</tr>
+<tr>
+<td>$db->concat_operator</td><td>Property that holds the concatenation operator
+</td>
+</tr>
+<tr><td>$db->length</td><td>Property that holds the name of the SQL strlen function.
+</td></tr>
+
+<tr><td>$db->upperCase</td><td>Property that holds the name of the SQL strtoupper function.
+</td></tr>
+<tr><td>$db->random</td><td>Property that holds the SQL to generate a random number between 0.00 and 1.00.
+</td>
+</tr>
+<tr><td>$db->substr</td><td>Property that holds the name of the SQL substring function.
+</td></tr>
+</table>
+<p> </p>
+<h2>DDL and Tuning</h2>
+There are database design tools such as ERWin or Dezign that allow you to generate data definition language commands such as ALTER TABLE or CREATE INDEX from Entity-Relationship diagrams.
+<p>
+However if you prefer to use a PHP-based table creation scheme, adodb provides you with this feature. Here is the code to generate the SQL to create a table with:
+<ol>
+ <li> Auto-increment primary key 'ID', </li>
+ <li>The person's 'NAME' VARCHAR(32) NOT NULL and defaults to '', </li>
+ <li>The date and time of record creation 'CREATED', </li>
+ <li> The person's 'AGE', defaulting to 0, type NUMERIC(16). </li>
+</ol>
+<p>
+Also create a compound index consisting of 'NAME' and 'AGE':
+<pre>
+$datadict = <strong>NewDataDictionary</strong>($connection);
+$flds = "
+<font color="#660000"> ID I AUTOINCREMENT PRIMARY,
+ NAME C(32) DEFAULT '' NOTNULL,
+ CREATED T DEFTIMESTAMP,
+ AGE N(16) DEFAULT 0</font>
+";
+$sql1 = $datadict-><strong>CreateTableSQL</strong>('tabname', $flds);
+$sql2 = $datadict-><strong>CreateIndexSQL</strong>('idx_name_age', 'tabname', 'NAME,AGE');
+</pre>
+
+<h3>Data Types</h3>
+<p>Stick to a few data types that are available in most databases. Char, varchar
+ and numeric/number are supported by most databases. Most other data types (including
+ integer, boolean and float) cannot be relied on being available. I recommend
+ using char(1) or number(1) to hold booleans. </p>
+<p>Different databases have different ways of representing dates and timestamps/datetime.
+ ADOdb attempts to display all dates in ISO (YYYY-MM-DD) format. ADOdb also provides
+ DBDate( ) and DBTimeStamp( ) to convert dates to formats that are acceptable
+ to that database. Both functions accept Unix integer timestamps and date strings
+ in ISO format.</p>
+<pre>$date1 = $connection->DBDate(time( ));<br>$date2 = $connection->DBTimeStamp('2002-02-23 13:03:33');</pre>
+<p>We also provide functions to convert database dates to Unix timestamps:</p>
+<pre>$unixts = $recordset->UnixDate('#2002-02-30#'); <font color="green"># MS Access date => unix timestamp</font></pre>
+<p>The maximum length of a char/varchar field is also database specific. You can
+ only assume that field lengths of up to 250 characters are supported. This is
+ normally impractical for web based forum or content management systems. You
+ will need to be familiar with how databases handle large objects (LOBs). ADOdb
+ implements two functions, UpdateBlob( ) and UpdateClob( ) that allow you to
+ update fields holding Binary Large Objects (eg. pictures) and Character Large
+ Objects (eg. HTML articles):</p>
+<pre><font color=green># for oracle </font>
+$conn->Execute('INSERT INTO blobtable (id, blobcol) VALUES (1,empty_blob())');
+$conn->UpdateBlob('blobtable','blobcol',$blobvalue,'id=1');
+
+<font color=green># non-oracle databases</font>
+$conn->Execute('INSERT INTO blobtable (id, blobcol) VALUES (1, null)');
+$conn->UpdateBlob('blobtable','blobcol',$blobvalue,'id=1');
+</pre>
+<p>Null handling is another area where differences can occur. This is a mine-field,
+ because 3-value logic is tricky.
+<p>In general, I avoid using nulls except for dates and default all my numeric
+ and character fields to 0 or the empty string. This maintains consistency with
+ PHP, where empty strings and zero are treated as equivalent, and avoids SQL
+ ambiguities when you use the ANY and EXISTS operators. However if your database
+ has significant amounts of missing or unknown data, using nulls might be a good
+ idea.
+ <p>
+ ADOdb also supports a portable <a href=http://phplens.com/adodb/reference.functions.concat.html#ifnull>IfNull</a> function, so you can define what to display
+ if the field contains a null.
+<h3><b>Stored Procedures</b></h3>
+<p>Stored procedures are another problem area. Some databases allow recordsets
+ to be returned in a stored procedure (Microsoft SQL Server and Sybase), and
+ others only allow output parameters to be returned. Stored procedures sometimes
+ need to be wrapped in special syntax. For example, Oracle requires such code
+ to be wrapped in an anonymous block with BEGIN and END. Also internal sql operators
+ and functions such as +, ||, TRIM( ), SUBSTR( ) or INSTR( ) vary between vendors.
+</p>
+<p>An example of how to call a stored procedure with 2 parameters and 1 return
+ value follows:</p>
+<pre> switch ($db->databaseType) {
+ case '<font color="#993300">mssql</font>':
+ $sql = <font color="#000000"><font color="#993333">'<font color="#993300">SP_RUNSOMETHING</font>'</font></font>; break;
+ case '<font color="#993300">oci8</font>':
+ $sql =
+<font color="#993300"> </font><font color="#000000"><font color="#993300">"declare RETVAL integer;begin :RETVAL := </font><font color="#000000"><font color="#993333"><font color="#993300">SP_RUNSOMETHING</font></font></font><font color="#993300">(:myid,:group);end;";
+</font> break;</font>
+ default:
+ die('<font color="#993300">Unsupported feature</font>');
+ }
+<font color="#000000"><font color="green"> # @RETVAL = SP_RUNSOMETHING @myid,@group</font>
+ $stmt = $db->PrepareSP($sql); <br> $db->Parameter($stmt,$id,'<font color="#993300">myid</font>');
+ $db->Parameter($stmt,$group,'<font color="#993300">group</font>');
+ <font color="green"># true indicates output parameter<br> </font>$db->Parameter($stmt,$ret,'<font color="#993300">RETVAL</font>',true);
+ $db->Execute($stmt); </font></pre>
+<p>As you can see, the ADOdb API is the same for both databases. But the stored
+ procedure SQL syntax is quite different between databases and is not portable,
+ so be forewarned! However sometimes you have little choice as some systems only
+ allow data to be accessed via stored procedures. This is when the ultimate portability
+ solution might be the only solution: <i>treating portable SQL as a localization
+ exercise...</i></p>
+<h3><b>SQL as a Localization Exercise</b></h3>
+<p> In general to provide real portability, you will have to treat SQL coding
+ as a localization exercise. In PHP, it has become common to define separate
+ language files for English, Russian, Korean, etc. Similarly, I would suggest
+ you have separate Sybase, Intebase, MySQL, etc files, and conditionally include
+ the SQL based on the database. For example, each MySQL SQL statement would be
+ stored in a separate variable, in a file called 'mysql-lang.inc.php'.</p>
+<pre>$sqlGetPassword = '<font color="#993300">select password from users where userid=%s</font>';
+$sqlSearchKeyword = "<font color="#993300">SELECT * FROM articles WHERE match (title,body) against (%s</font>)";</pre>
+<p>In our main PHP file:</p>
+<pre><font color=green># define which database to load...</font>
+<b>$database = '<font color="#993300">mysql</font>';
+include_once("<font color="#993300">$database-lang.inc.php</font>");</b>
+
+$db = &NewADOConnection($database);
+$db->PConnect(...) or die('<font color="#993300">Failed to connect to database</font>');
+
+<font color=green># search for a keyword $word</font>
+$rs = $db->Execute(sprintf($sqlSearchKeyWord,$db->qstr($word)));</pre>
+<p>Note that we quote the $word variable using the qstr( ) function. This is because
+ each database quotes strings using different conventions.</p>
+<p>
+<h3>Final Thoughts</h3>
+<p>The best way to ensure that you have portable SQL is to have your data tables designed using
+sound principles. Learn the theory of normalization and entity-relationship diagrams and model
+your data carefully. Understand how joins and indexes work and how they are used to tune performance.
+<p> Visit the following page for more references on database theory and vendors:
+ <a href="http://php.weblogs.com/sql_tutorial">http://php.weblogs.com/sql_tutorial</a>.
+ Also read this article on <a href=http://phplens.com/lens/php-book/optimizing-debugging-php.php>Optimizing PHP</a>.
+<p>
+<font size=1>(c) 2002-2003 John Lim.</font>
+
+</body>
+</html>
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