library/adodb/tips_portable_sql.htm

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   5         <title>Tips on Writing Portable SQL for Multiple Databases for PHP</title>
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   9 <table width=100% border=0><tr><td><h2>Tips on Writing Portable SQL &nbsp;</h2></td><td>
  10  <div align=right><img src="cute_icons_for_site/adodb.gif"></div></td></tr></table>
  11   <p>Updated 18 Sep 2003. Added Portable Native SQL section.
  12 <p>
  13
  14  If you are writing an application that is used in multiple environments and
  15   operating systems, you need to plan to support multiple databases. This article
  16   is based on my experiences with multiple database systems, stretching from 4th
  17   Dimension in my Mac days, to the databases I currently use, which are: Oracle,
  18   FoxPro, Access, MS SQL Server and MySQL. Although most of the advice here applies
  19   to using SQL with Perl, Python and other programming languages, I will focus on PHP and how
  20   the <a href="http://php.weblogs.com/adodb">ADOdb</a> database abstraction library
  21   offers some solutions.<p></p>
  22 <p>Most database vendors practice product lock-in. The best or fastest way to
  23   do things is often implemented using proprietary extensions to SQL. This makes
  24   it extremely hard to write portable SQL code that performs well under all conditions.
  25   When the first ANSI committee got together in 1984 to standardize SQL, the database
  26   vendors had such different implementations that they could only agree on the
  27   core functionality of SQL. Many important application specific requirements
  28   were not standardized, and after so many years since the ANSI effort began,
  29   it looks as if much useful database functionality will never be standardized.
  30   Even though ANSI-92 SQL has codified much more, we still have to implement portability
  31   at the application level.</p>
  32 <h3><b>Selects</b></h3>
  33 <p>The SELECT statement has been standardized to a great degree. Nearly every
  34   database supports the following:</p>
  35 <p>SELECT [cols] FROM [tables]<br>
  36   &nbsp;&nbsp;[WHERE conditions]<br>
  37   &nbsp; [GROUP BY cols]<br>
  38   &nbsp; [HAVING conditions] <br>
  39   &nbsp; [ORDER BY cols]</p>
  40 <p>But so many useful techniques can only be implemented by using proprietary
  41   extensions. For example, when writing SQL to retrieve the first 10 rows for
  42   paging, you could write...</p>
  43 <table width="80%" border="1" cellspacing="0" cellpadding="0" align="center">
  44   <tr>
  45         <td><b>Database</b></td>
  46         <td><b>SQL Syntax</b></td>
  47   </tr>
  48   <tr>
  49         <td>DB2</td>
  50         <td>select * from table fetch first 10 rows only</td>
  51   </tr>
  52   <tr>
  53         <td>Informix</td>
  54         <td>select first 10 * from table</td>
  55   </tr>
  56   <tr>
  57         <td>Microsoft SQL Server and Access</td>
  58         <td>select top 10 * from table</td>
  59   </tr>
  60   <tr>
  61         <td>MySQL and PostgreSQL</td>
  62         <td>select * from table limit 10</td>
  63   </tr>
  64   <tr>
  65         <td>Oracle 8i</td>
  66         <td>select * from (select * from table) where rownum &lt;= 10</td>
  67   </tr>
  68 </table>
  69 <p>This feature of getting a subset of data is so useful that in the PHP class
  70   library ADOdb, we have a SelectLimit( ) function that allows you to hide the
  71   implementation details within a function that will rewrite your SQL for you:</p>
  72 <pre>$connection-&gt;SelectLimit('select * from table', 10);
  73 </pre>
  74 <p><b>Selects: Fetch Modes</b></p>
  75 <p>PHP allows you to retrieve database records as arrays. You can choose to have
  76   the arrays indexed by field name or number. However different low-level PHP
  77   database drivers are inconsistent in their indexing efforts. ADOdb allows you
  78   to determine your prefered mode. You set this by setting the variable $ADODB_FETCH_MODE
  79   to either of the constants ADODB_FETCH_NUM (for numeric indexes) or ADODB_FETCH_ASSOC
  80   (using field names as an associative index).</p>
  81 <p>The default behaviour of ADOdb varies depending on the database you are using.
  82   For consistency, set the fetch mode to either ADODB_FETCH_NUM (for speed) or
  83   ADODB_FETCH_ASSOC (for convenience) at the beginning of your code. </p>
  84 <p><b>Selects: Counting Records</b></p>
  85 <p>Another problem with SELECTs is that some databases do not return the number
  86   of rows retrieved from a select statement. This is because the highest performance
  87   databases will return records to you even before the last record has been found.
  88 </p>
  89 <p>In ADOdb, RecordCount( ) returns the number of rows returned, or will emulate
  90   it by buffering the rows and returning the count after all rows have been returned.
  91   This can be disabled for performance reasons when retrieving large recordsets
  92   by setting the global variable $ADODB_COUNTRECS = false. This variable is checked
  93   every time a query is executed, so you can selectively choose which recordsets
  94   to count.</p>
  95 <p>If you prefer to set $ADODB_COUNTRECS = false, ADOdb still has the PO_RecordCount(
  96   ) function. This will return the number of rows, or if it is not found, it will
  97   return an estimate using SELECT COUNT(*):</p>
  98 <pre>$rs = $db-&gt;Execute(&quot;select * from table where state=$state&quot;);
  99 $numrows = $rs-&gt;PO_RecordCount('table', &quot;state=$state&quot;);</pre>
 100 <p><b>Selects: Locking</b> </p>
 101 <p>SELECT statements are commonly used to implement row-level locking of tables.
 102   Other databases such as Oracle, Interbase, PostgreSQL and MySQL with InnoDB
 103   do not require row-level locking because they use versioning to display data
 104   consistent with a specific point in time.</p>
 105 <p>Currently, I recommend encapsulating the row-level locking in a separate function,
 106   such as RowLock($table, $where):</p>
 107 <pre>$connection-&gt;BeginTrans( );
 108 $connection-&gt;RowLock($table, $where); </pre>
 109 <pre><font color=green># some operation</font></pre>
 110 <pre>if ($ok) $connection-&gt;CommitTrans( );
 111 else $connection-&gt;RollbackTrans( );
 112 </pre>
 113 <p><b>Selects: Outer Joins</b></p>
 114 <p>Not all databases support outer joins. Furthermore the syntax for outer joins
 115   differs dramatically between database vendors. One portable (and possibly slower)
 116   method of implementing outer joins is using UNION.</p>
 117 <p>For example, an ANSI-92 left outer join between two tables t1 and t2 could
 118   look like:</p>
 119 <pre>SELECT t1.col1, t1.col2, t2.cola <br>  FROM t1 <i>LEFT JOIN</i> t2 ON t1.col = t2.col</pre>
 120 <p>This can be emulated using:</p>
 121 <pre>SELECT t1.col1, t1.col2, t2.cola FROM t1, t2 <br>     WHERE t1.col = t2.col
 122    UNION ALL
 123 SELECT col1, col2, null FROM t1 <br>       WHERE t1.col not in (select distinct col from t2)
 124 </pre>
 125 <p>Since ADOdb 2.13, we provide some hints in the connection object as to legal
 126   join variations. This is still incomplete and sometimes depends on the database
 127   version you are using, but is useful as a general guideline:</p>
 128 <p><font face="Courier New, Courier, mono">$conn-&gt;leftOuter</font>: holds the
 129   operator used for left outer joins (eg. '*='), or false if not known or not
 130   available.<br>
 131   <font face="Courier New, Courier, mono">$conn-&gt;rightOuter</font>: holds the
 132   operator used for right outer joins (eg '=*'), or false if not known or not
 133   available.<br>
 134   <font face="Courier New, Courier, mono">$conn-&gt;ansiOuter</font>: boolean
 135   that if true means that ANSI-92 style outer joins are supported, or false if
 136   not known.</p>
 137 <h3><b>Inserts</b> </h3>
 138 <p>When you create records, you need to generate unique id's for each record.
 139   There are two common techniques: (1) auto-incrementing columns and (2) sequences.
 140 </p>
 141 <p>Auto-incrementing columns are supported by MySQL, Sybase and Microsoft Access
 142   and SQL Server. However most other databases do not support this feature. So
 143   for portability, you have little choice but to use sequences. Sequences are
 144   special functions that return a unique incrementing number every time you call
 145   it, suitable to be used as database keys. In ADOdb, we use the GenID( ) function.
 146   It has takes a parameter, the sequence name. Different tables can have different
 147   sequences. </p>
 148 <pre>$id = $connection-&gt;GenID('sequence_name');<br>$connection-&gt;Execute(&quot;insert into table (id, firstname, lastname) <br>                       values ($id, $firstname, $lastname)&quot;);</pre>
 149 <p>For databases that do not support sequences natively, ADOdb emulates sequences
 150   by creating a table for every sequence.</p>
 151 <h3><b>Binding</b></h3>
 152 <p>Binding variables in an SQL statement is another tricky feature. Binding is
 153   useful because it allows pre-compilation of SQL. When inserting multiple records
 154   into a database in a loop, binding can offer a 50% (or greater) speedup. However
 155   many databases such as Access and MySQL do not support binding natively and
 156   there is some overhead in emulating binding. Furthermore, different databases
 157   (specificly Oracle!) implement binding differently. My recommendation is to
 158   use binding if your database queries are too slow, but make sure you are using
 159   a database that supports it like Oracle. </p>
 160 <p>ADOdb supports portable Prepare/Execute with:</p>
 161 <pre>$stmt = $db-&gt;Prepare('select * from customers where custid=? and state=?');
 162 $rs = $db-&gt;Execute($stmt, array($id,'New York'));</pre>
 163 <p>Oracle uses named bind placeholders, not "?", so to support portable binding, we have Param() that generates
 164 the correct placeholder (available since ADOdb 3.92):
 165 <pre><font color="#000000">$sql = <font color="#993300">'insert into table (col1,col2) values ('</font>.$DB-&gt;Param('a').<font color="#993300">','</font>.$DB-&gt;Param('b').<font color="#993300">')'</font>;
 166 <font color="#006600"># generates 'insert into table (col1,col2) values (?,?)'
 167 # or        'insert into table (col1,col2) values (:a,:b)</font>'
 168 $stmt = $DB-&gt;Prepare($sql);
 169 $stmt = $DB-&gt;Execute($stmt,array('one','two'));
 170 </font></pre>
 171 <a name="native"></a>
 172 <h2>Portable Native SQL</h2>
 173 <p>ADOdb provides the following functions for portably generating SQL functions
 174   as strings to be merged into your SQL statements (some are only available since
 175   ADOdb 3.92): </p>
 176 <table width="75%" border="1" align=center>
 177   <tr>
 178     <td width=30%><b>Function</b></td>
 179     <td><b>Description</b></td>
 180   </tr>
 181   <tr>
 182     <td>DBDate($date)</td>
 183     <td>Pass in a UNIX timestamp or ISO date and it will convert it to a date
 184       string formatted for INSERT/UPDATE</td>
 185   </tr>
 186   <tr>
 187     <td>DBTimeStamp($date)</td>
 188     <td>Pass in a UNIX timestamp or ISO date and it will convert it to a timestamp
 189       string formatted for INSERT/UPDATE</td>
 190   </tr>
 191   <tr>
 192     <td>SQLDate($date, $fmt)</td>
 193     <td>Portably generate a date formatted using $fmt mask, for use in SELECT
 194       statements.</td>
 195   </tr>
 196   <tr>
 197     <td>OffsetDate($date, $ndays)</td>
 198     <td>Portably generate a $date offset by $ndays.</td>
 199   </tr>
 200   <tr>
 201     <td>Concat($s1, $s2, ...)</td>
 202     <td>Portably concatenate strings. Alternatively, for mssql use mssqlpo driver,
 203       which allows || operator.</td>
 204   </tr>
 205   <tr>
 206     <td>IfNull($fld, $replaceNull)</td>
 207     <td>Returns a string that is the equivalent of MySQL IFNULL or Oracle NVL.</td>
 208   </tr>
 209   <tr>
 210     <td>Param($name)</td>
 211     <td>Generates bind placeholders, using ? or named conventions as appropriate.</td>
 212   </tr>
 213   <tr><td>$db->sysDate</td><td>Property that holds the SQL function that returns today's date</td>
 214 </tr>
 215 <tr><td>$db->sysTimeStamp</td><td>Property that holds the SQL function that returns the current
 216 timestamp (date+time).
 217 </td>
 218 </tr>
 219 <tr>
 220 <td>$db->concat_operator</td><td>Property that holds the concatenation operator
 221 </td>
 222 </tr>
 223 <tr><td>$db->length</td><td>Property that holds the name of the SQL strlen function.
 224 </td></tr>
 225
 226 <tr><td>$db->upperCase</td><td>Property that holds the name of the SQL strtoupper function.
 227 </td></tr>
 228 <tr><td>$db->random</td><td>Property that holds the SQL to generate a random number between 0.00 and 1.00.
 229 </td>
 230 </tr>
 231 <tr><td>$db->substr</td><td>Property that holds the name of the SQL substring function.
 232 </td></tr>
 233 </table>
 234 <p>&nbsp; </p>
 235 <h2>DDL and Tuning</h2>
 236 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.
 237 <p>
 238 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:
 239 <ol>
 240         <li> Auto-increment primary key 'ID', </li>
 241         <li>The person's 'NAME' VARCHAR(32) NOT NULL and defaults to '', </li>
 242         <li>The date and time of record creation 'CREATED', </li>
 243         <li> The person's 'AGE', defaulting to 0, type NUMERIC(16). </li>
 244 </ol>
 245 <p>
 246 Also create a compound index consisting of 'NAME' and 'AGE':
 247 <pre>
 248 $datadict = <strong>NewDataDictionary</strong>($connection);
 249 $flds = "
 250 <font color="#660000">  ID I AUTOINCREMENT PRIMARY,
 251   NAME C(32) DEFAULT '' NOTNULL,
 252   CREATED T DEFTIMESTAMP,
 253   AGE N(16) DEFAULT 0</font>
 254 ";
 255 $sql1 = $datadict-><strong>CreateTableSQL</strong>('tabname', $flds);
 256 $sql2 = $datadict-><strong>CreateIndexSQL</strong>('idx_name_age', 'tabname', 'NAME,AGE');
 257 </pre>
 258
 259 <h3>Data Types</h3>
 260 <p>Stick to a few data types that are available in most databases. Char, varchar
 261   and numeric/number are supported by most databases. Most other data types (including
 262   integer, boolean and float) cannot be relied on being available. I recommend
 263   using char(1) or number(1) to hold booleans. </p>
 264 <p>Different databases have different ways of representing dates and timestamps/datetime.
 265   ADOdb attempts to display all dates in ISO (YYYY-MM-DD) format. ADOdb also provides
 266   DBDate( ) and DBTimeStamp( ) to convert dates to formats that are acceptable
 267   to that database. Both functions accept Unix integer timestamps and date strings
 268   in ISO format.</p>
 269 <pre>$date1 = $connection-&gt;DBDate(time( ));<br>$date2 = $connection-&gt;DBTimeStamp('2002-02-23 13:03:33');</pre>
 270 <p>We also provide functions to convert database dates to Unix timestamps:</p>
 271 <pre>$unixts = $recordset-&gt;UnixDate('#2002-02-30#'); <font color="green"># MS Access date =&gt; unix timestamp</font></pre>
 272 <p>The maximum length of a char/varchar field is also database specific. You can
 273   only assume that field lengths of up to 250 characters are supported. This is
 274   normally impractical for web based forum or content management systems. You
 275   will need to be familiar with how databases handle large objects (LOBs). ADOdb
 276   implements two functions, UpdateBlob( ) and UpdateClob( ) that allow you to
 277   update fields holding Binary Large Objects (eg. pictures) and Character Large
 278   Objects (eg. HTML articles):</p>
 279 <pre><font color=green># for oracle </font>
 280 $conn->Execute('INSERT INTO blobtable (id, blobcol) VALUES (1,empty_blob())');
 281 $conn->UpdateBlob('blobtable','blobcol',$blobvalue,'id=1');
 282
 283 <font color=green># non-oracle databases</font>
 284 $conn->Execute('INSERT INTO blobtable (id, blobcol) VALUES (1, null)');
 285 $conn->UpdateBlob('blobtable','blobcol',$blobvalue,'id=1');
 286 </pre>
 287 <p>Null handling is another area where differences can occur. This is a mine-field,
 288   because 3-value logic is tricky.
 289 <p>In general, I avoid using nulls except for dates and default all my numeric
 290   and character fields to 0 or the empty string. This maintains consistency with
 291   PHP, where empty strings and zero are treated as equivalent, and avoids SQL
 292   ambiguities when you use the ANY and EXISTS operators. However if your database
 293   has significant amounts of missing or unknown data, using nulls might be a good
 294   idea.
 295   <p>
 296   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
 297   if the field contains a null.
 298 <h3><b>Stored Procedures</b></h3>
 299 <p>Stored procedures are another problem area. Some databases allow recordsets
 300   to be returned in a stored procedure (Microsoft SQL Server and Sybase), and
 301   others only allow output parameters to be returned. Stored procedures sometimes
 302   need to be wrapped in special syntax. For example, Oracle requires such code
 303   to be wrapped in an anonymous block with BEGIN and END. Also internal sql operators
 304   and functions such as +, ||, TRIM( ), SUBSTR( ) or INSTR( ) vary between vendors.
 305 </p>
 306 <p>An example of how to call a stored procedure with 2 parameters and 1 return
 307   value follows:</p>
 308 <pre>   switch ($db->databaseType) {
 309         case '<font color="#993300">mssql</font>':
 310           $sql = <font color="#000000"><font color="#993333">'<font color="#993300">SP_RUNSOMETHING</font>'</font></font>; break;
 311         case '<font color="#993300">oci8</font>':
 312           $sql =
 313 <font color="#993300">    </font><font color="#000000"><font color="#993300">&quot;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;&quot;;
 314 </font>   break;</font>
 315         default:
 316           die('<font color="#993300">Unsupported feature</font>');
 317         }
 318 <font color="#000000"><font color="green">      # @RETVAL = SP_RUNSOMETHING @myid,@group</font>
 319         $stmt = $db-&gt;PrepareSP($sql);        <br>    $db-&gt;Parameter($stmt,$id,'<font color="#993300">myid</font>');
 320         $db-&gt;Parameter($stmt,$group,'<font color="#993300">group</font>');
 321         <font color="green"># true indicates output parameter<br>       </font>$db-&gt;Parameter($stmt,$ret,'<font color="#993300">RETVAL</font>',true);
 322         $db-&gt;Execute($stmt); </font></pre>
 323 <p>As you can see, the ADOdb API is the same for both databases. But the stored
 324   procedure SQL syntax is quite different between databases and is not portable,
 325   so be forewarned! However sometimes you have little choice as some systems only
 326   allow data to be accessed via stored procedures. This is when the ultimate portability
 327   solution might be the only solution: <i>treating portable SQL as a localization
 328   exercise...</i></p>
 329 <h3><b>SQL as a Localization Exercise</b></h3>
 330 <p> In general to provide real portability, you will have to treat SQL coding
 331   as a localization exercise. In PHP, it has become common to define separate
 332   language files for English, Russian, Korean, etc. Similarly, I would suggest
 333   you have separate Sybase, Intebase, MySQL, etc files, and conditionally include
 334   the SQL based on the database. For example, each MySQL SQL statement would be
 335   stored in a separate variable, in a file called 'mysql-lang.inc.php'.</p>
 336 <pre>$sqlGetPassword = '<font color="#993300">select password from users where userid=%s</font>';
 337 $sqlSearchKeyword = &quot;<font color="#993300">SELECT * FROM articles WHERE match (title,body) against (%s</font>)&quot;;</pre>
 338 <p>In our main PHP file:</p>
 339 <pre><font color=green># define which database to load...</font>
 340 <b>$database = '<font color="#993300">mysql</font>';
 341 include_once(&quot;<font color="#993300">$database-lang.inc.php</font>&quot;);</b>
 342
 343 $db = &amp;NewADOConnection($database);
 344 $db->PConnect(...) or die('<font color="#993300">Failed to connect to database</font>');
 345
 346 <font color=green># search for a keyword $word</font>
 347 $rs = $db-&gt;Execute(sprintf($sqlSearchKeyWord,$db-&gt;qstr($word)));</pre>
 348 <p>Note that we quote the $word variable using the qstr( ) function. This is because
 349   each database quotes strings using different conventions.</p>
 350 <p>
 351 <h3>Final Thoughts</h3>
 352 <p>The best way to ensure that you have portable SQL is to have your data tables designed using
 353 sound principles. Learn the theory of normalization and entity-relationship diagrams and model
 354 your data carefully. Understand how joins and indexes work and how they are used to tune performance.
 355 <p> Visit the following page for more references on database theory and vendors:
 356   <a href="http://php.weblogs.com/sql_tutorial">http://php.weblogs.com/sql_tutorial</a>.
 357   Also read this article on <a href=http://phplens.com/lens/php-book/optimizing-debugging-php.php>Optimizing PHP</a>.
 358 <p>
 359 <font size=1>(c) 2002-2003 John Lim.</font>
 360
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