Google Choice: HTML5 - Web SQL Database

Thursday, February 23, 2017

HTML5 - Web SQL Database

The Web SQL Database API isn't actually part of the HTML5 specification but it is a separate specification which introduces a set of APIs to manipulate client-side databases using SQL.
I'm assuming you are a great web developer and if that is the case then no doubt, you would be well aware of SQL and RDBMS concepts.
If you still want to have a session with SQL then, you can go through our SQL Tutorial.
Web SQL Database will work in latest version of Safari, Chrome and Opera.

The Core Methods

There are following three core methods defined in the spec that I.m going to cover in this tutorial −

openDatabase − This method creates the database object either using existing database or creating new one.
transaction − This method give us the ability to control a transaction and performing either commit or roll-back based on the situation.
executeSql − This method is used to execute actual SQL query.

Opening Database

The openDatabase method takes care of opening a database if it already exists, this method will create it if it already does not exist.
To create and open a database, use the following code −

var db = openDatabase('mydb', '1.0', 'Test DB', 2 * 1024 * 1024);

Above method took following five parameters −

Database name
Version number
Text description
Size of database
Creation callback

The last and 5th argument, creation callback will be called if the database is being created. Without this feature, however, the databases are still being created on the fly and correctly version.

Executing queries

To execute a query you use the database.transaction() function. This function needs a single argument, which is a function that takes care of actually executing the query as follows −

var db = openDatabase('mydb', '1.0', 'Test DB', 2 * 1024 * 1024);

db.transaction(function (tx) {  
   tx.executeSql('CREATE TABLE IF NOT EXISTS LOGS (id unique, log)');
});

The above query will create a table called LOGS in 'mydb' database.

INSERT Operation

To create eateries into the table we add simple SQL query in the above example as follows −

var db = openDatabase('mydb', '1.0', 'Test DB', 2 * 1024 * 1024);
db.transaction(function (tx) {
   tx.executeSql('CREATE TABLE IF NOT EXISTS LOGS (id unique, log)');
   tx.executeSql('INSERT INTO LOGS (id, log) VALUES (1, "foobar")');
   tx.executeSql('INSERT INTO LOGS (id, log) VALUES (2, "logmsg")');
});

We can pass dynamic values while creating entering as follows −

var db = openDatabase('mydb', '1.0', 'Test DB', 2 * 1024 * 1024);

db.transaction(function (tx) {  
   tx.executeSql('CREATE TABLE IF NOT EXISTS LOGS (id unique, log)');
   tx.executeSql('INSERT INTO LOGS (id,log) VALUES (?, ?'), [e_id, e_log];
});

Here e_id and e_log are external variables, and executeSql maps each item in the array argument to the "?"s.

READ Operation

To read already existing records we use a callback to capture the results as follows −

var db = openDatabase('mydb', '1.0', 'Test DB', 2 * 1024 * 1024);

db.transaction(function (tx) {
   tx.executeSql('CREATE TABLE IF NOT EXISTS LOGS (id unique, log)');
   tx.executeSql('INSERT INTO LOGS (id, log) VALUES (1, "foobar")');
   tx.executeSql('INSERT INTO LOGS (id, log) VALUES (2, "logmsg")');
});

db.transaction(function (tx) {
   tx.executeSql('SELECT * FROM LOGS', [], function (tx, results) {
      var len = results.rows.length, i;
      msg = "<p>Found rows: " + len + "</p>";
      document.querySelector('#status').innerHTML +=  msg;
 
      for (i = 0; i < len; i++){
         alert(results.rows.item(i).log );
      }
 
   }, null);
});

Final Example

So finally, let us keep this example in full fledged HTML5 document as follows and try to run it with Safari browser.

<!DOCTYPE HTML>
<html>

   <head>
 
      <script type="text/javascript">
  
         var db = openDatabase('mydb', '1.0', 'Test DB', 2 * 1024 * 1024);
         var msg;
   
         db.transaction(function (tx) {
            tx.executeSql('CREATE TABLE IF NOT EXISTS LOGS (id unique, log)');
            tx.executeSql('INSERT INTO LOGS (id, log) VALUES (1, "foobar")');
            tx.executeSql('INSERT INTO LOGS (id, log) VALUES (2, "logmsg")');
            msg = '<p>Log message created and row inserted.</p>';
            document.querySelector('#status').innerHTML =  msg;
         });

         db.transaction(function (tx) {
            tx.executeSql('SELECT * FROM LOGS', [], function (tx, results) {
               var len = results.rows.length, i;
               msg = "<p>Found rows: " + len + "</p>";
               document.querySelector('#status').innerHTML +=  msg;
     
               for (i = 0; i < len; i++){
                  msg = "<p><b>" + results.rows.item(i).log + "</b></p>";
                  document.querySelector('#status').innerHTML +=  msg;
               }
            }, null);
         });
   
      </script>
  
   </head>
 
   <body>
      <div id="status" name="status">Status Message</div>
   </body>
 
</html>

It will produce the following result −

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