added a SQLite example program

examples/sqlite_ex.cpp

 // The contents of this file are in the public domain. See LICENSE_FOR_EXAMPLE_PROGRAMS.txt
 
 /*
+    This example gives a quick overview of dlib's C++ API for the popular SQLite library.
 */
 
 
@@ -12,48 +13,119 @@ using namespace dlib;
 using namespace std;
 
 // ----------------------------------------------------------------------------------------
+
 bool table_exists (
     database& db,
     const std::string& tablename
 )
 {
+    // Sometimes you want to just run a query that returns one thing.  In this case, we
+    // want to see how many tables are in our database with the given tablename.  The only
+    // possible outcomes are 1 or 0 and we can do this by looking in the special
+    // sqlite_master table that records such database metadata.  For these kinds of "one
+    // result" queries we can use the query_int() method which executes a SQL statement
+    // against a database and returns the result as an int.
     return query_int(db, "select count(*) from sqlite_master where name = '"+tablename+"'")==1;
 }
 
+// ----------------------------------------------------------------------------------------
+
 int main() try
 {
-
+    // Open the SQLite database in the stuff.db file (or create an empty database in
+    // stuff.db if it doesn't exist).
     database db("stuff.db");
 
-    if (!table_exists(db,"davis"))
-        db.exec("create table davis (name, age, data)");
+    // Create a people table that records a person's name, age, and their "data".
+    if (!table_exists(db,"people"))
+        db.exec("create table people (name, age, data)");
+
 
-    statement st(db, "insert into davis VALUES(?,?,?)");
+    // Now let's add some data to this table.  We can do this by making a statement object
+    // as shown.  Here we use the special ? character to indicate bindable arguments and
+    // below we will use st.bind() statements to populate those fields with values.  
+    statement st(db, "insert into people VALUES(?,?,?)");
 
-    string name = "davis";
+    // The data for Davis
+    string name = "Davis";
     int age = 32;
-    matrix<double> m = randm(3,3);
+    matrix<double> m = randm(3,3); // some random "data" for Davis
+
+    // You can bind any of the built in scalar types (e.g. int, float) or std::string and
+    // they will go into the table as the appropriate SQL types (e.g. INT, TEXT).  If you
+    // try to bind any other object it will be saved as a binary blob if the type has an
+    // appropriate void serialize(const T&, std::ostream&) function defined for it.  The
+    // matrix has such a serialize function (as do most dlib types) so the bind below saves
+    // the matrix as a binary blob.
+    st.bind(1, name);
+    st.bind(2, age);
+    st.bind(3, m); 
+    st.exec(); // execute the SQL statement.  This does the insert.
+
 
+    // We can reuse the statement to add more data to the database.  In fact, if you have a
+    // bunch of statements to execute it is fastest if you reuse them in this manner. 
+    name = "John";
+    age = 82;
+    m = randm(2,3); 
     st.bind(1, name);
     st.bind(2, age);
-    st.bind(3, m);
+    st.bind(3, m); 
     st.exec();
+    
 
 
-
-    statement st2(db, "select * from davis");
+    // Now lets print out all the rows in the people table.
+    statement st2(db, "select * from people");
     st2.exec();
+    // Loop over all the rows obtained by executing the statement with .exec().
     while(st2.move_next())
     {
         string name;
         int age;
         matrix<double> m;
+        // Analogously to bind, we can grab the columns straight into C++ types.  Here the
+        // matrix is automatically deserialized by calling its deserialize() routine.
         st2.get_column(0, name);
         st2.get_column(1, age);
         st2.get_column(2, m);
         cout << name << " " << age << "\n" << m << endl << endl;
     }
 
+
+
+    // Finally, if you want to make a bunch of atomic changes to a database then you should
+    // do so inside a transaction.  Here, either all the database modifications that occur
+    // between the creation of my_trans and the invocation of my_trans.commit() will appear
+    // in the database or none of them will.  This way, if an exception or other error
+    // happens halfway though your transaction you won't be left with your database in an
+    // inconsistent state.  
+    // 
+    // Additionally, if you are going to do a large amount of inserts or updates then it is
+    // much faster to group them into a transaction.  
+    transaction my_trans(db);
+
+    name = "Dude";
+    age = 49;
+    m = randm(4,2); 
+    st.bind(1, name);
+    st.bind(2, age);
+    st.bind(3, m); 
+    st.exec();
+
+    name = "Bob";
+    age = 29;
+    m = randm(2,2); 
+    st.bind(1, name);
+    st.bind(2, age);
+    st.bind(3, m); 
+    st.exec();
+
+    // If you comment out this line then you will see that these inserts do not take place.
+    // Specifically, what happens is that when my_trans is destructed it rolls back the
+    // entire transaction unless commit() has been called.
+    my_trans.commit();
+
 }
 catch (std::exception& e)
 {