Sqlite Support for Jim Tcl


The sqlite3 extension makes it possible to work with sqlite databases from Jim. SQLite is a small C library that implements a self-contained, embeddable, zero-configuration SQL database engine. This means it is perfect for embedded systems, and for stand-alone applications that need the power of SQL without to use an external server like Mysql.

Basic usage

The Sqlite extension exports an Object Based interface for databases. In order to open a database use:

set f [sqlite3.open dbname]

The sqlite3.open command returns a db handle, that is a command name that can be used to perform operations on the database. A real example:

. set db [sqlite3.open test.db]
. $db query "SELECT * from tbl1" 
{one hello! two 10} {one goodbye two 20}

In the second line the handle is used as a command name, followed by the ‘method’ or ‘subcommand’ (“query” in the example), and the arguments.

The query method

The query method has the following signature:

$db query SqlQuery ?args?

The sql query may contain occurrences of “%s” that are substituted in the actual query with the following arguments, quoted in order to make sure that the query is correct even if this arguments contain “’” characters. So for example it is possible to write:

. $db query "SELECT * from tbl1 WHERE one='%s'" hello!
{one hello! two 10}

Instead of hello! it is possible to use a string with embedded “’”:

. $db query "SELECT * from tbl1 WHERE one='%s'" a'b
(no matches - the empty list is returned)

This does not work instead using the Tcl variable expansion in the string:

. $db query "SELECT * from tbl1 WHERE one='$foo'"
near "b": syntax error

In order to obtain an actual ‘%’ character in the query, there is just to use two, like in “foo %% bar”. This is the same as the [format] argument.

Specification of query results

In one of the above examples, the following query was used:

. $db query "SELECT * from tbl1" 
{one hello! two 10} {one goodbye two 20}

As you can see the result of a query is a list of lists. Every element of the list represents a row, as a list of key/value pairs, so actually every row is a Jim dictionary.

The following example and generated output show how to take advantage of this representation:

. set res [$db query "SELECT * from tbl1"]
{one hello! two 10} {one goodbye two 20}
. foreach row $res {puts "One: $row(one), Two: $row(two)"}
One: hello!, Two: 10
One: goodbye, Two: 20

To access every row sequentially is very simple, and field of a row can be accessed using the $row(field) syntax.

The close method

In order to close the db, use the ‘close’ method that will have as side effect to close the db and to remove the command associated with the db. Just use:

$db close

Handling NULL values

In the SQL language there is a special value NULL that is not the empty string, so how to represent it in a typeless language like Tcl? For default this extension will use the empty string, but it is possible to specify a different string for the NULL value.

In the above example there were two rows in the ‘tbl1’ table. Now we can add using the “sqlite” command line client another one with a NULL value:

sqlite> INSERT INTO tbl1 VALUES(NULL,30);
sqlite> .exit

That’s what the sqlite extension will return for default:

. $db query "SELECT * from tbl1"
{one hello! two 10} {one goodbye two 20} {one {} two 30}

As you can see in the last row, the NULL is represented as {}, that’s the empty string. Using the -null option of the ‘query’ command we can change this default, and tell the sqlite extension to represent the NULL value as a different string:

. $db query -null <<NULL>> "SELECT * from tbl1"
{one hello! two 10} {one goodbye two 20} {one <<NULL>> two 30}

This way if the empty string has some semantical value for your dataset you can change it.

Finding the ID of the last inserted row

This is as simple as:

. $db lastid

Number of rows changed by the most recent query

This is also very simple, there is just to use the ‘changes’ method without arguments.

. $db changes

Note that if you drop an entire table the number of changes will be reported as zero, because of details of the sqlite implementation.

That’s all, Enjoy! Salvatore Sanfilippo

p.s. this extension is just the work of some hour thanks to the cool clean C API that sqlite exports. Thanks to the author of sqlite for this great work.

In memory databases

SQLite is able to create in-memory databases instead of using files. This is faster and does not need the ability to write to the filesystem, however is only useful for transient data.

In-memory DBs are used just like regular databases, just the name used to open the database is :memory:. That’s an example that does not use the filesystem at all to create and work with the db.

package require sqlite3
set db [sqlite3.open :memory:]
$db query {CREATE TABLE plays (id, author, title)}
$db query {INSERT INTO plays (id, author, title) VALUES (1, 'Goethe', 'Faust');}
$db query {INSERT INTO plays (id, author, title) VALUES (2, 'Shakespeare', 'Hamlet');}
$db query {INSERT INTO plays (id, author, title) VALUES (3, 'Sophocles', 'Oedipus Rex');}
set res [$db query "SELECT * FROM plays"]
$db close
foreach r $res {puts $r(author)}

Of course once the Jim process is destroyed the database will no longer exist.