riscv-openocd-wch/jimtcl/README.sqlite

Jim Sqlite extension documentation.
Copyright 2005 Salvatore Sanfilippo <antirez@invece.org>


Overview
~~~~~~~~

The Sqlite extension makes possible to work with sqlite (http://www.sqlite.org)
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]
    sqlite.handle0
    . $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 these 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'"
    Runtime error, file "?", line 1:
        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
    10

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
    5

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 to use files.
This is of course faster and does not need the ability to write
to the filesystem. Of course this databases are only useful for
temp 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
exists.