The Tcl interface to the SQLite library

(This page was last modified on 2002/04/12 10:09:00 UTC)

The SQLite library is designed to be very easy to use from a Tcl or Tcl/Tk script. This document gives an overview of the Tcl programming interface.


The interface to the SQLite library consists of single tcl command named sqlite. Because there is only this one interface command, the interface is not placed in a separate namespace.

The sqlite command is used as follows:

sqlite  dbcmd  database-name

The sqlite command opens the database named in the second argument. If the database does not already exist, it is automatically created. The sqlite command also creates a new Tcl command to control the database. The name of the new Tcl command is given by the first argument. This approach is similar to the way widgets are created in Tk.

The name of the database is just the name of a disk file in which the database is stored.

Once an SQLite database is open, it can be controlled using methods of the dbcmd. There are currently 7 methods defined:

  • busy
  • changes
  • close
  • complete
  • eval
  • last_insert_rowid
  • timeout

We will explain all of these methods, though not in that order. We will be begin with the "close" method.

The "close" method

As its name suggests, the "close" method to an SQLite database just closes the database. This has the side-effect of deleting the dbcmd Tcl command. Here is an example of opening and then immediately closing a database:

sqlite db1 ./testdb
db1 close

If you delete the dbcmd directly, that has the same effect as invoking the "close" method. So the following code is equivalent to the previous:

sqlite db1 ./testdb
rename db1 {}

The "eval" method

The most useful dbcmd method is "eval". The eval method is used to execute SQL on the database. The syntax of the eval method looks like this:

dbcmd  eval  sql   ?array-name  script?

The job of the eval method is to execute the SQL statement or statements given in the second argument. For example, to create a new table in a database, you can do this:

sqlite db1 ./testdb
db1 eval {CREATE TABLE t1(a int, b text)}

The above code creates a new table named t1 with columns a and b. What could be simpler?

Query results are returned as a list of column values. If a query requests 2 columns and there are 3 rows matching the query, then the returned list will contain 6 elements. For example:

db1 eval {INSERT INTO t1 VALUES(1,'hello')}
db1 eval {INSERT INTO t1 VALUES(2,'goodbye')}
db1 eval {INSERT INTO t1 VALUES(3,'howdy!')}
set x [db1 eval {SELECT * FROM t1 ORDER BY a}]

The variable $x is set by the above code to

1 hello 2 goodbye 3 howdy!

You can also process the results of a query one row at a time by specifying the name of an array variable and a script following the SQL code. For each row of the query result, the value of each column will be inserted into the array variable and the script will be executed. For instance:

db1 eval {SELECT * FROM t1 ORDER BY a} values {
    parray values
    puts ""

This last code will give the following output:

values(*) = a b
values(a) = 1
values(b) = hello

values(*) = a b
values(a) = 2
values(b) = goodbye

values(*) = a b
values(a) = 3
values(b) = howdy!

For each column in a row of the result, the name of that column is used as an index in to array. The value of the column is stored in the corresponding array entry. The special array index * is used to store a list of column names in the order that they appear.

If the array variable name is the empty string, then the value of each column is stored in a variable with the same name as the column itself. For example:

db1 eval {SELECT * FROM t1 ORDER BY a} {} {
    puts "a=$a b=$b"

From this we get the following output

a=1 b=hello
a=2 b=goodbye
a=3 b=howdy!

The "complete" method

The "complete" method takes a string of supposed SQL as its only argument. It returns TRUE if the string is a complete statement of SQL and FALSE if there is more to be entered.

The "complete" method is useful when building interactive applications in order to know when the user has finished entering a line of SQL code. This is really just an interface to the sqlite_complete() C function. Refer to the C/C++ interface specification for additional information.

The "timeout" method

The "timeout" method is used to control how long the SQLite library will wait for locks to clear before giving up on a database transaction. The default timeout is 0 millisecond. (In other words, the default behavior is not to wait at all.)

The SQlite database allows multiple simultaneous readers or a single writer but not both. If any process is writing to the database no other process is allows to read or write. If any process is reading the database other processes are allowed to read but not write. The entire database shared a single lock.

When SQLite tries to open a database and finds that it is locked, it can optionally delay for a short while and try to open the file again. This process repeats until the query times out and SQLite returns a failure. The timeout is adjustable. It is set to 0 by default so that if the database is locked, the SQL statement fails immediately. But you can use the "timeout" method to change the timeout value to a positive number. For example:

db1 timeout 2000

The argument to the timeout method is the maximum number of milliseconds to wait for the lock to clear. So in the example above, the maximum delay would be 2 seconds.

The "busy" method

The "busy" method, like "timeout", only comes into play when the database is locked. But the "busy" method gives the programmer much more control over what action to take. The "busy" method specifies a callback Tcl procedure that is invoked whenever SQLite tries to open a locked database. This callback can do whatever is desired. Presumably, the callback will do some other useful work for a short while then return so that the lock can be tried again. The callback procedure should return "0" if it wants SQLite to try again to open the database and should return "1" if it wants SQLite to abandon the current operation.

The "last_insert_rowid" method

The "last_insert_rowid" method returns an integer which is the ROWID of the most recently inserted database row.

The "changes" method

The "changes" method returns an integer which is the number of rows in the database that were inserted, deleted, and/or modified by the most recent "eval" method.

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