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<html><head><title>The Tcl interface to the SQLite library</title></head>
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<h2>The Tcl interface to the SQLite library</h2>

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 (version 2.8) or sqlite3
(version 3.0). Because there is only this
one command, the interface is not placed in a separate
namespace.

The sqlite3 command is used as follows:

<blockquote>
sqlite3&nbsp;&nbsp;dbcmd&nbsp;&nbsp;database-name
</blockquote>

The sqlite3 command opens the database named in the second
argument. If the database does not already exist, it is
automatically created.
The sqlite3 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. If the name of the database is an empty
string or the special name ":memory:" then a new database is created
in memory.

Once an SQLite database is open, it can be controlled using 
methods of the dbcmd. There are currently 21 methods
defined:

The sqlite3 also accepts an optional third argument called
the "mode". This argument is a legacy from SQLite version 2 and is
currently ignored.

<ul>

<li><a href="#authorizer">authorizer</a></li>
<li><a href="#busy">busy</a></li>
<li><a href="#cache">cache</a></li>
<li><a href="#changes">changes</a></li>
<li><a href="#close">close</a></li>
<li><a href="#collate">collate</a></li>
<li><a href="#collation_needed">collation_needed</a></li>
<li><a href="#commit_hook">commit_hook</a></li>
<li><a href="#complete">complete</a></li>
<li><a href="#copy">copy</a></li>
<li><a href="#errorcode">errorcode</a></li>
<li><a href="#eval">eval</a></li>
<li><a href="#exists">exists</a></li>
<li><a href="#function">function</a></li>
<li><a href="#last_insert_rowid">last_insert_rowid</a></li>
<li><a href="#nullvalue">nullvalue</a></li>
<li><a href="#onecolumn">onecolumn</a></li>
<li><a href="#progress">progress</a></li>
<li><a href="#timeout">timeout</a></li>
<li><a href="#total_changes">total_changes</a></li>
<li><a href="#trace">trace</a></li>
<li><a href="#transaction">transaction</a></li>

</ul>

The use of each of these methods will be explained in the sequel, though
not in the order shown above.

<a name="eval">
<h3>The "eval" method</h3>

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:

<blockquote>
dbcmd&nbsp;&nbsp;eval&nbsp;&nbsp;sql
&nbsp;&nbsp;&nbsp;&nbsp;?array-name&nbsp;?&nbsp;?script?
</blockquote>

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:

<blockquote>
sqlite3 db1 ./testdb 
db1 eval {CREATE TABLE t1(a int, b text)}
</blockquote>

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:

<blockquote>
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}]
</blockquote>

The variable $x is set by the above code to

<blockquote>
1 hello 2 goodbye 3 howdy!
</blockquote>

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 values of all
columns will be inserted into the array variable and the script will
be executed. For instance:

<blockquote>
db1 eval {SELECT * FROM t1 ORDER BY a} values { 
&nbsp;&nbsp;&nbsp;&nbsp;parray values 
&nbsp;&nbsp;&nbsp;&nbsp;puts "" 
}
</blockquote>

This last code will give the following output:

<blockquote>
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!
</blockquote>

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 omitted or 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:

<blockquote>
db1 eval {SELECT * FROM t1 ORDER BY a} { 
&nbsp;&nbsp;&nbsp;&nbsp;puts "a=$a b=$b" 
}
</blockquote>

From this we get the following output

<blockquote>
a=1 b=hello 
a=2 b=goodbye 
a=3 b=howdy!
</blockquote>

Tcl variable names can appear in the SQL statement of the second argument
in any position where it is legal to put a string or number literal. The
value of the variable is substituted for the variable name. If the
variable does not exist a NULL values is used. For example:

<blockquote>
db1 eval {INSERT INTO t1 VALUES(5,$bigblob)}
</blockquote>

Note that it is not necessary to quote the $bigblob value. That happens
automatically. If $bigblob is a large string or binary object, this
technique is not only easier to write, it is also much more efficient
since it avoids making a copy of the content of $bigblob.

<a name="close">
<h3>The "close" method</h3>

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:

<blockquote>
sqlite3 db1 ./testdb 
db1 close
</blockquote>

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:

<blockquote>
sqlite3 db1 ./testdb 
rename db1 {}
</blockquote>

<a name="transaction">
<h3>The "transaction" method</h3>

The "transaction" method is used to execute a TCL script inside an SQLite
database transaction. The transaction is committed when the script completes,
or it rolls back if the script fails. If the transaction occurs within
another transaction (even one that is started manually using BEGIN) it
is a no-op.

The transaction command can be used to group together several SQLite
commands in a safe way. You can always start transactions manually using
BEGIN, of
course. But if an error occurs so that the COMMIT or ROLLBACK are never
run, then the database will remain locked indefinitely. Also, BEGIN
does not nest, so you have to make sure no other transactions are active
before starting a new one. The "transaction" method takes care of
all of these details automatically.

The syntax looks like this:

<blockquote>
dbcmd&nbsp;&nbsp;transaction&nbsp;&nbsp;?transaction-type?
&nbsp;&nbsp;SCRIPT,
</blockquote>

The transaction-type can be one of deferred,
exclusive or immediate. The default is deferred.

<a name="cache">
<h3>The "cache" method</h3>

The "eval" method described <a href="#eval">above</a> keeps a cache of
<a href="capi3ref.html#sqlite3_prepare">prepared statements</a>
for recently evaluated SQL commands. 
The "cache" method is used to control this cache.
The first form of this command is:

<blockquote>
dbcmd&nbsp;&nbsp;cache size&nbsp;&nbsp;N
</blockquote>

This sets the maximum number of statements that can be cached.
The upper limit is 100. The default is 10. If you set the cache size
to 0, no caching is done.

The second form of the command is this:

<blockquote>
dbcmd&nbsp;&nbsp;cache flush
</blockquote>

The cache-flush method 
<a href="capi3ref.html#sqlite3_finalize">finalizes</a>
all prepared statements currently
in the cache.

<a name="complete">
<h3>The "complete" method</h3>

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 
<a href="capi3ref.html#sqlite3_complete">sqlite3_complete()</a> C
function.

<a name="copy">
<h3>The "copy" method</h3>

The "copy" method copies data from a file into a table.
It returns the number of rows processed successfully from the file.
The syntax of the copy method looks like this:

<blockquote>
dbcmd&nbsp;&nbsp;copy&nbsp;&nbsp;conflict-algorithm
&nbsp;&nbsp;table-name&nbsp;&nbsp;&nbsp;file-name&nbsp;
&nbsp;&nbsp;&nbsp;&nbsp;?column-separator&nbsp;?
&nbsp;&nbsp;?null-indicator?
</blockquote>

Conflict-alogrithm must be one of the SQLite conflict algorithms for
the INSERT statement: rollback, abort,
fail,ignore, or replace. See the SQLite Language
section for <a href="lang.html#conflict">ON CONFLICT</a> for more information.
The conflict-algorithm must be specified in lower case.

Table-name must already exists as a table. File-name must exist, and
each row must contain the same number of columns as defined in the table.
If a line in the file contains more or less than the number of columns defined,
the copy method rollbacks any inserts, and returns an error.

Column-separator is an optional column separator string. The default is
the ASCII tab character \t.

Null-indicator is an optional string that indicates a column value is null.
The default is an empty string. Note that column-separator and
null-indicator are optional positional arguments; if null-indicator
is specified, a column-separator argument must be specifed and
precede the null-indicator argument.

The copy method implements similar functionality to the .import
SQLite shell command. 
The SQLite 2.x <a href="lang.html#copy">COPY</a> statement 
(using the PostgreSQL COPY file format)
can be implemented with this method as:

<blockquote>
dbcmd&nbsp;&nbsp;copy&nbsp;&nbsp;$conflictalgo
&nbsp;&nbsp;$tablename&nbsp;&nbsp;&nbsp;$filename&nbsp;
&nbsp;&nbsp;&nbsp;&nbsp;\t&nbsp;
&nbsp;&nbsp;\\N
</blockquote>

<a name="timeout">
<h3>The "timeout" method</h3>

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:

<blockquote>db1 timeout 2000</blockquote>

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.

<a name="busy">
<h3>The "busy" method</h3>

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 (such as service
GUI events) 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.

<a name="exists">
<h3>The "exists" method</h3>

The "exists" method is similar to "onecolumn" and "eval" in that
it executes SQL statements. The difference is that the "exists" method
always returns a boolean value which is TRUE if a query in the SQL
statement it executes returns one or more rows and FALSE if the SQL
returns an empty set.

The "exists" method is often used to test for the existance of
rows in a table. For example:

<blockquote>
if {[db exists {SELECT 1 FROM table1 WHERE user=$user}]} { 
&nbsp;&nbsp;&nbsp;# Processing if $user exists 
} else { 
&nbsp;&nbsp;&nbsp;# Processing if $user does not exist 
}
</blockquote>

<a name="last_insert_rowid">
<h3>The "last_insert_rowid" method</h3>

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

<a name="function">
<h3>The "function" method</h3>

The "function" method registers new SQL functions with the SQLite engine.
The arguments are the name of the new SQL function and a TCL command that
implements that function. Arguments to the function are appended to the
TCL command before it is invoked.

The following example creates a new SQL function named "hex" that converts
its numeric argument in to a hexadecimal encoded string:

<blockquote>
db function hex {format 0x%X}
</blockquote>

<a name="nullvalue">
<h3>The "nullvalue" method</h3>

The "nullvalue" method changes the representation for NULL returned
as result of the "eval" method.

<blockquote>
db1 nullvalue NULL
</blockquote>

The "nullvalue" method is useful to differ between NULL and empty
column values as Tcl lacks a NULL representation. The default
representation for NULL values is an empty string.

<a name="onecolumn">
<h3>The "onecolumn" method</h3>

The "onecolumn" method works like 
"<a href="#eval">eval</a>" in that it evaluates the
SQL query statement given as its argument. The difference is that
"onecolumn" returns a single element which is the first column of the
first row of the query result.

This is a convenience method. It saves the user from having to
do a "<tt>[lindex&nbsp;...&nbsp;0]</tt>" on the results of an "eval"
in order to extract a single column result.

<a name="changes">
<h3>The "changes" method</h3>

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.

<a name="total_changes">
<h3>The "total_changes" method</h3>

The "total_changes" method returns an integer which is the number of rows
in the database that were inserted, deleted, and/or modified since the
current database connection was first opened.

<a name="authorizer">
<h3>The "authorizer" method</h3>

The "authorizer" method provides access to the 
<a href="capi3ref.html#sqlite3_set_authorizer">sqlite3_set_authorizer</a>
C/C++ interface. The argument to authorizer is the name of a procedure that
is called when SQL statements are being compiled in order to authorize
certain operations. The callback procedure takes 5 arguments which describe
the operation being coded. If the callback returns the text string
"SQLITE_OK", then the operation is allowed. If it returns "SQLITE_IGNORE",
then the operation is silently disabled. If the return is "SQLITE_DENY"
then the compilation fails with an error.

If the argument is an empty string then the authorizer is disabled.
If the argument is omitted, then the current authorizer is returned.

<a name="progress">
<h3>The "progress" method</h3>

This method registers a callback that is invoked periodically during
query processing. There are two arguments: the number of SQLite virtual
machine opcodes between invocations, and the TCL command to invoke.
Setting the progress callback to an empty string disables it.

The progress callback can be used to display the status of a lengthy
query or to process GUI events during a lengthy query.

<a name="collate">
<h3>The "collate" method</h3>

This method registers new text collating sequences. There are
two arguments: the name of the collating sequence and the name of a
TCL procedure that implements a comparison function for the collating
sequence.

For example, the following code implements a collating sequence called
"NOCASE" that sorts in text order without regard to case:

<blockquote>
proc nocase_compare {a b} { 
&nbsp;&nbsp;&nbsp;&nbsp;return [string compare [string tolower $a] [string tolower $b]] 
} 
db collate NOCASE nocase_compare 
</blockquote>

<a name="collation_needed">
<h3>The "collation_needed" method</h3>

This method registers a callback routine that is invoked when the SQLite
engine needs a particular collating sequence but does not have that
collating sequence registered. The callback can register the collating
sequence. The callback is invoked with a single parameter which is the
name of the needed collating sequence.

<a name="commit_hook">
<h3>The "commit_hook" method</h3>

This method registers a callback routine that is invoked just before
SQLite tries to commit changes to a database. If the callback throws
an exception or returns a non-zero result, then the transaction rolls back
rather than commit.

<a name="errorcode">
<h3>The "errorcode" method</h3>

This method returns the numeric error code that resulted from the most
recent SQLite operation.

<a name="trace">
<h3>The "trace" method</h3>

The "trace" method registers a callback that is invoked as each SQL
statement is compiled. The text of the SQL is appended as a single string
to the command before it is invoked. This can be used (for example) to
keep a log of all SQL operations that an application performs.

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This page last modified on 2006/01/05 15:50:07
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