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The Tcl interface to the SQLite library

The SQLite library is designed to be very easy to use from a Tcl or Tcl/Tk script. SQLite began as a Tcl extension and the primary test suite for SQLite is written in TCL. SQLite can be used with any programming language, but its connections to TCL run deep.

This document gives an overview of the Tcl programming interface for SQLite.

The API

The interface to the SQLite library consists of single tcl command named sqlite3 Because there is only this one command, the interface is not placed in a separate namespace.

The sqlite3 command is used as follows:

sqlite3  dbcmd  database-name

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 the special name ":memory:" then a new database is created in memory. If the name of the database is an empty string, then the database is created in an empty file that is automatically deleted when the database connection closes.

Once an SQLite database is open, it can be controlled using methods of the dbcmd. There are currently 22 methods defined.

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

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:

sqlite3 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 values of all columns 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 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:

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!

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:

db1 eval {INSERT INTO t1 VALUES(5,$bigstring)}

Note that it is not necessary to quote the $bigstring value. That happens automatically. If $bigstring 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 $bigstring.

If the $bigstring variable has both a string and a "bytearray" representation, then TCL inserts the value as a string. If it has only a "bytearray" representation, then the value is inserted as a BLOB. To force a value to be inserted as a BLOB even if it also has a text representation, us a "@" character to in place of the "$". Like this:

db1 eval {INSERT INTO t1 VALUES(5,@bigstring)}

If the variable does not have a bytearray representation, then "@" works just like "$". Note that ":" works like "$" in all cases so the following is another way to express the same statement:

db1 eval {INSERT INTO t1 VALUES(5,:bigstring)}

The use of ":" instead of "$" before the name of a variable can sometimes be useful if the SQL text is enclosed in "..." instead of {...}. When the SQL is contained within "..." then TCL will do the substitution of $-variables, which can lead to SQL injection if extreme care is not used. But TCL will never substitute a :-variable regardless of whether "..." or {...} are used to enclose the SQL, so the use of :-variables adds an extra measure of defense against SQL injection.

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:

sqlite3 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:

sqlite3 db1 ./testdb
rename db1 {}

The "transaction" method

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:

dbcmd  transaction  ?transaction-type?   SCRIPT,

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

The "cache" method

The "eval" method described above keeps a cache of prepared statements for recently evaluated SQL commands. The "cache" method is used to control this cache. The first form of this command is:

dbcmd  cache size  N

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:

dbcmd  cache flush

The cache-flush method finalizes all prepared statements currently in the cache.

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 sqlite3_complete() C function.

The "copy" method

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:

dbcmd  copy  conflict-algorithm   table-name   file-name      ?column-separator ?   ?null-indicator?

Conflict-algorithm must be one of the SQLite conflict algorithms for the INSERT statement: rollback, abort, fail,ignore, or replace. See the SQLite Language section for ON CONFLICT 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 specified and precede the null-indicator argument.

The copy method implements similar functionality to the .import SQLite shell command. The SQLite 2.x COPY statement (using the PostgreSQL COPY file format) can be implemented with this method as:

dbcmd  copy  $conflictalgo   $tablename   $filename      \t    \\N

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 (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.

The "enable_load_extension" method

The extension loading mechanism of SQLite (accessed using the load_extension() SQL function) is turned off by default. This is a security precaution. If an application wants to make use of the load_extension() function it must first turn the capability on using this method.

This method takes a single boolean argument which will turn the extension loading functionality on or off.

This method maps to the sqlite3_enable_load_extension() C/C++ interface.

The "exists" method

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 existence of rows in a table. For example:

if {[db exists {SELECT 1 FROM table1 WHERE user=$user}]} {
   # Processing if $user exists
} else {
   # Processing if $user does not exist
}

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 "function" method

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:

db function hex {format 0x%X}

The "nullvalue" method

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

db1 nullvalue NULL

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.

The "onecolumn" method

The "onecolumn" method works like "eval" 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 "[lindex ... 0]" on the results of an "eval" in order to extract a single column result.

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.

The "total_changes" method

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.

The "authorizer" method

The "authorizer" method provides access to the sqlite3_set_authorizer 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.

The "progress" method

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.

The "collate" method

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:

proc nocase_compare {a b} {
    return [string compare [string tolower $a] [string tolower $b]]
}
db collate NOCASE nocase_compare

The "collation_needed" method

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.

The "commit_hook" method

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.

The "rollback_hook" method

This method registers a callback routine that is invoked just before SQLite tries to do a rollback. The script argument is run without change.

The "status" method

This method returns status information from the most recently evaluated SQL statement. The status method takes a single argument which should be either "steps" or "sorts". If the argument is "steps", then the method returns the number of full table scan steps that the previous SQL statement evaluated. If the argument is "sorts", the method returns the number of sort operations. This information can be used to detect queries that are not using indices to speed search or sorting.

The status method is basically a wrapper on the sqlite3_stmt_status() C-language interface.

The "update_hook" method

This method registers a callback routine that is invoked just before each row is modified by an UPDATE, INSERT, or DELETE statement. Four arguments are appended to the callback before it is invoked:

The "incrblob" method

This method opens a TCL channel that can be used to read or write into a preexisting BLOB in the database. The syntax is like this:

dbcmd  incrblob  ?-readonly??   ?DB?  TABLE  COLUMN  ROWID

The command returns a new TCL channel for reading or writing to the BLOB. The channel is opened using the underlying sqlite3_blob_open() C-language interface. Close the channel using the close command of TCL.

The "errorcode" method

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

The "trace" method

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.

The "backup" method

The "backup" method makes a backup copy of a live database. The command syntax is like this:

dbcmd  backup  ?source-database?  backup-filename

The optional source-database argument tells which database in the current connection should be backed up. The default value is main (or, in other words, the primary database file). To back up TEMP tables use temp. To backup an auxiliary database added to the connection using the ATTACH command, use the name of that database as it was assigned in the ATTACH command.

The backup-filename is the name of a file into which the backup is written. Backup-filename does not have to exist ahead of time, but if it does, it must be a well-formed SQLite database.

The "restore" method

The "restore" method copies the content a separate database file into the current database connection, overwriting any preexisting content. The command syntax is like this:

dbcmd  restore  ?target-database?  source-filename

The optional target-database argument tells which database in the current connection should be overwritten with new content. The default value is main (or, in other words, the primary database file). To repopulate the TEMP tables use temp. To overwrite an auxiliary database added to the connection using the ATTACH command, use the name of that database as it was assigned in the ATTACH command.

The source-filename is the name of an existing well-formed SQLite database file from which the content is extracted.

The "version" method

Return the current library version. For example, "3.6.17".

The "profile" method

This method is used to profile the execution of SQL statements run by the application. The syntax is as follows:

dbcmd  profile  ?script

Unless script is an empty string, this method arranges for the script to be evaluated after the execution of each SQL statement. Two arguments are appended to script before it is invoked: the text of the SQL statement executed and the time elapsed while executing the statement, in nanoseconds.

A database handle may only have a single profile script registered at any time. If there is already a script registered when the profile method is invoked, the previous profile script is replaced by the new one. If the script argument is an empty string, any previously registered profile callback is canceled but no new profile script is registered.