Next, I demonstrate the simple sign function and the amazingly powerful trick of using this function in SQL select statements to solve complex queries with a single pass through the data, after making a brief mathematical case for how the sign function defines the absolute value and IF conditions.

Although the sign function currently does not exist in sqlite3, it is very easy to create in the "/src/func.c" file so that this function will be permanently available to all sqlite applications. Normally, user functions are created in C, Perl, or C++, which is also documented in this article. sqlite3 has the ability to store "blob", binary data. The sample program in the download, "eatblob.c", reads a binary file of any size into memory and stores the data in a user-specified field.

All examples can be found in sqlite_examples.tar.gz, and I encourage you to download these examples as you read this document.

This tutorial was made with sqlite3 version 3.0.8.

Common Commands

To create a database file, run the command "sqlite3", followed by the database name. For example, to create the database "test.db", run the sqlite3 command as follows:

$ sqlite3 test.db
SQLite version 3.0.8
Enter ".help" for instructions
sqlite> .quit
$

The database file test.db will be created, if it does not already exist. Running this command will leave you in the sqlite3 environment. There are three ways to safely exit this environment: .q, .quit, and .exit.

You do not have to enter the sqlite3 interactive environment. Instead, you could perform all commands at the shell prompt, which is ideal when running bash scripts and commands in an ssh string. Here is an example of how you would create a simple table from the command prompt:

$ sqlite3 test.db "create table t1 (t1key INTEGER
PRIMARY KEY,data TEXT,num double,timeEnter DATE);"

After table t1 has been created, data can be inserted as follows:

$ sqlite3 test.db "insert into t1 (data,num) values ('This is sample data',3);"
$ sqlite3 test.db "insert into t1 (data,num) values ('More sample data',6);"
$ sqlite3 test.db "insert into t1 (data,num) values ('And a little more',9);"

As expected, doing a select returns the data in the table. Note that the primary key "t1key" auto increments; however, there are no default values for timeEnter. To populate the timeEnter field with the time, an update trigger is needed. Note that you should not use the abbreviation "INT" when working with the PRIMARY KEY. You must use "INTEGER" for the primary key to update.

$ sqlite3 test.db "select * from t1 limit 2";
1|This is sample data|3|
2|More sample data|6|

In the statement above, the limit clause is used, and only two rows are displayed. For a quick reference of SQL syntax statements available with SQLite, see the syntax page. There is an offset option for the limit clause. For instance, the third row is equal to the following: "limit 1 offset 2".

$ sqlite3 test.db "select * from t1 order by t1key limit 1 offset 2";
3|And a little more|9|

The ".table" command shows the table names. For a more comprehensive list of tables, triggers, and indexes created in the database, query the master table "sqlite_master", as shown below.

$ sqlite3 test.db ".table"
t1

$ sqlite3 test.db "select * from sqlite_master"
table|t1|t1|2|CREATE TABLE t1 (t1key INTEGER
PRIMARY KEY,data TEXT,num double,timeEnter DATE)

All SQL information and data inserted into a database can be extracted with the ".dump" command. Also, you might want to look for the "~/.sqlite_history" file.

$ sqlite3 test.db ".dump"
BEGIN TRANSACTION;
CREATE TABLE t1 (t1key INTEGER
PRIMARY KEY,data TEXT,num double,timeEnter DATE);
INSERT INTO "t1" VALUES(1, 'This is sample data', 3, NULL);
INSERT INTO "t1" VALUES(2, 'More sample data', 6, NULL);
INSERT INTO "t1" VALUES(3, 'And a little more', 9, NULL);
COMMIT;

The contents of the ".dump" can be filtered and piped to another database. Below, table t1 is changed to t2 with the sed command, and it is piped into the test2.db database.

$ sqlite3 test.db ".dump"|sed -e s/t1/t2/|sqlite3 test2.db

Triggers

An insert trigger is created below in the file "trigger1". The Coordinated Universal Time (UTC) will be entered into the field "timeEnter", and this trigger will fire after a row has been inserted into the table t1.

-- ********************************************************************
-- Creating a trigger for timeEnter
-- Run as follows:
-- $ sqlite3 test.db < trigger1
-- ********************************************************************
CREATE TRIGGER insert_t1_timeEnter AFTER INSERT ON t1
BEGIN
UPDATE t1 SET timeEnter = DATETIME('NOW') WHERE rowid = new.rowid;
END;
-- ********************************************************************

The AFTER specification in ..."insert_t1_timeEnter AFTER..." is necessary. Without the AFTER keyword, the rowid would not have been generated. This is a common source of errors with triggers, since AFTER is not the default, so it must be specified. If your trigger depends on newly-created data in any of the fields from the created row (which was the case in this example, since we need the rowid), the AFTER specification is needed. Otherwise, the trigger is a BEFORE trigger, and will fire before rowid or other pertinent data is entered into the field.

Comments are preceded by "--". If this script were created in the file "trigger1", you could easily execute it as follows.

$ sqlite3 test.db < trigger1

Now try entering a new record as before, and you should see the time in the field timeEnter.

$ sqlite3 test.db "insert into t1 (data,num) values ('First entry with timeEnter',19);"

$ sqlite3 test.db "select * from t1";
1|This is sample data|3|
2|More sample data|6|
3|And a little more|9|
4|First entry with timeEnter|19|2004-10-02 15:12:19

The last value has timeEnter filled automatically with Coordinated Universal Time, or UTC. If you want localtime, use select datetime('now','localtime'). See the note at the end of this section regarding UTC and localtime.

For the examples that follow, the table "exam" and the database "examScript" will be used. The table and trigger are defined below. Just like the trigger above, UTC time will be used.

-- *******************************************************************
-- examScript: Script for creating exam table
-- Usage:
-- $ sqlite3 examdatabase < examScript
--
-- Note: The trigger insert_exam_timeEnter
-- updates timeEnter in exam
-- *******************************************************************
-- *******************************************************************
CREATE TABLE exam (ekey INTEGER PRIMARY KEY,
fn VARCHAR(15),
ln VARCHAR(30),
exam INTEGER,
score DOUBLE,
timeEnter DATE);

CREATE TRIGGER insert_exam_timeEnter AFTER INSERT ON exam
BEGIN

UPDATE exam SET timeEnter = DATETIME('NOW')
WHERE rowid = new.rowid;
END;
-- *******************************************************************
-- *******************************************************************
Here's an example usage:

$ sqlite3 examdatabase < examScript
$ sqlite3 examdatabase "insert into exam (ln,fn,exam,score)
values ('Anderson','Bob',1,75)"

$ sqlite3 examdatabase "select * from exam"

1|Bob|Anderson|1|75|2004-10-02 15:25:00

As you can see, the PRIMARY KEY and current UTC time have been updated correctly.