Android Tutorial SQL tutorial

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Erel

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Update 2018:

New video tutorial:


The source code is available here: https://www.b4x.com/android/forum/t...ming-video-tutorials.88787/page-2#post-577932

Async methods: [B4X] SQL with Wait For

The following tutorial is obsolete:

This tutorial covers the SQL library and its usage with Basic4android.
There are many general SQL tutorials that cover the actual SQL language. If you are not familiar with SQL it is recommended to start with such a tutorial.
SQL Introduction

A new code module is available named DBUtils. It contains methods for common tasks which you can use and also learn from the code.

Android uses SQLite which is an open source SQL implementation.
Each implementation has some nuances. The following two links cover important information regarding SQLite.
SQLite syntax: Query Language Understood by SQLite
SQLite data types: Datatypes In SQLite Version 3

SQL in Basic4android
The first step is to add a reference to the SQL library. This is done by going to the Libraries tab and checking SQL.
There are two types in this library.
An SQL object gives you access to the database.
The Cursor object allows you to process queries results.

Usually you will want to declare the SQL object as a process global object. This way it will be kept alive when the activity is recreated.

SQLite stores the database in a single file.
When we initialize the SQL object we pass the path to a database file (which can be created if needed).
B4X:
Sub Process_Globals
    Dim SQL1 As SQL
End Sub

Sub Globals

End Sub

Sub Activity_Create(FirstTime As Boolean)
    If FirstTime Then
        SQL1.Initialize(File.DirDefaultExternal, "test1.db", True)
    End If
    CreateTables
    FillSimpleData
    LogTable1
    InsertManyRows
    Log("Number of rows = " & SQL1.ExecQuerySingleResult("SELECT count(*) FROM table1"))
 
    InsertBlob 'stores an image in the database.
    ReadBlob 'load the image from the database and displays it.
End Sub
The SQL1 object will only be initialized once when the process starts.
In our case we are creating it in the sd card. The last parameter (CreateIfNecessary) is True so the file will be created if it doesn't exist.

There are three types of methods that execute SQL statements.
ExecNonQuery - Executes a "writing" statement and doesn't return any result. This can be for example: INSERT, UPDATE or CREATE TABLE.

ExecQuery - Executes a query statement and returns a Cursor object that is used to process the results.

ExecQuerySingleResult - Executes a query statement and returns the value of the first column in the first row in the result set. This method is a shorthand for using ExecQuery and reading the value with a Cursor.

We will analyze the example code:
B4X:
Sub CreateTables
    SQL1.ExecNonQuery("DROP TABLE IF EXISTS table1")
    SQL1.ExecNonQuery("DROP TABLE IF EXISTS table2")
    SQL1.ExecNonQuery("CREATE TABLE table1 (col1 TEXT , col2 INTEGER, col3 INTEGER)")
    SQL1.ExecNonQuery("CREATE TABLE table2 (name TEXT, image BLOB)")
End Sub
The above code first deletes the two tables if they exist and then creates them again.

B4X:
Sub FillSimpleData
    SQL1.ExecNonQuery("INSERT INTO table1 VALUES('abc', 1, 2)")
    SQL1.ExecNonQuery2("INSERT INTO table1 VALUES(?, ?, ?)", Array As Object("def", 3, 4))
End Sub
In this code we are adding two rows. SQL.ExecNonQuery2 receives two parameters. The first parameter is the statement which includes question marks. The question marks are then replaced with values from the second List parameter. The List can hold numbers, strings or arrays of bytes (blobs).
Arrays are implicitly converted to lists so instead of creating a list we are using the Array keyword to create an array of objects.

B4X:
Sub LogTable1
    Dim Cursor1 As Cursor
    Cursor1 = SQL1.ExecQuery("SELECT col1, col2, col3 FROM table1")
    For i = 0 To Cursor1.RowCount - 1
        Cursor1.Position = i
        Log("************************")
        Log(Cursor1.GetString("col1"))
        Log(Cursor1.GetInt("col2"))
        Log(Cursor1.GetInt("col3"))
    Next
    Cursor1.Close
End Sub
This code uses a Cursor to log the two rows that were previously added.
SQL.ExecQuery returns a Cursor object.
Then we are using the For loop to iterate over all the results.
Note that before reading values from the Cursor we are first setting its position (the current row).

B4X:
Sub InsertManyRows
    SQL1.BeginTransaction
    Try
        For i = 1 To 500
            SQL1.ExecNonQuery2("INSERT INTO table1 VALUES ('def', ?, ?)", Array As Object(i, i))
        Next
        SQL1.TransactionSuccessful
    Catch
        Log(LastException.Message)
    End Try
    SQL1.EndTransaction
End Sub
This code is an example of adding many rows. Internally a lock is acquired each time a "writing" operation is done.
By explicitly creating a transaction the lock is acquired once.
The above code took less than half a second to run on a real device.
Without the BeginTransaction / EndTransaction block it took about 70 seconds.
A transaction block can also be used to guarantee that a set of changes were successfully done. Either all changes are made or none are made.
By calling SQL.TransactionSuccessful we are marking this transaction as a successful transaction. If you omit this line, all the 500 INSERTS will be ignored.
It is very important to call EndTransaction eventually.
Therefore the transaction block should usually look like:
B4X:
SQL1.BeginTransaction
Try
  'Execute the sql statements.
SQL1.TransactionSuccessful
Catch
'the transaction will be cancelled
End Try
SQL1.EndTransaction
Note that using transactions is only relevant when doing "writing" operations.

Blobs
The last two methods write an image file to the database and then read it and set it as the activity background.
B4X:
Sub InsertBlob
    Dim Buffer() As Byte = File.ReadBytes(File.DirAssets, "smiley.gif")
    'write the image to the database
    SQL1.ExecNonQuery2("INSERT INTO table2 VALUES('smiley', ?)", Array As Object(Buffer))
End Sub
Here we are using a special type of OutputStream which writes to a dynamic bytes array.
File.Copy2 copies all available data from the input stream into the output stream.
Then the bytes array is written to the database.

B4X:
Sub ReadBlob
    Dim Cursor1 As Cursor = SQL1.ExecQuery2("SELECT image FROM table2 WHERE name = ?", Array As String("smiley"))
    Cursor1.Position = 0
    Dim Buffer() As Byte = Cursor1.GetBlob("image")
    Dim InputStream1 As InputStream
    InputStream1.InitializeFromBytesArray(Buffer, 0, Buffer.Length)
    Dim Bitmap1 As Bitmap
    Bitmap1.Initialize2(InputStream1)
    InputStream1.Close
    Activity.SetBackgroundImage(Bitmap1)
End Sub
Using a Cursor.GetBlob we fetch the previously stored image.
Now we are using an input stream that reads from this array and load the image.

Asynchronous queries
SQL library v1.20 supports asynchronous select queries and asynchronous batch inserts.

Asynchronous means that the task will be processed in the background and an event will be raised when the task completes. This is useful when you need to issue a slow query and keep your application responsive.

The usage is quite simple:
B4X:
sql1.ExecQueryAsync("SQL", "SELECT * FROM table1", Null)
...
Sub SQL_QueryComplete (Success As Boolean, Crsr As Cursor)
    If Success Then
        For i = 0 To Crsr.RowCount - 1
            Crsr.Position = i
            Log(Crsr.GetInt2(0))
        Next
    Else
        Log(LastException)
    End If
End Sub
The first parameter is the "event name". It determines which sub will handle the QueryComplete event.

Batch inserts
SQL.AddNonQueryToBatch / ExecNonQueryBatch allow you to asynchronously process a batch of non-query statements (such as INSERT statements).
You should add the statements by calling AddNonQueryToBatch and eventually call ExecNonQueryBatch.
The task will be processed in the background. The NonQueryComplete event will be raised after all the statements execute.
B4X:
For i = 1 To 10000
        sql1.AddNonQueryToBatch("INSERT INTO table1 VALUES (?)", Array As Object(Rnd(0, 100000)))
Next
sql1.ExecNonQueryBatch("SQL")
...
Sub SQL_NonQueryComplete (Success As Boolean)
    Log("NonQuery: " & Success)
    If Success = False Then Log(LastException)
End Sub
 

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nfordbscndrd

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Licensed User
Indexes

I have written a data-intensive program in VB6 using databases/tables created in Access2007. The database has over a half-dozen tables totalling over a gig.

The Archos 70 has a 250GB hard drive, so storing the databases should not be a problem, but each table has multiple indexes, most of them indexing 2+ fields combined. I have looked at the links you gave for SQLite, but cannot figure out if it offers the same ability of indexing multiple fields.

Also, it was not clear to me if an index can be created and stored in advance of running the app, or if it is created each time the app is run, which seems like it would really be time consuming with such large files. I'm also concerned about running such an app with the A70 only having 256MB of memory.

I downloaded a user's app (from the B4a site) which uses SQLite, but the demo version will not run it, so I can't experiment with SQLite. I don't have high hopes for getting my app onto the Archos 70, but this is my only need for Basic4android, so I wouldn't buy it without knowing if this will work.
 

Erel

Administrator
Staff member
Licensed User
SQLite supports multiple columns in the index. See the CREATE INDEX command: SQLite Query Language: CREATE INDEX
The index is stored in the database file. There is no need to recreate it each time.

I cannot estimate the performance of such a database on your device.
 

nfordbscndrd

Well-Known Member
Licensed User
I am trying to run the following:

Cursor = SQL1.ExecQuery("SELECT * FROM Words")

I compile the app to a device, enter some text, click a button to execute this code, and get this error:

android.database.sqlite.SQLiteException: no such table: Words: , while compiling:
SELECT * FROM Words

I did an If File.Exists... to make sure the database is really there. The only table in the db is "Words".

If it helps, here is the Log:
 

Erel

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Are you sure that the Words table exists in the database?
Can you upload the database (in a new thread preferably)?
 

Erel

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You can issue the following query to list all the tables:
B4X:
SELECT name FROM sqlite_master WHERE type='table' ORDER BY name;
 

nfordbscndrd

Well-Known Member
Licensed User
You can issue the following query to list all the tables:
B4X:
SELECT name FROM sqlite_master WHERE type='table' ORDER BY name;
When I insert that code and compile, I get an error message saying that "End Select" is missing.
Also, where does this display the table names? Do I have to add some code to do that?
 
Last edited:

nfordbscndrd

Well-Known Member
Licensed User
After staring at this a dozen times, it dawned on me that the code you gave me needs to be in a Query, so I put in:

B4X:
Cursor = SQL1.ExecQuery("Select name from sqlite_master _
               WHERE  Type='table' ORDER BY name;")
Cursor.Position = 0
Msgbox(Cursor.GetString("name"), "table:")
I got the response of "android_metadata".
Ooo-kay...
 

karmacomposer

Member
Licensed User
Can we access mysql databases on a hosted server via internet commands in Basic4Android?

I have a MASSIVE database project I have been working on for a client that uses PHP and MySql. If I could write a app for android that accesses and utilizes the data from the existing database, it would be fantastic.

However, this database is protected and must remain on the protected server (since it has sensitive data and must be private for governmental reasons).

How would I do this?

Mike
 

Erel

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Staff member
Licensed User
You can use the HTTP library to communicate with web services.
 

markbarrett_1

Member
Licensed User
Hi,

I agree with Erel, using http services would be much better.

In my humble opinion, use https and spend a little bit of money to get a signed certificate for your server from a certificate authority.

Cheers.

M
 

asawyer13

Member
Licensed User
Mark,
Do you know of a simple step by step guide of what would be needed to setup an https webservice and get the certificate set up?
I can write the php webservice, but I don't know how to set up the server, etc?
Alan
 

eps

Well-Known Member
Licensed User
Success!

I Googled for "android_metadata" and found


Followed the directions there and my Words table is now showing up.

Time for me to find a fresh piece of wall for me to bang my head against...
That's a great post!! Very helpful :)

On my way to creating a SQLite DB and then manipulating the data - it's only going to be a piddly little DB, but it may grow and grow..
 

markbarrett_1

Member
Licensed User
Mark,
Do you know of a simple step by step guide of what would be needed to setup an https webservice and get the certificate set up?
I can write the php webservice, but I don't know how to set up the server, etc?
Alan
Sorry for the delayed response, I've been otherwise occupied.

What web server are you going to use? Both lighttpd and apache have heaps of examples (favourite search engine is your friend). Most of the questions resolve around using a self signed certificate, but how you configure your server doesn't change, its the way of getting your certificate that does if you self sign. Usually its a 5 or so line configuration in your httpd.conf which is essentially specifying your SSL key locations and which part of the wwwroot is to use this key information.

The bit about getting your certificate signed depends on which certificate authority you are going to use (be it verisign, godaddy, thawte etc etc). Each of them have their own instructions and procedures. You probably just want to double check the certificate authority you choose is one of the ones trusted by the android libraries before you fork out your cash to get the key signed.

Hope this is of some help.

Cheers.

M
 

HankAllen

New Member
Licensed User
Unable to access "test1.db"

When I try to run the SQL sample project, I get error says I'm unable to open the database file. This makes no sense because I am an admin user on this PC.
Any help would be greatly appreciated.
 
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