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i'd like to deduct quantity from database but it doesn't work.

What I have tried:

this is the code i have wrote..

("UPDATE tbl_medicine SET quantity = quantity - '" & textbox1.Text & "' WHERE Item_code= '" & txtCode.Text & "'", conn)

where textbox1 is the quantity i want to deduct. (from user)

Posted 2-Aug-17 22:56pm

Member 13343055

Updated 3-Aug-17 0:29am

v2

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Comments

Richard Deeming 3-Aug-17 6:25am

Your code is vulnerable to SQL Injection[^]. NEVER use string concatenation to build a SQL query. ALWAYS use a parameterized query.

Everything you wanted to know about SQL injection (but were afraid to ask) | Troy Hunt[^]
How can I explain SQL injection without technical jargon? | Information Security Stack Exchange[^]
Query Parameterization Cheat Sheet | OWASP[^]

2 solutions

Solution 1

Never put direct user accessible values in your queries, you are just asking for trouble with sql injection attacks : SQL injection - Wikipedia[^]

Use SQL Parameters : Using SQLParameters with VB.NET/C#[^]

Try not putting the numeric values in single quotes :

"UPDATE tbl_medicine SET quantity = quantity - " & qvalue & " WHERE Item_code= '" & itemcode & "'"

Posted 2-Aug-17 23:22pm

Mehdi Gholam

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Member 13343055 3-Aug-17 9:12am

thanks for the info :)

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Richard Deeming · Accepted Answer · 2017-08-03T00:29:00

Use parameters, and make sure the quantity input is numeric:

VB.NET

Dim quantity As Integer
If Not Integer.TryParse(textbox1.Text, quantity) Then
    ' TODO: Show an error message to the user
    Return
End If

Using con As New SqlConnection("...")
    Using cmd As New SqlCommand("UPDATE tbl_medicine SET quantity = quantity - @quantity WHERE Item_code = @ItemCode", con)
        cmd.Parameters.AddWithValue("@quantity", quantity)
        cmd.Parameters.AddWithValue("@ItemCode", txtCode.Text)
        
        connection.Open()
        cmd.ExecuteNonQuery()
    End Using
End Using