Question

I’m trying to join 3 tables in 1 SQL query but I’m really a ‘noob’ with the join query, this is what I’ve ‘created’ so far :

SELECT m.Mod_ID as Modelnr, m.Mod_Naam as Modelnaam, m.Mod_Omschrijving as Omschrijving, m.Taal_ID as Taal, m.User_ID as Ontwerper 
FROM Model as m 
INNER JOIN Login as l ON m.User_ID = l.User_ID 
INNER JOIN Taal as t ON m.Taal_ID = t.Taal_ID
WHERE m.User_ID = '" + userid + "' 

The DB was created inside Visual studio (asp.net c#), the Tables : Model, Taal and Login.

• Model : Mod_ID, Mod_Naam, Mod_Omschrijving, Taal_ID, User_ID
• Taal : Taal_ID, Taal
• Login : User_ID, U_Naam, U_Achternaam

Tables & Relations : http://i52.tinypic.com/2upxmbk.jpg

Code behind to fill the gridview :

SqlCommand objCommand = new SqlCommand("SELECT m.Mod_ID as Modelnr, m.Mod_Naam as Modelnaam, m.Mod_Omschrijving as Omschrijving, m.Taal_ID as Taal, m.User_ID as Ontwerper from Model as m INNER JOIN Login as l ON m.User_ID = l.User_ID INNER JOIN Taal as t ON t.Taal_ID = t.Taal_ID WHERE m.User_ID = '" + userid + "' ", con);
dr = objCommand.ExecuteReader();
gvModel.DataSource = dr;
gvModel.HeaderStyle.HorizontalAlign = HorizontalAlign.Left;
gvModel.DataBind();

Trying to show Model in a gridview with the corresponding language from Taal (Taal.Taal) and user from Login (Login.U_Naam).

What I’m getting atm (only 3 rows in Model inside the DB) : http://i55.tinypic.com/2wbrcd2.jpg

But I’m still getting just number ID’s in my gridview.
Anyone who can help me out ?

thanks !

Answer 1

It looks to me like your query is doing exactly what it’s supposed to – the problem is in the “select” part of your query – everything is coming from the Model table.

After you do the “from” and the joins, the data that you’re selecting looks like this:

|         these columns come from the Model table          |                          these come from the User table                              |   Taal table   |
| Mod_ID | Mod_Naam | Mod_Omschrijving | Taal_ID | User_ID | User_ID | Username | Password | U_Naam | U_Voornaam | U_Geboorte | U_Dienst | (....) | Taal_ID | Taal |

…you can think of it kind of like “one big table”. So you want to select things from the User and Taal tables too – otherwise you made the database do all the work of joining them for nothing. I’m pretty sure that what you want is:

SELECT m.Mod_ID as Modelnr, m.Mod_Naam as Modelnaam, m.Mod_Omschrijving as Omschrijving,
    t.Taal as Taal, l.U_Naam as Ontwerper -- <-- this is the only bit that's different
FROM Model as m 
INNER JOIN Login as l ON m.User_ID = l.User_ID 
INNER JOIN Taal as t ON m.Taal_ID = t.Taal_ID
WHERE m.User_ID = @user_ID

I also fixed the SQL injection problem you have, to call this in .NET, do this:

SqlCommand objCommand = new SqlCommand("SELECT .... etc", con);
objCommand.Parameters.AddWithValue("@user_ID", userid);
dr = objCommand.ExecuteReader();
// as before