How to join multiple tables and display in a single datagrid using C# winforms

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I have an application that stores students results from Grade 1 - 9. Each grade has 3 terms so there are 27 tables in total to store the grades of a student. A student can start from any grade, therefore it might be that a student has results for grade 4-9 because he/she started from grade 4. So my tables tables are names as follows, Grade1FT,Grade1ST,Grade1TT,Grade2FT,Grade2ST,Grade3TT etc. i.e it follows such a pattern right to Grade9FT,Grade9ST,Grade9TT. Now each grade table contains 10 columns, so the column names are ID,English,Maths,Science,Social,RME,ICT,Drawing,History and Geography.
I have another table called register where all students register their names and are given their unique ID's.

I want the ability where a student can search through all his results from grade 1 - 9 and display in a single datagrid view using his unique student ID which he/she inputs into a textbox. I put in a sample code trying to run but l got an error saying "error near LEFT"

Here is what l tried but with no luck.

What I have tried:

C#

private void btnsearchall_Click(object sender, EventArgs e)
        {
            cn.Open();
            cmd = new SqlCommand(" SELECT register.StudentId LEFT JOIN grade1FT_results,grade1ST_results,grade1TT_results,grade2FT_results,
grade2ST_results,grade2TT_results,grade3FT_results,grade3ST_results,
grade3TT_results,
grade4FT_results, grade4ST_results,grade4TT_results,grade5FT_results,
grade5ST_results,grade5TT_results,
grade6FT_results,grade6ST_results,grade6TT_results, grade7FT_results,grade7ST_results,grade7TT_results,
grade8FT_results,grade8ST_results,grade8TT_results ,grade9FT_results,
grade9ST_results,
grade9TT_results WHERE StudentId=@Id ", cn);
            cmd.Parameters.AddWithValue("@Id", txtid.Text);
            cmd.ExecuteNonQuery();

            DataTable dtable;
            SqlDataAdapter da = new SqlDataAdapter();
            da.SelectCommand = cmd;
            dtable = new DataTable();
            da.Fill(dtable);
            BindingSource dsource = new BindingSource();

            dsource.DataSource = dtable;
            datagrid.DataSource = dsource;
            da.Update(dtable);

            cn.Close();

           

        }

Posted 31-May-18 14:47pm

KingDolly

Updated 31-May-18 21:51pm

Herman<T>.Instance

v3

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2 solutions

Solution 1

The SELECT statement works as follows;

SQL

SELECT
<Column Names>
FROM
<Table Name> <alias>
<Join Type> JOIN <Table Name> <alias> ON <join condition>
WHERE
<Conditions>

An example of this is as follows;

SQL

SELECT
a.StudentName, b.Grade AS Yr1Term1, c.Grade AS Yr1Term2
FROM
Register a
LEFT JOIN grade1FT_results b ON a.StudentId = b.StudentId
LEFT JOIN grade1ST_results c ON a.StudentId = c.StudentId
WHERE
a.StudentId = 123

Refer the following link for a full description of the Select statement; Select Statement [^]

The error you are getting is because you have not specified your FROM statement correctly

Kind Regards

Posted 31-May-18 15:13pm

an0ther1

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#realJSOP · Accepted Answer · 2018-05-31T15:16:00

0) Your query won’t run.

1) I would create a view in the database that performs a union on all of those tables

SQL

SELECT * FROM mydatabase.dbo.grade1FT_results
UNION ALL SELECT * FROM mydatabase.dbo.grade1ST_results
...
...
UNION ALL SELECT * FROM mydatabase.dbo.grade9TT_results

...and the query in your code would end up looking something like this:

SQL

SELECT * FROM mydatabase.dbo.myview WHERE StudentID = @id

Keep in mind that I'm assuming all of the tables have exactly the same schemas, but if they don't, you'll have to select each individual column by name instead of using SELECT *

There are other ways to do it, so if you don't like this way, feel free to search out other ways.