Deleting Data From Tables

This tutorial describes how to delete data from tables using the TMyQuery and TMyTable components.

1. Requirements
2. General information
3. Using DataSet Functionality
4. Building DML Statements Manually
   • 4.1 DML Statements With Parameters
   • 4.2 DML Statements As Plain Text
5. Additional Information

This walkthrough supposes that you know how to connect to server (tutorials "Connecting To MySQL" and "Connecting To MySQL Embedded"), how to create necessary objects on the server (tutorial "Creating Database Objects"), and how to insert data to created tables (tutorial "Inserting Data Into Tables").

Data on server can be deleted using Data Manipulation Language (DML), which is a part of SQL. DML statements can be executed on server by an account that has necessary privileges. There are two ways to manipulate a database. You can build DML statements manually and run them within some component like TMyQuery. Another way is to use the dataset functionality (the Delete method) of the TMyQuery and TMyTable components. We will discuss both ways. The goal of this tutorial is to delete a record in the table dept.

The Delete method of the TMyQuery and TMyTable components allows deleting data without using DML statements. DML statements are generated by MyDAC components internally. The code below demonstrates using this method:

[Delphi]

var
  q: TMyQuery;
begin
  q := TMyQuery.Create(nil);
  try
    // con is either TMyConnection or TMyEmbConnection already set up
    q.Connection := con; 

    // retrieve data
    q.SQL.Text := 'SELECT * FROM dept';
    q.Open;

    // delete the current record
    q.Delete; 
  finally
    q.Free;
  end;
end;

[C++Builder]

{
  TMyQuery* q = new TMyQuery(NULL);
  try
  {
    // con is either TMyConnection or TMyEmbConnection already set up
    q->Connection = con; 

    // retrieve data
    q->SQL->Text = "SELECT * FROM dept";
    q->Open();

    // delete the current record
    q->Delete();
  }
  __finally
  {
    q->Free();
  }
}

DML Statements can contain plain text and text with parameters. This section describes both ways.

[Delphi]

var
  q: TMyQuery;
begin
  q := TMyQuery.Create(nil);
  try
    // con is either TMyConnection or TMyEmbConnection already set up
    q.Connection := con; 
    
    // set SQL query for delete record
    q.SQL.Clear;
    q.SQL.Add('DELETE FROM dept WHERE deptno = :deptno;');

    // set parameters
    q.ParamByName('deptno').AsInteger := 10;

    // execute query
    q.Execute;
  finally
    q.Free;
  end;
end;

[C++Builder]

{
  TMyQuery* q = new TMyQuery(NULL);
  try
  {
    // con is either TMyConnection or TMyEmbConnection already set up
    q->Connection = con;

    // set SQL query for delete record
    q->SQL->Clear();
    q->SQL->Add("DELETE FROM dept WHERE deptno = :deptno;");

    // set parameters
    q->ParamByName("deptno")->AsInteger = 10;

    // execute query
    q->Execute();
  }
  __finally
  {
    q->Free();
  }
}

[Delphi]

var
  q: TMyQuery;
begin
  q := TMyQuery.Create(nil);
  try
    // con is either TMyConnection or TMyEmbConnection already set up
    q.Connection := con;

    // set SQL query for delete record
    q.SQL.Clear;
    q.SQL.Add('DELETE FROM dept WHERE deptno = 10;');

    // execute query 
    q.Execute;
  finally
    q.Free;
  end;
end;

[C++Builder]

{
  TMyQuery* q = new TMyQuery(NULL);
  try
  {
    // con is either TMyConnection or TMyEmbConnection already set up
    q->Connection = con;

    // set SQL query for delete record
    q->SQL->Clear();    
    q->SQL->Add("DELETE FROM dept WHERE deptno = 10;");

    // execute query 
    q->Execute();
  }
  __finally
  {
    q->Free();
  }
}

It is also possible to use stored procedures for deleting data. In this case, all data manipulation logic is defined on server. You can find more about using stored procedures in the tutorial "Stored Procedures".