Programming

Types of joins in SQL (cheatsheet for beginners)

Submitted by st on

This article is used in my book "Programming with databases".

Better to see once than to hear a hundred times. This small article explains what are the different kinds of joins in SQL.

At the first time you should take into account that SQL is a declarative 4th generation language. So the best way to use it (especially for beginners) is to write the query and let SQL optimizer do its job. That means thinking in set-oriented terms and avoiding sub-queries.

Let's take two example tables: Companies and Contacts.

Bit-packed structure (record) in Free Pascal and Delphi

Submitted by st on

Here is the code explaining some methods to pack bit accessing logic in your program.

program project1;
 
type
  TRec1 = packed record // Doesn't pack at all
    b1, b2, b3, b4, b5, b6, b7, b8, b9, b10, b11, b12, b13, b14, b15, b16: boolean;
  end;
 
  TRec2 = bitpacked record // Free Pascal specific
    b1, b2, b3, b4, b5, b6, b7, b8, b9, b10, b11, b12, b13, b14, b15, b16: boolean;
  end;
 
  TRecByte = (b1, b2, b3, b4, b5, b6, b7, b8, b9, b10, b11, b12, b13, b14, b15, b16);
  TRecBytes = set of TRecByte;
var
  r1: TRec1;
  r2: TRec2;
  s1: TRecBytes;

Crack C# namespaces in 30 seconds

Submitted by st on

The namespaces conception in C# seems to be exhaustive and insufficient at the same time compared with a strong module notation in Oberon-2 and even with a modular programming approaches in Free Pascal/Delphi.

Why insufficient? You cannot declare constants and functions within namespaces but you should add a class with constant field or static method for it.

Why exhaustive? The using directive with such introduced classes is the source of unexpected errors. Here is an example below.

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