Types of joins in SQL (cheatsheet for beginners)

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.

Delphi Starter doesn't support command line

Delphi feature matrix has two important points:

  • Delphi 32-bit native code optimizing compiler (dcc32) for Windows, including high performance x86
    Assembler
  • Delphi toolchains for Windows

Regardless indicated options dcc32 doesn't work in Delphi Berlin Starter with the error message "This version of the product does not support command line compiling".

Happy tickets benchmark

Some countries have integer numbers printed on the transport tickets, for example, in ex-USSR. The ticket is "happy" when the sum of the first half of digits equals to the second one. Then you should eat it and make a wish.

Example:

123456 and 111222 are not happy tickets
123123 and 123222 are the happy tickets

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

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;

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