====== DISTINCT ======


===== Версии сервера =====
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===== Доступно в =====
[[raznovidnosti_jazyka_sql|DSQL]] [[raznovidnosti_jazyka_sql|PSQL]]

===== Формат =====
DISTINCT  используется в конструкции [[SELECT]] и как самостоятельный оператор 

//переменная// IS [NOT] DISTINCT FROM //переменная2//

FIXME
===== Описание =====

сравнивает 2 значения невзирая на NULL

если на поле существует индекс то он может быть использован при NOT DISTINCT

FIXME

===== Пример =====
Блок 
<code sql>
execute block
returns (s varchar(100))
as
begin
  if (null = null) then
  begin
    s = 'null = null';
    suspend;
  end

  if (null <> null) then
  begin
    s = 'null <> null';
    suspend;
  end

  if (null = 12) then
  begin
    s = 'null = 12';
    suspend;
  end

  if (null <> 12) then
  begin
    s = 'null <> 12';
    suspend;
  end

  if (12 <> 12) then
  begin
    s = '12 <> 12';
    suspend;
  end

  if (12 = 12) then
  begin
    s = '12 = 12';
    suspend;
  end

  if (null is distinct from null) then
  begin
    s = 'null is distinct from null';
    suspend;
  end

  if (null is not distinct from null) then
  begin
    s = 'null is not distinct from null';
    suspend;
  end

  if (null is distinct from 12) then
  begin
    s = 'null is distinct from 12';
    suspend;
  end

  if (12 is distinct from null) then
  begin
    s = '12 is distinct from null';
    suspend;
  end

  if (12 is distinct from 12) then
  begin
    s = '12 is distinct from 12';
    suspend;
  end

  if (12 is not distinct from 12) then
  begin
    s = '12 is not distinct from 12';
    suspend;
  end

  if (12 is distinct from 13) then
  begin
    s = '12 is distinct from 13';
    suspend;
  end

end;
</code>
результат
<code>
12 = 12
null is not distinct from null
null is distinct from 12
12 is distinct from null
12 is not distinct from 12
12 is distinct from 13
</code>



<code sql>SELECT * FROM T1 JOIN T2 ON T1.NAME IS NOT DISTINCT FROM T2.NAME;</code>
<code sql>SELECT * FROM T WHERE T.MARK IS DISTINCT FROM 'test'</code>

===== См. также =====
[[COALESCE]] [[nullif]]

===== Источник =====
README.distinct.txt