TSQL – How to find missing or deleted table identity autoincrement values

As a database designer, I often create on database tables the primary key as identity autoincrement and sometimes I needed to find missing or deleted IDs thet interrupt the sequence of autoincrement number on the primary key.
To do this you can create a sequence from 1 (or 0 based on setted RESEED) to MAX ID available on the table and than make a LEFT JOIN of this sequence with start table.

The following snippet get the list of IDs of identity autoincrement column that interrupt the sequence of autoincrement values:

DECLARE @START_ID INT,  @MAXID INT

SET @START_ID = 1
SET @MAXID = (SELECT MAX(ID) FROM [TABLENAME_TO_CHECK])

DECLARE @SEQUENCE_TABLE TABLE (ID BIGINT)

--CREATE TABLE WITH ID FROM [@START_ID] TO [MAX TABLE ID]
WHILE @START_ID < @MAXID
	BEGIN
		INSERT INTO @SEQUENCE_TABLE VALUES(@START_ID)

		SET @START_ID = @START_ID + 1
	END

SELECT S.ID AS [MISSING ID] 
FROM @SEQUENCE_TABLE S 
		LEFT JOIN [TABLENAME_TO_CHECK] AS T ON  S.ID = T.ID
WHERE T.ID IS NULL

In qualità di database designer, spesso creo la chiave primaria come identity autoincrement sulle tabelle del database ed alcune volte mi è capitato di dover trovare gli ID che sono stati cancellati, per cui interrompono la sequenza dell’identity autoincrement su campo chiave.
Per fare questo si può semplicemente creare una sequenza che va da 1 (oppure 0 in base al RESEED impostato) al MAX ID presente sulla tabella e, successivamente, mettere in LEFT JOIN questa sequenza con la tabella originaria.

Di seguito lo snippet per recuperare gli ID di un campo identity autoincrement che interrompono la sequenza di valori:

DECLARE @START_ID INT,  @MAXID INT

SET @START_ID = 1
SET @MAXID = (SELECT MAX(ID) FROM [TABLENAME_TO_CHECK])

DECLARE @SEQUENCE_TABLE TABLE (ID BIGINT)

--CREATE TABLE WITH ID FROM [@START_ID] TO [MAX TABLE ID]
WHILE @START_ID < @MAXID
	BEGIN
		INSERT INTO @SEQUENCE_TABLE VALUES(@START_ID)

		SET @START_ID = @START_ID + 1
	END

SELECT S.ID AS [MISSING ID] 
FROM @SEQUENCE_TABLE S 
		LEFT JOIN [TABLENAME_TO_CHECK] AS T ON  S.ID = T.ID
WHERE T.ID IS NULL