SQL with Manoj

In MS BOL for IDENTITY columns it is mentioned that [link]: “It creates an identity column in a table. This property is used with the CREATE TABLE and ALTER TABLE Transact-SQL statements.”

But experimenting with IDENTITY function I learnt this new thing with is not mentioned (or I could not find) anywhere in MS BOL that: “IDENTITY function can also be used with a SELECT statement but with some condition.”

Let’s check what’s that condition is:

USE [AdventureWorks]
GO

-- Let's use Person.Contact table in our SELECT statement and add a new column as IDENTITY function to it:
SELECT 
	IDENTITY (INT, 100, 5) AS NEW_ID, 
	ContactID, Title, 
	FirstName, 
	MiddleName, 
	LastName, 
	EmailAddress, 
	Phone
FROM Person.Contact

Error Msg thrown:
Msg 177, Level 15, State 1, Line 1
The IDENTITY function can only be used when the SELECT statement has an INTO clause.

As per the error msg we cannot add an IDENTITY column to a SELECT query. The SELECT should be followed by an INTO clause. This way a new table will be created and records will be entered with the new IDENTITY column.

-- Let's add INTO clause in SELECT statement:
SELECT 
	IDENTITY (INT, 100, 5) AS NEW_ID, 
	ContactID, 
	Title, 
	FirstName, 
	MiddleName, 
	LastName, 
	EmailAddress, 
	Phone
INTO #tempTable
FROM Person.Contact

Again an Error Msg:
Msg 8108, Level 16, State 1, Line 1
Cannot add identity column, using the SELECT INTO statement, to table ‘#tempTable’, which already has column ‘ContactID’ that inherits the identity property.

Completely agree with this!!! You cannot have 2 IDENTITY columns in a single table.

-- So we will remove the ContactID column as we want to have new ID column.
SELECT 
	IDENTITY (INT, 100, 5) AS NEW_ID, 
	Title, 
	FirstName, 
	MiddleName, 
	LastName, 
	EmailAddress, 
	Phone
INTO #tempTable
FROM Person.Contact
-- This worked perfectly.

-- Now let's check the newly created temp table and inserted records:
select * from #tempTable

-- Output below shows selected records from Person.Contact table with a new column having values starting from 100 with a gap of 5:

-- Final Cleanup
DROP TABLE #tempTable

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