How to update a row with another row if column values are null in SQL

Introduction

If you need to update one row columns from another row columns in the same table following approach can be useful.

Background

To test queries and concept I am using SQL Server 2008 R2.

Queries

Create a sample table.

CREATE TABLE [dbo].[Employee](
[Title] [varchar](50) NULL,
[FirstName] [varchar](50) NULL,
[Surname] [varchar](50) NULL,
[DOB] [varchar](50) NULL,
[Datestarted] [varchar](50) NULL,
[EmployeeId] [int] IDENTITY(1,1) NOT NULL,
[Interest] [varchar](50) NULL,
[EmpAddress] [varchar](500) NULL
) ON [PRIMARY]
GO

Add some sample rows as follows.

I am just updating 2 columns. Of course you can update more in similar fashion. Here updating two columns of 1^st row with 2^nd row column values.

UPDATE n
SET n.EmpAddress = p.EmpAddress
,n.[Interest] = p.Interest
FROM [Employee] n inner join [Employee] p
ON n.employeeid = 1 AND p.employeeid = 2
GO
Select * from Employee

Please see next query if you wish to update only when value in the column is null.

Update column when value is null

I am going to use COALESCE. This is function similar to Case.

It returns the first nonnull expression among its arguments. Example:

select coalesce(Interest,'FUN') from [Employee] where EmployeeId = 5 

Updating two columns of row 4^th when they are null with column values of row 3^rd.

Precise query can look like

UPDATE n
SET n.EmpAddress = coalesce(n.EmpAddress, p.EmpAddress)
,n.[Interest] = coalesce(n.Interest, p.Interest)
FROM [Employee] n inner join [Employee] p
ON n.employeeid = 4 AND p.employeeid = 3
GO
Select * from Employee

Summary

In this article I have suggested queries to update row with anther row and how you can use coalesce to restrict modification to null fields only. I hope you can use these queries for other purposes too.