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Update table from a join in TSQL

Often we find ourselves in the situation where we need to do updates of data.

The update function in TSQL is fantastic and does the job, but what happens if you need to update data depends on foreign keys.

In TSQL you can write a query that joins different tables and only updates records from the selected dataset as such:

Update <table or alias> set
   <column 0 > = value,
<column 1> = value
From dbo.<table> <alias>
Join dbo.<second table<alias1> on alias1.<column 1> = alias.<column 1>



If we use the AdventureWorks Database we can quickly create a scenario that is quite like something we would be likely to seen in the real world (well sort of).

Let say we have a customer that has just found out that due to new regulations, all employees that work with bikes should get a 10% raise (as if) and it should be pay back in time(even more unlikely).
What needs to happen is that all Standard Cost of a bike should be updated by 10% for all products in the  in the products table but only those that belong to the Bikes category.
Sounds like a big manual job , but wait , TSQL to the rescue.

The Code

update p
set p.StandardCost =  ( (StandardCost * 0.10 ) + StandardCost)
from Production.Product p
join Production.ProductCategory pc on pc.ProductCategoryID = p.ProductSubcategoryID
where pc.Name = ‘Bikes’


So what is going on here?

Line 1
The <p> is an alias to the table Production.Product , that means that we are updating the Production.Product  table.

Line 2
p.StandardCost =  ( (StandardCost * 0.10 ) + StandardCost)]

The update is on the column StandardCost or <p.StandardCost> | <tablealias.column>
and the value of 10% is added to the value or 1/10 of an 100.

Line 3
Specify witch table we will be updating and allows us to reference columns in the Production.Product table by typing p.<column name> by adding the alias p

[from Production.Product p]

Line 4
In order to only update the products in bikes we need to find out in what category the products are in. The Product.ProductCategory Table should provide that information. In order to get that into the update statement we use the following Join.

[join Production.ProductCategory pc on pc.ProductCategoryID = p.ProductSubcategoryID]

This tells the SQL Server to join the tables Production.ProductCategory pc and set the alias of the Production.ProductCategory to pc. And the relationship between the tables are the columns pc.ProductCategoryID and p.ProductSubcategoryID with their alias in front.

Line 5
And last but not least we need to specify our filter and that is done with the where clause
where pc.Name = ‘Bikes’
This clause tells the SQL Server only to offer up rows where the name of the category is ‘Bikes’

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