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Editorial Team
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Asked: 2026-05-26T03:22:45+00:00

As a result of a mistake during an import to the a test table

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As a result of a mistake during an import to the a test table called CUSTOMERS, I found myself needing to switch the values of two columns in SQL Server.

I mistakenly imported the respective values for LASTNAME and FIRSTNAME into the opposite fields (i.e. the customer last name was imported into first name, and vice versa). To remedy this, I ran the following query in SQL Server 2008 R2, of course not expecting it to work:

UPDATE CUSTOMERS
SET LASTNAME=FIRSTNAME, FIRSTNAME=LASTNAME

Surprisingly, it worked! The limited programming experience I’ve had (high school, a few college courses) always followed the paradigm that switching two values required the presence of a third variable to “hold” the value of one of the initial values. In other words, I expected to have to run the following query: 

UPDATE CUSTOMERS
SET SOMEOTHERFIELD = LASTNAME
SET LASTNAME = FIRSTNAME
SET FIRSTNAME = SOMEOTHERFIELD

Is this behavior only seen in SQL Server 2008 R2, or does this represent some other underlying facet of relational theory that I’m missing?

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1 Answer

  1. Editorial Team

    Its because the way a update works:

    In the set clause are construct a pseudo-table. The rows in this table are build by copying values from the columns that are not mentioned from the original row to a new row. The columns are assigned all at once. That is, the unit of work is a row, not one column at a time. The last step is to delete the olds rows and insert the new rows. Internally, a update is select, delete, insert.

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