SQL Maniak

Merging Tables Without Downtime: When Two Data Models Become One

Wed, 04 Nov 2026 08:00:00 +0100

Splitting a table is one direction of schema evolution. Sometimes, after years of development, we need to do the opposite: combine several structures into a single model.

Example

We have:

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RetailCustomer
BusinessCustomer

And we want to move to:

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Customer

The New Model

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CREATE TABLE dbo.CustomerV2
(
 CustomerId bigint NOT NULL PRIMARY KEY,
 CustomerType varchar(20) NOT NULL,
 DisplayName nvarchar(200) NOT NULL,
 TaxId nvarchar(30) NULL
);

Map the Semantics First

Merging tables is not simply a matter of running UNION ALL.

Splitting a Table Without a Big Bang: From One Data Model to Two

Wed, 28 Oct 2026 08:00:00 +0100

Imagine that, over the years, a Customer table has grown to contain dozens of columns.

We want to move some of the address information into a separate table.

The New Table

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CREATE TABLE dbo.CustomerAddress
(
 CustomerId int NOT NULL,
 AddressType varchar(20) NOT NULL,
 AddressLine1 nvarchar(200) NOT NULL,
 City nvarchar(100) NOT NULL,
 PostalCode nvarchar(20) NULL,
 CONSTRAINT PK_CustomerAddress
 PRIMARY KEY (CustomerId, AddressType)
);

Step 1 — Introduce the New Structure Without Removing the Old One

The existing columns remain in place. The new table extends the data model.

About

Mon, 01 Jan 0001 00:00:00 +0000

Hi, I’m Marcin 👋

I’m a DBA and engineer working with SQL Server 2016–2022, including Query Store, Extended Events, Always On, performance tuning, and security.

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