LocalDB is created specifically for developers. It is very easy to install and requires no management, yet it offers the same T-SQL language, programming surface and client-side providers as the regular SQL Server Express. In effect the developers that target SQL Server no longer have to install and manage a full instance of SQL Server Express on their laptops and other development machines.

Localdb is a minified version of SQL Express that require less number of pre-req to install and offers a fast, zero-configuration installation. – Chirag Shah

In SQL Server "Denali" we decided to change the approach and introduce a dedicated version of SQL Express for developers - LocalDB that delivers the simplicity and yet is compatible with other editions of SQL Server at the API level.

LocalDBManager.exe was the original name of the tool. Just before shipping CTP3 we decided to change the name to SqlLocalDB.exe, since LocalDBManager.exe seemed a little bit over the top for a simple command line tool. "Manager" part of the name suggests something much more comprehensive than SqlLocalDB.exe, something like the old SQL Server Enterprise Manager or IIS Manager. - Krzysztof Kozielczyk

Also, by making LocalDB a better SQL Express for developers, we hope to be able to improve the regular SQL Express to be a better free SQL Server. We'd be very happy to hear your feedback in this area, especially if you're using SQL Express as a database server and find any issues caused by the new features that were introduced to fit the needs of developers and desktop environment.

At a very high level, LocalDB has the following key properties:

1. LocalDB uses the same sqlservr.exe as the regular SQL Express and other editions of SQL Server. The application is using the same client-side providers (ADO.NET, ODBC, PDO and others) to connect to it and operates on data using the same T-SQL language as provided by SQL Express.

2. LocalDB is installed once on a machine (per major SQL Server version). Multiple applications can start multiple LocalDB processes, but they are all started from the same sqlservr.exe executable file from the same disk location.

3. LocalDB doesn't create any database services; LocalDB processes are started and stopped automatically when needed. The application is just connecting to "Data Source=(localdb)\v11.0" and LocalDB process is started as a child process of the application. A few minutes after the last connection to this process is closed the process shuts down.

4. LocalDB connections support AttachDbFileName property, which allows developers to specify a database file location. LocalDB will attach the specified database file and the connection will be made to it.

SQL Express

LocalDB is not a replacement for SQL Server Express -- it is an addition to SQL Server Express lineup. While LocalDB is meant for developers, the regular SQL Server Express will continue to exist as a free SQL Server edition, fully compatible with and easily upgradeable to higher SQL Server editions.

SQL Express User Instance

Users familiar with today's User Instances of SQL Server Express should feel right at home with LocalDB. In essence LocalDB offers the ability to create and start a "user instance" without the hassle of installing and maintaining the parent SQL Express instance. In that respect LocalDB could be seen as an upgrade of the User Instances feature of SQL Server Express. Let me also take this opportunity to remind everyone that User Instances are on a deprecation path since SQL Server 2008.

SQL 2012 LocalDB

As we said before, at the heart of LocalDB is the same sqlservr.exe as in the regular SQL Express and other editions of SQL Server. In case of LocalDB, it is installed into one central location together with all necessary DLLs. By default it is located at "C:\Program Files\Microsoft SQL Server\110\LocalDB\Binn".

When an application uses any of the client-side providers (like ADO.NET, ODBC or PDO) to connect to "Data Source=(localdb)\v11.0", the provider will first check if LocalDB instance for the current user is started. If it's already started the application will connect to it. Otherwise the LocalDB instance for the current user will be started and then the provider proceeds to connect to it. Note that each user (Windows login) may have their own LocalDB instance that is isolated from instances of other users.

SQL Server Compact

Small and simple database, lightweight installation, connecting to a database file -- this will sound familiar to any developer using SQL Server Compact. The similarities are not accidental, as our goal for LocalDB was to be as easy to use as SQL Server Compact (while being as powerful and compatible with full SQL Server as SQL Express).

There are significant differences between LocalDB and SQL Server Compact:

1. Execution mode: SQL Server Compact is an in-proc DLL, while LocalDB runs as a separate process.

2. Disk usage: all SQL Server Compact binaries amount to some 4MBs, while LocalDB installation takes 140MBs.

3. Features: SQL Server Compact offers core RDBMS functionality like querying, while LocalDB provides a much richer set of features, including Stored Procedures, Geometry and Geography data types, etc.

Read the article here.

Feature Comparison

Erik Jensen has put together an excellent table to compare features as we know them today. Read his article here.

Feature

CE

v3.5 SP2

CE

v4.0

Express

2008 R2

”Denali”

LocalDB

Deployment/Installation Features

Download Size

2.5MB

2.5MB

74MB

32MB

Size On Disk 12MB 18MB 300MB 160MB ?

ClickOnce

Yes

Yes

Yes

Yes

Installed with MSI

Yes

Yes

Yes

Yes

Embedded w/App

Yes

Yes

No

No

Non-admin install

Yes

Yes

No

No

On ASP.Net

No

Yes

Yes

Yes

On Windows Phone

Yes

No

No

No

Runs in-process

Yes

Yes

No

No

64-bit support

Yes

Yes

Yes

Yes

Runs as a service

No

No

Yes

No

Data file features

Single File

Yes

Yes

Multiple

Multiple

File on Net share

No

No

No

No

Support for different file extensions

Yes

Yes

No

No

Max Size

4 GB

4 GB

10 GB

10 GB

XML storage

As ntext

As ntext

Yes

Yes

BLOB storage

As image

As image

Yes

Yes

FileStream storage

No

No

Yes

No

Code free, document safe, file format

Yes

Yes

No

No

Programmability

Transact-SQL

Yes

Yes

Yes

Yes

Procedural T-SQL

No

No

Yes

Yes

Remote Data Access

Yes

No

No

No

ADO.NET Sync

Yes

No

Yes

Yes

LINQ to SQL

Yes

No

Yes

Yes

ADO.NET EF 4.1

Yes

Yes

Yes

Yes

Merge replication

Yes

No

Yes

No

Simple
Transactions

Yes

Yes

Yes

Yes

Distributed
Transactions

No

No

Yes

Yes

XQuery / XPath

No

No

Yes

Yes

Stored Procedures, Views, Triggers

No

No

Yes

Yes

Role-based security

No

No

Yes

Yes

Concurrent connections

256 (100)

256

Unlimited

Unlimited (local)