SQL Server CTEs: Why the Leading Semicolon?

CTE: Why the Semicolon Conundrum?

Common Table Expressions (CTEs) often begin with a semicolon (;), puzzling some developers. We delve into why this is the case.

Rationale for the Semicolon

The semicolon plays a crucial role in resolving ambiguity and ensuring statement termination. The WITH keyword is used in various contexts:

• FROM..WITH: Uses CTEs after the FROM clause.
• RESTORE..WITH MOVE: Specifies options in RESTORE statements.

To avoid confusion, a semicolon before the CTE (e.g., ;WITH OrderedOrders AS) is recommended. It ensures that the previous statement is terminated before the CTE begins.

Optional Semicolons

While semicolons are generally optional for statement termination in SQL Server, they become mandatory before CTEs and MERGE commands. This is a protective measure to prevent errors. For instance:

DECLARE @foo int;

WITH OrderedOrders AS
(
    SELECT SalesOrderID, OrderDate,
    ...

is equivalent to:

DECLARE @foo int

;WITH OrderedOrders AS
(
    SELECT SalesOrderID, OrderDate,
    ...

Therefore, using a semicolon before CTEs is a good practice to avoid potential ambiguity and ensure proper execution.

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