Combining datasets with EXCEPT versus checking on IS NULL in a LEFT JOIN

Question

I'm currently working my way through the Microsoft SQL Server 2008 - Database Development (MCTS Exam 70-433) certification. In one of the earlier chapters on Combining Datasets, I came across the EXCEPT (and INTERSECT) commands. One example shows how to use EXCEPT to get all values from one table that doesn't have a related value in a second table like this:

SELECT EmployeeKey FROM DimEmployee
EXCEPT
SELECT EmployeeKey FROM FactResellerSales

The EXCEPT command was new to me, but with what I knew before today I would still easily solve the problem using a LEFT JOIN and check for IS NULL on the join constraint the following way:

SELECT DISTINCT DimEmployee.EmployeeKey FROM DimEmployee
LEFT JOIN FactResellerSales ON FactResellerSales.EmployeeKey = DimEmployee.EmployeeKey
WHERE FactResellerSales.EmployeeKey IS NULL

Now I started to wonder which one of these have the best performance. I tried to study the query execution plans, but I'm not too good at reading those, so it didn't make me any wiser. For the query using EXCEPT, the plan looks like this:

|--Nested Loops(Left Anti Semi Join, OUTER REFERENCES:([AdventureWorksDW2008].[dbo].[DimEmployee].[EmployeeKey], [Expr1006]) WITH UNORDERED PREFETCH)
    |--Index Scan(OBJECT:([AdventureWorksDW2008].[dbo].[DimEmployee].[IX_DimEmployee_SalesTerritoryKey]))
    |--Top(TOP EXPRESSION:((1)))
        |--Index Seek(OBJECT:([AdventureWorksDW2008].[dbo].[FactResellerSales].[IX_FactResellerSales_EmployeeKey]), SEEK:([AdventureWorksDW2008].[dbo].[FactResellerSales].[EmployeeKey]=[AdventureWorksDW2008].[dbo].[DimEmployee].[EmployeeKey]) ORDERED FORWARD)

And for the one using the LEFT JOIN it looks like this:

|--Stream Aggregate(GROUP BY:([AdventureWorksDW2008].[dbo].[DimEmployee].[EmployeeKey]))
    |--Filter(WHERE:([AdventureWorksDW2008].[dbo].[FactResellerSales].[EmployeeKey] IS NULL))
        |--Merge Join(Left Outer Join, MERGE:([AdventureWorksDW2008].[dbo].[DimEmployee].[EmployeeKey])=([AdventureWorksDW2008].[dbo].[FactResellerSales].[EmployeeKey]), RESIDUAL:([AdventureWorksDW2008].[dbo].[FactResellerSales].[EmployeeKey]=[AdventureWorksDW2008].[dbo].[DimEmployee].[EmployeeKey]))
            |--Clustered Index Scan(OBJECT:([AdventureWorksDW2008].[dbo].[DimEmployee].[PK_DimEmployee_EmployeeKey]), ORDERED FORWARD)
            |--Index Scan(OBJECT:([AdventureWorksDW2008].[dbo].[FactResellerSales].[IX_FactResellerSales_EmployeeKey]), ORDERED FORWARD)

The tables used in the query are from the AdventureWorksDW2008 example database, so below I also include the create scripts for the two tables in case that is needed to answer the question correctly:

USE [AdventureWorksDW2008]
GO

/****** Object:  Table [dbo].[DimEmployee]    Script Date: 11/22/2010 20:30:20 ******/
SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

CREATE TABLE [dbo].[DimEmployee](
    [EmployeeKey] [int] IDENTITY(1,1) NOT NULL,
    [ParentEmployeeKey] [int] NULL,
    [EmployeeNationalIDAlternateKey] [nvarchar](15) NULL,
    [ParentEmployeeNationalIDAlternateKey] [nvarchar](15) NULL,
    [SalesTerritoryKey] [int] NULL,
    [FirstName] [nvarchar](50) NOT NULL,
    [LastName] [nvarchar](50) NOT NULL,
    [MiddleName] [nvarchar](50) NULL,
    [NameStyle] [bit] NOT NULL,
    [Title] [nvarchar](50) NULL,
    [HireDate] [date] NULL,
    [BirthDate] [date] NULL,
    [LoginID] [nvarchar](256) NULL,
    [EmailAddress] [nvarchar](50) NULL,
    [Phone] [nvarchar](25) NULL,
    [MaritalStatus] [nchar](1) NULL,
    [EmergencyContactName] [nvarchar](50) NULL,
    [EmergencyContactPhone] [nvarchar](25) NULL,
    [SalariedFlag] [bit] NULL,
    [Gender] [nchar](1) NULL,
    [PayFrequency] [tinyint] NULL,
    [BaseRate] [money] NULL,
    [VacationHours] [smallint] NULL,
    [SickLeaveHours] [smallint] NULL,
    [CurrentFlag] [bit] NOT NULL,
    [SalesPersonFlag] [bit] NOT NULL,
    [DepartmentName] [nvarchar](50) NULL,
    [StartDate] [date] NULL,
    [EndDate] [date] NULL,
    [Status] [nvarchar](50) NULL,
 CONSTRAINT [PK_DimEmployee_EmployeeKey] PRIMARY KEY CLUSTERED 
(
    [EmployeeKey] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

ALTER TABLE [dbo].[DimEmployee]  WITH CHECK ADD  CONSTRAINT [FK_DimEmployee_DimEmployee] FOREIGN KEY([ParentEmployeeKey])
REFERENCES [dbo].[DimEmployee] ([EmployeeKey])
GO

ALTER TABLE [dbo].[DimEmployee] CHECK CONSTRAINT [FK_DimEmployee_DimEmployee]
GO

ALTER TABLE [dbo].[DimEmployee]  WITH CHECK ADD  CONSTRAINT [FK_DimEmployee_DimSalesTerritory] FOREIGN KEY([SalesTerritoryKey])
REFERENCES [dbo].[DimSalesTerritory] ([SalesTerritoryKey])
GO

ALTER TABLE [dbo].[DimEmployee] CHECK CONSTRAINT [FK_DimEmployee_DimSalesTerritory]
GO

And the second table:

USE [AdventureWorksDW2008]
GO

/****** Object:  Table [dbo].[FactResellerSales]    Script Date: 11/22/2010 20:30:47 ******/
SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

CREATE TABLE [dbo].[FactResellerSales](
    [ProductKey] [int] NOT NULL,
    [OrderDateKey] [int] NOT NULL,
    [DueDateKey] [int] NOT NULL,
    [ShipDateKey] [int] NOT NULL,
    [ResellerKey] [int] NOT NULL,
    [EmployeeKey] [int] NOT NULL,
    [PromotionKey] [int] NOT NULL,
    [CurrencyKey] [int] NOT NULL,
    [SalesTerritoryKey] [int] NOT NULL,
    [SalesOrderNumber] [nvarchar](20) NOT NULL,
    [SalesOrderLineNumber] [tinyint] NOT NULL,
    [RevisionNumber] [tinyint] NULL,
    [OrderQuantity] [smallint] NULL,
    [UnitPrice] [money] NULL,
    [ExtendedAmount] [money] NULL,
    [UnitPriceDiscountPct] [float] NULL,
    [DiscountAmount] [float] NULL,
    [ProductStandardCost] [money] NULL,
    [TotalProductCost] [money] NULL,
    [SalesAmount] [money] NULL,
    [TaxAmt] [money] NULL,
    [Freight] [money] NULL,
    [CarrierTrackingNumber] [nvarchar](25) NULL,
    [CustomerPONumber] [nvarchar](25) NULL,
 CONSTRAINT [PK_FactResellerSales_SalesOrderNumber_SalesOrderLineNumber] PRIMARY KEY CLUSTERED 
(
    [SalesOrderNumber] ASC,
    [SalesOrderLineNumber] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

ALTER TABLE [dbo].[FactResellerSales]  WITH CHECK ADD  CONSTRAINT [FK_FactResellerSales_DimCurrency] FOREIGN KEY([CurrencyKey])
REFERENCES [dbo].[DimCurrency] ([CurrencyKey])
GO

ALTER TABLE [dbo].[FactResellerSales] CHECK CONSTRAINT [FK_FactResellerSales_DimCurrency]
GO

ALTER TABLE [dbo].[FactResellerSales]  WITH CHECK ADD  CONSTRAINT [FK_FactResellerSales_DimDate] FOREIGN KEY([OrderDateKey])
REFERENCES [dbo].[DimDate] ([DateKey])
GO

ALTER TABLE [dbo].[FactResellerSales] CHECK CONSTRAINT [FK_FactResellerSales_DimDate]
GO

ALTER TABLE [dbo].[FactResellerSales]  WITH CHECK ADD  CONSTRAINT [FK_FactResellerSales_DimDate1] FOREIGN KEY([DueDateKey])
REFERENCES [dbo].[DimDate] ([DateKey])
GO

ALTER TABLE [dbo].[FactResellerSales] CHECK CONSTRAINT [FK_FactResellerSales_DimDate1]
GO

ALTER TABLE [dbo].[FactResellerSales]  WITH CHECK ADD  CONSTRAINT [FK_FactResellerSales_DimDate2] FOREIGN KEY([ShipDateKey])
REFERENCES [dbo].[DimDate] ([DateKey])
GO

ALTER TABLE [dbo].[FactResellerSales] CHECK CONSTRAINT [FK_FactResellerSales_DimDate2]
GO

ALTER TABLE [dbo].[FactResellerSales]  WITH CHECK ADD  CONSTRAINT [FK_FactResellerSales_DimEmployee] FOREIGN KEY([EmployeeKey])
REFERENCES [dbo].[DimEmployee] ([EmployeeKey])
GO

ALTER TABLE [dbo].[FactResellerSales] CHECK CONSTRAINT [FK_FactResellerSales_DimEmployee]
GO

ALTER TABLE [dbo].[FactResellerSales]  WITH CHECK ADD  CONSTRAINT [FK_FactResellerSales_DimProduct] FOREIGN KEY([ProductKey])
REFERENCES [dbo].[DimProduct] ([ProductKey])
GO

ALTER TABLE [dbo].[FactResellerSales] CHECK CONSTRAINT [FK_FactResellerSales_DimProduct]
GO

ALTER TABLE [dbo].[FactResellerSales]  WITH CHECK ADD  CONSTRAINT [FK_FactResellerSales_DimPromotion] FOREIGN KEY([PromotionKey])
REFERENCES [dbo].[DimPromotion] ([PromotionKey])
GO

ALTER TABLE [dbo].[FactResellerSales] CHECK CONSTRAINT [FK_FactResellerSales_DimPromotion]
GO

ALTER TABLE [dbo].[FactResellerSales]  WITH CHECK ADD  CONSTRAINT [FK_FactResellerSales_DimReseller] FOREIGN KEY([ResellerKey])
REFERENCES [dbo].[DimReseller] ([ResellerKey])
GO

ALTER TABLE [dbo].[FactResellerSales] CHECK CONSTRAINT [FK_FactResellerSales_DimReseller]
GO

ALTER TABLE [dbo].[FactResellerSales]  WITH CHECK ADD  CONSTRAINT [FK_FactResellerSales_DimSalesTerritory] FOREIGN KEY([SalesTerritoryKey])
REFERENCES [dbo].[DimSalesTerritory] ([SalesTerritoryKey])
GO

ALTER TABLE [dbo].[FactResellerSales] CHECK CONSTRAINT [FK_FactResellerSales_DimSalesTerritory]
GO

This question is using a very specific example, but I'm also interested in general information about when/if it would be appropriate to use EXCEPT instead of a LEFT JOIN and check on IS NULL.

I also noticed, that if I tried running the 1st query with INTERCEPT, the equivalent in the 2nd query would be to use a standard JOIN and SELECT DISTINCT DimEmployee.EmployeeKey (and no WHERE clause at all). In this case however, the execution plan was exactly the same in the two cases.

Update
Minor update to the 2nd query (see revision history), which resulted in a bit more complex query plan for that one. I would guess that a bigger query plan indicates a less optimal query, but I'd still like this question to be answered.

Answer 1

The 2 queries will be different in cases where the LEFT JOIN gives multiple rows. That is, FactResellerSales is a child of DimEmployee with many rows per rows in DimEmployee. So you need DISTINCT as you noted for your JOIN example.

If you change the query to use NOT EXISTS you get the same plan (left anti semi join is typical for NOT EXISTS)

SELECT EmployeeKey
FROM DimEmployee DE 
WHERE
NOT EXISTS (SELECT * FROM 
        FactResellerSales FRS
    WHERE FRS.EmployeeKey = DE.EmployeeKey)

In addition and for the same reason, INTERSECT/EXISTS will most likely give the same plan.

It's another aspect to the JOIN/EXISTS/IN or OUTER JOIN/NOT EXISTS/NOT IN debates. INTERSECT/EXCEPT is a slightly more elegant construct of (NOT) EXISTS is you like

Edit:

There isn't an obvious question...

Personally, I don't use OUTER JOIN for "existence" checking: I use EXISTS or NOT EXISTS (or INTERSECT/EXCEPT if I remember) because it's more obvious what you're trying to do. AKA, if I don't need rows from the "outer" table, I don't use it to avoid having a DISTINCT.

There is no CASE to use OUTER JOIN/IS NULL IMHO if in this case. Of course, I use OUTER JOIN when needed: this answer is for one specific case only.