Common Table Expressions

This feature was introduced on SQL Server 2005. It’s a great way to query another query on the fly. I prefer using these over derived tables (DTs) because it provides more flexibility. Some people report better performance using Common Table Expressions (CTEs). I’ve seen and heard both though (that DTs are faster), but I suppose it depends. Just test it out and see for yourself.

Anywhoot, let’s play with CTEs. First let’s create two tables with dummy data.

CREATE TABLE Records 
(
  RecordID INT IDENTITY(1, 1) PRIMARY KEY,
  RandomData VARCHAR(100)
)

DECLARE @t INT 
SET @t = 0
WHILE @t < 1000
BEGIN
  SET @t = @t + 1
  INSERT INTO Records VALUES( NEWID() ) 
END


CREATE TABLE Information 
(
  RecordID INT IDENTITY(1, 1) PRIMARY KEY,
  RandomData VARCHAR(100)
)
DECLARE @t INT 
SET @t = 0
WHILE @t < 1000
BEGIN
  SET @t = @t + 1
  INSERT INTO Information VALUES( NEWID() ) 
END

Now that we’ve create the dummy tables, here’s a barebones example of a CTE:

WITH Slice1 AS 
(
  -- The results for this query gets put into Slice1
  -- It persists for the life of this query.
  SELECT * FROM Records
  WHERE RecordID BETWEEN 5 AND 400

) -- Done creating a virtual table called Slice1, now let's 
  -- query it:
  SELECT * FROM Slice1
  WHERE RecordID > 300

CTE’s real power comes when you create multiple virtual tables then finally query them, joining any virtual table you created:

-- This whole thing is 1 query:
WITH    Slice1
          AS ( SELECT   RecordID,
                        RandomData
               FROM     Records
             ) , -- done creating Table Slice1
             
        Slice2
          AS ( SELECT   RecordID
               FROM     Records
               WHERE    RecordID BETWEEN 10 AND 20
             ) , -- done creating Table Slice2
             
        Info
          AS ( SELECT   RecordID
               FROM     Information
               WHERE    RecordID IN ( 5, 6, 7, 9, 15, 18 )
             ) -- done creating Table Info
             
  -- Now that we've created all these virtual tables, let's use them together in
  -- one single query:             
    SELECT  RecordID,
            RandomData
    FROM    Slice1
    WHERE   Slice1.RecordID IN ( SELECT *
                                 FROM   Info
                                 WHERE  RecordID IN ( SELECT    RecordID
                                                      FROM      Slice2 ) )