Here is another option:

SELECT

Audit_ID,

EntID,

Field_A,

Field_B,

ActionDate,

rn3 = ROW_NUMBER() OVER (PARTITION BY EntID ORDER BY ActionDate) - ROW_NUMBER() OVER (PARTITION BY EntID, Field_B ORDER BY ActionDate)

INTO #test

FROM

(

SELECT 1 as Audit_ID, 123 as EntID, 100 as Field_A, 10 as Field_B, CAST('01-Jan-12' AS DATETIME) as ActionDate UNION

SELECT 2 as Audit_ID, 123 as EntID, 110 as Field_A, 10 as Field_B, CAST('15-Feb-12' AS DATETIME) as ActionDate UNION

SELECT 3 as Audit_ID, 123 as EntID, 150 as Field_A, 15 as Field_B, CAST('20-Mar-12' AS DATETIME) as ActionDate UNION

SELECT 4 as Audit_ID, 123 as EntID, 100 as Field_A, 10 as Field_B, CAST('12-Jun-12' AS DATETIME) as ActionDate UNION

SELECT 5 as Audit_ID, 445 as EntID, 50 as Field_A, 4 as Field_B, CAST('02-Apr-12' AS DATETIME) as ActionDate UNION

SELECT 6 as Audit_ID, 445 as EntID, 60 as Field_A, 3 as Field_B, CAST('15-May-12' AS DATETIME) as ActionDate UNION

SELECT 7 as Audit_ID, 445 as EntID, 30 as Field_A, 3 as Field_B, CAST('18-Aug-12' AS DATETIME) as ActionDate UNION

SELECT 8 as Audit_ID, 552 as EntID, 500 as Field_A, 1 as Field_B, CAST('10-Jan-12' AS DATETIME) as ActionDate UNION

SELECT 9 as Audit_ID, 552 as EntID, 450 as Field_A, 3 as Field_B, CAST('15-Mar-12' AS DATETIME) as ActionDate UNION

SELECT 10 as Audit_ID, 552 as EntID, 320 as Field_A, 4 as Field_B, CAST('29-Aug-12' AS DATETIME) as ActionDate

)x

GO

WITH BaseData AS (

SELECT

Audit_ID,

EntID,

Field_A,

Field_B,

ActionDate,

rn3 = ROW_NUMBER() OVER (PARTITION BY EntID ORDER BY ActionDate) - ROW_NUMBER() OVER (PARTITION BY EntID, Field_B ORDER BY ActionDate)

FROM

#test

), IntermediateData AS (

SELECT

*,

rn = ROW_NUMBER() OVER (PARTITION BY EntID, rn3 ORDER BY ActionDate)

FROM

BaseData

)

SELECT

*

FROM

IntermediateData

WHERE

rn = 1

ORDER BY

EntID,

ActionDate;

GO

DROP TABLE #test;

GO