Month End Join Query, Sql Server

Language: Layout:

CREATE TABLE #MetricFirstGroupReading (Patient_Key INT, Metric_Group VARCHAR(100), 
                                       Observation_Date DATE)
ALTER TABLE #MetricFirstGroupReading 
ADD CONSTRAINT UQ_MetricFirst UNIQUE (Patient_Key, Metric_Group);

INSERT INTO #MetricFirstGroupReading
SELECT 1, 'Albumin', '2018-11-15' UNION
SELECT 1, 'Potassium', '2018-12-10' UNION
SELECT 2, 'Albumin', '2018-10-20' UNION
SELECT 2, 'Potassium', '2018-11-25'

CREATE TABLE #MonthEndDates (Month_End_Value DATE)
                                       
INSERT INTO #MonthEndDates
VALUES ('2018-01-31'),
       ('2018-02-28'),
       ('2018-03-31'),
       ('2018-04-30'),
       ('2018-05-31'),
       ('2018-04-30'),
       ('2018-06-30'),
       ('2018-07-31'),
       ('2018-08-31'),
       ('2018-09-30'),
       ('2018-10-31'),
       ('2018-11-30'),
       ('2018-12-31')

SELECT m.Patient_Key, m.Metric_Group, m.Observation_Date, 
       DateDiff(month,  m.Observation_Date, d.Month_End_Value) AS Month_Diff, 
       d.Month_End_Value
        
FROM #MetricFirstGroupReading m
INNER JOIN #MonthEndDates d
  ON m.Observation_Date < d.Month_End_Value

View schema
Execution time: 0,03 sec, rows selected: 8, rows affected: 17, absolute service time: 0,27 sec, absolute service time: 0,26 sec