`--AVG used only in Select correctly does the average calculation from the detail (child) table SELECT sd.customer, AVG(score) AS avg_score_group from score_detail as sd
        JOIN score_master as sm ON sd.customer = sm.customer
        GROUP BY sd.customer
    --Using AVG to actually update the master table with averages from child
    table has incorrect calculations for some grouped records
        UPDATE score_master
        SET avg_score = t.avg_score_group
        FROM (
        SELECT AVG(score) AS avg_score_group from score_detail as sd
        JOIN
        score_master as sm
        ON
        sd.customer = sm.customer
        GROUP BY sd.customer
        ) AS t
    --Let us explore master table
        SELECT * FROM  score_master;
--Table structures:
CREATE TABLE [dbo].[score_detail](
[customer] [float] NULL,
[score] [float] NULL
) ON [PRIMARY]


CREATE TABLE [dbo].[score_master](
[customer] [float] NULL,
[avg_score] [float] NULL
) ON [PRIMARY]
-- Data to calculate average has 17 Decimal points:
INSERT [dbo].[score_detail] ([customer], [score]) VALUES (2, -0.07216878364870323)`