MySQL 8.0

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CREATE TABLE test ( 1_player INT,
                    2_player INT,
                    3_player INT,
                    id1_TA FLOAT,
                    id2_TA FLOAT,
                    id3_TA FLOAT,
                    avg_TA FLOAT
);

INSERT INTO test VALUES
(2,	16,	30,	85.00000,	100.00000,	100.00000,	100.00000),
(10,	9,	9,	100.00000,	100.00000,	100.00000,	100.00000),
(11,	2,	15,	100.00000,	85.00000,	100.00000,	100.00000);

SELECT * FROM test;


1_player
2_player
3_player
id1_TA
id2_TA
id3_TA
avg_TA


2
16
30
85
100
100
100

10
9
9
100
100
100
100

11
2
15
100
85
100
100


ALTER TABLE test
    MODIFY COLUMN id1_TA DECIMAL(8,5),
    MODIFY COLUMN id2_TA DECIMAL(8,5),
    MODIFY COLUMN id3_TA DECIMAL(8,5),
    DROP COLUMN avg_TA,
    ADD COLUMN avg_TA DECIMAL(8,5) AS ((id1_TA + id2_TA + id3_TA) / 3);
    
SELECT * FROM test;


1_player
2_player
3_player
id1_TA
id2_TA
id3_TA
avg_TA


2
16
30
85.00000
100.00000
100.00000
95.00000

10
9
9
100.00000
100.00000
100.00000
100.00000

11
2
15
100.00000
85.00000
100.00000
95.00000


INSERT INTO test VALUES
(5,	10,	20,	90.00000,	90.00000,	85.00000,	DEFAULT);

SELECT * FROM test;


1_player
2_player
3_player
id1_TA
id2_TA
id3_TA
avg_TA


2
16
30
85.00000
100.00000
100.00000
95.00000

10
9
9
100.00000
100.00000
100.00000
100.00000

11
2
15
100.00000
85.00000
100.00000
95.00000

5
10
20
90.00000
90.00000
85.00000
88.33333

1_player	2_player	3_player	id1_TA	id2_TA	id3_TA	avg_TA
2	16	30	85.00000	100.00000	100.00000	95.00000
10	9	9	100.00000	100.00000	100.00000	100.00000
11	2	15	100.00000	85.00000	100.00000	95.00000