MySQL 8.0

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select version();
CREATE TABLE TableName(Prob DOUBLE);
INSERT INTO TableName ( Prob ) VALUES ( RAND());
INSERT INTO TableName ( Prob ) SELECT IF(RAND()<0.1,NULL,RAND()) FROM TableName;
INSERT INTO TableName ( Prob ) SELECT IF(RAND()<0.1,NULL,RAND()) FROM TableName;
INSERT INTO TableName ( Prob ) SELECT IF(RAND()<0.1,NULL,RAND()) FROM TableName;
INSERT INTO TableName ( Prob ) SELECT IF(RAND()<0.1,NULL,RAND()) FROM TableName;
INSERT INTO TableName ( Prob ) SELECT IF(RAND()<0.1,NULL,RAND()) FROM TableName;
INSERT INTO TableName ( Prob ) SELECT IF(RAND()<0.1,NULL,RAND()) FROM TableName;
INSERT INTO TableName ( Prob ) SELECT IF(RAND()<0.1,NULL,RAND()) FROM TableName;
INSERT INTO TableName ( Prob ) SELECT IF(RAND()<0.1,NULL,RAND()) FROM TableName;
INSERT INTO TableName ( Prob ) SELECT IF(RAND()<0.1,NULL,RAND()) FROM TableName;
INSERT INTO TableName ( Prob ) SELECT IF(RAND()<0.1,NULL,RAND()) FROM TableName;
SELECT TRUNCATE(table_s.Prob,1) AS p,COUNT(*) AS c
FROM TableName table_s
GROUP BY TRUNCATE(table_s.Prob,1)
WITH ROLLUP;
SELECT TRUNCATE(table_s.Prob,1) AS p,COUNT(*) AS c
FROM TableName table_s
GROUP BY TRUNCATE(table_s.Prob,1);

version()
8.0.22

p	c
null	91
0	94
0.	99
0.	92
0.	101
0.	88
0.	89
1	82
1	97
1	92
1	99
null	1024

p	c
0.1	99
0.2	92
0	94
0.6	82
0.5	89
0.7	97
0.8	92
0.3	101
null	91
0.4	88
0.9	99