您需要LEFT()和COUNT()和GROUP BY才能获得预期的结果：

请找到示例执行：

CREATE TABLE TestTable (String VARCHAR (100));

INSERT INTO TestTable (String) VALUES
('Apple'),('Ace'),('Benedict'),('Been'),('Broom'),('Car'),('Make'),('Safe'),('Thanksgiving'),('Zappers');

DECLARE @TCount AS INT = 0;
SELECT @TCount = COUNT(*) FROM TestTable;

SELECT LEFT(String,1) AS FChar,CONCAT(COUNT(LEFT(String,1)) *100 / @TCount,'%') AS CharPercent
FROM TestTable 
GROUP BY LEFT(String,1)
ORDER BY LEFT(String,1)

Working demo on db<>fiddle

您可以使用窗口功能。正确的逻辑是：

SELECT LEFT(name,1) AS first_char,COUNT(*) * 100.0 / SUM(COUNT(*)) OVER() AS pct
FROM t
GROUP BY LEFT(name,1)
ORDER BY first_char;