CREATE TABLE #Letter
(
Letter char(1) PRIMARY KEY NOT NULL,
Number int NOT NULL
)

INSERT #Letter
VALUES ('A', 1)

INSERT #Letter
VALUES ('B', 2)

INSERT #Letter
VALUES ('C', 3)

CREATE FUNCTION dbo.GetFirstLetter(@text varchar(100)) RETURNS CHAR(1)
AS
BEGIN
RETURN LEFT(@text, 1)
END

-- Option 1
DECLARE @firstLetter char(1)
SET @firstLetter = dbo.GetFirstLetter('Apple')
SELECT * FROM #Letter WHERE Letter = @firstLetter

-- Option 2
SELECT * FROM #Letter WHERE Letter = dbo.GetFirstLetter('Apple')

-- Option 3
SELECT * FROM #Letter WHERE dbo.GetFirstLetter(Letter)  = 'A'