IF EXISTS (SELECT 1 FROM SYSOBJECTS WHERE name = 'A')
BEGIN
    DROP TABLE A
END
GO
CREATE TABLE A
(    A_ID INT,
    A_数值 INT,
    A_条件 VARCHAR(100)
)
GO
IF EXISTS (SELECT 1 FROM SYSOBJECTS WHERE name = 'B')
BEGIN
    DROP TABLE B
END
GO 
CREATE TABLE B
(
    B_ID INT,
    B_A权值 INT,
    A_ID INT,
    B_条件 VARCHAR(100)
)
GO

INSERT INTO A
SELECT 1,4,'haha' UNION
SELECT 2,4,'hihi' UNION
SELECT 3,2,'hoho'

INSERT INTO B
SELECT 1,40,1,'haha' UNION
SELECT 2,60,2,'hihi' UNION
SELECT 3,20,3,'hoho'


SELECT SUM(B_A权值 * A_数值)/SUM(A_数值) AS Avg
FROM A , B
WHERE A.A_ID = B.A_ID

Avg
44