AVG??????

AVG????????????????????????????????????????????????????????????????????????(?????? (????????????)??????????????????????????????) AVG??????????????????????????????????????????????????????????????????????????????????????????

??????

AVG ( [ DISTINCT | ALL ] ??? )

DISTICT?????????????????????????????????????????????????????? ALL?????????????????????????????????????????????ALL???DISTINCT???????????????????????????????????????ALL????????? ????????????????????????1.0???1.0???1.0???1.0???2.0???????????????????????????????????????AVG(col)???AVG(DISTINCT col)????????????????????????????????????

SELECT AVG (DISTINCT flying_time), SUM (DISTINCT miles)
FROM Flights

???????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????? ?????????????????????SQL-92????????????????????????????????????????????????????????????SQL-92???????????????????????????????????????????????????????????????????????????????????????NULL???????????????????????????????????????????????????????????????

SELECT AVG(c1)
FROM (VALUES (1), (1), (1), (1), (2)) AS myTable (c1)

SELECT AVG(CAST (c1 AS DOUBLE PRECISION))
FROM (VALUES (1), (1), (1), (1), (2)) AS myTable (c1)