我有一个下表，其中包含id和ts的组合主键以实现历史记录：
create table "author" (
"id" bigint not null,
"ts" timestamp not null default now(),
"login" text unique not null,
primary key ("id", "ts")
);
现在，我只对最新的login值感兴趣。因此，我按id分组：
select "id", max("ts"), "login" from "author" group by "id";
但这会引发错误：聚合函数中应使用login。
id和max("ts")唯一地标识一行，因为连音符（id，ts）是主键。我需要login与id和max("ts")所标识的行匹配。
我可以写一个子选择来找到login：
select ao."id", max(ao."ts"),
(select ai.login from "author" ai
where ao."id" = ai."id" and max(ao."ts") = ai.ts)
from "author" ao
group by "id";
这是可行的，但是它很嘈杂并且不是很聪明，因为它可以搜索整个表，尽管搜索组就足够了。
是否存在一个聚合函数，该函数避免了子选择，而是给了我剩下的login，它们属于id和max("ts")？
答案 0 :(得分：1)
您必须标识正确的密钥才能从表中获取所需的值。
正确的密钥是：
##main loop
man = player (100,430,100,100)
bullet = bullets (50,50)
game = True
idle = [(py.transform.scale(idle[0],(man.width,man.lenght))),(py.transform.scale(idle[1],(man.width,man.lenght))),(py.transform.scale(idle[2],(man.width,man.lenght))),(py.transform.scale(idle[3],(man.width,man.lenght))),(py.transform.scale(idle[4],(man.width,man.lenght))),(py.transform.scale(idle[5],(man.width,man.lenght))),(py.transform.scale(idle[6],(man.width,man.lenght))),(py.transform.scale(idle[7],(man.width,man.lenght))),(py.transform.scale(idle[8],(man.width,man.lenght))),(py.transform.scale(idle[9],(man.width,man.lenght)))]
run_right = [(py.transform.scale(run_right[0],(man.width,man.lenght))),(py.transform.scale(run_right[1],(man.width,man.lenght))),(py.transform.scale(run_right[2],(man.width,man.lenght))),(py.transform.scale(run_right[3],(man.width,man.lenght))),(py.transform.scale(run_right[4],(man.width,man.lenght))),(py.transform.scale(run_right[5],(man.width,man.lenght))),(py.transform.scale(run_right[6],(man.width,man.lenght))),(py.transform.scale(run_right[7],(man.width,man.lenght)))]
jump = [(py.transform.scale(jump[0],(man.width,man.lenght))),(py.transform.scale(jump[1],(man.width,man.lenght))),(py.transform.scale(jump[2],(man.width,man.lenght))),(py.transform.scale(jump[3],(man.width,man.lenght))),(py.transform.scale(jump[4],(man.width,man.lenght))),(py.transform.scale(jump[5],(man.width,man.lenght))),(py.transform.scale(jump[6],(man.width,man.lenght))),(py.transform.scale(jump[7],(man.width,man.lenght))),(py.transform.scale(jump[8],(man.width,man.lenght))),(py.transform.scale(jump[9],(man.width,man.lenght)))]
run_left = [(py.transform.flip(run_right[0],True,False)),(py.transform.flip(run_right[1],True,False)),(py.transform.flip(run_right[2],True,False)),(py.transform.flip(run_right[3],True,False)),(py.transform.flip(run_right[4],True,False)),(py.transform.flip(run_right[5],True,False)),(py.transform.flip(run_right[6],True,False)),(py.transform.flip(run_right[7],True,False))]
idle2 = [(py.transform.flip(idle[0],True,False)),(py.transform.flip(idle[1],True,False)),(py.transform.flip(idle[2],True,False)),(py.transform.flip(idle[3],True,False)),(py.transform.flip(idle[4],True,False)),(py.transform.flip(idle[5],True,False)),(py.transform.flip(idle[6],True,False)),(py.transform.flip(idle[7],True,False)),(py.transform.flip(idle[8],True,False)),(py.transform.flip(idle[9],True,False))]
jump2 = [(py.transform.flip(jump[0],True,False)),(py.transform.flip(jump[1],True,False)),(py.transform.flip(jump[2],True,False)),(py.transform.flip(jump[3],True,False)),(py.transform.flip(jump[4],True,False)),(py.transform.flip(jump[5],True,False)),(py.transform.flip(jump[6],True,False)),(py.transform.flip(jump[7],True,False)),(py.transform.flip(jump[8],True,False)),(py.transform.flip(jump[9],True,False))]
shoot_idle = [(py.transform.scale(shoot_idle[0],(man.width,man.lenght))),(py.transform.scale(shoot_idle[1],(man.width,man.lenght))),(py.transform.scale(shoot_idle[2],(man.width,man.lenght))),(py.transform.scale(shoot_idle[3],(man.width,man.lenght)))]
shoot_idle2 = [(py.transform.flip(shoot_idle[0],True,False)),(py.transform.flip(shoot_idle[1],True,False)),(py.transform.flip(shoot_idle[2],True,False)),(py.transform.flip(shoot_idle[3],True,False))]
pows = [(py.transform.scale(pows[0],(bullet.width,bullet.lenght))),(py.transform.scale(pows[1],(bullet.width,bullet.lenght))),(py.transform.scale(pows[2],(bullet.width,bullet.lenght))),(py.transform.scale(pows[3],(bullet.width,bullet.lenght))),(py.transform.scale(pows[4],(bullet.width,bullet.lenght)))]
pows2 = [(py.transform.flip(pows[0],True,False)),(py.transform.flip(pows[1],True,False)),(py.transform.flip(pows[2],True,False)),(py.transform.flip(pows[3],True,False)),(py.transform.flip(pows[4],True,False))]
while game:
clock.tick (30)
for event in py.event.get():
if event == py.QUIT:
game = False
keys = py.key.get_pressed ()
if keys[py.K_RIGHT] and man.x <= 700:
man.x += man.vel
man.right = True
man.left = False
man.standing = False
man.idlecount = 0
man.direction = 1
elif keys[py.K_LEFT] and man.x >= 0:
man.x -= man.vel
man.right = False
man.left = True
man.standing = False
man.idlecount = 0
man.direction = -1
elif keys [py.K_SPACE]:
if man.direction == 1:
bullet.x = man.x + 75
bullet.y = man.y + 20
man.shooting = True
bullet.shoot = True
elif man.direction == -1:
bullet.x = man.x - 25
bullet.y = man.y + 20
man.shooting = True
bullet.shoot = True
else:
man.standing = True
man.shooting = False
if not(man.jumping):
if keys[py.K_UP]:
man.jumping = True
man.right = False
man.left = False
man.standing = False
man.walkcount = 0
else:
if man.jumpcount >= -14:
neg = 1
if man.jumpcount < 0:
neg = -1
man.y -= (man.jumpcount ** 2) * 0.2 * neg
man.jumpcount -= 1
else:
man.jumping = False
man.jumpcount = 14
drawGameScreen ()
并使用它来获取所需的登录名：
select "id", max("ts") from "author" group by "id";
或者使用窗口函数：
select a1."id", a1.ts, a1.login
from "author" a1
inner join (select "id", max("ts") maxts, "login" from "author" group by "id") a2
ON a1.id = a2.id AND a1.ts = a2.maxts;
还有其他几种方法可以给这只猫剥皮，但这基本上就是要点。