PostgreSQL的，从最大ID

Question

使用选择的libpq在PG 9.1，我想写查询从排得值与指数最高的“添加my_id”：

SELECT my_id, col2, col3 
FROM mytable 
WHERE my_id = MAX(my_id) 

这给了我错误：

ERROR: aggregates not allowed in WHERE clause...

如何正确编写这样的查询？

Answer 1

如果你的目标是获得最高添加my_id值的行，那么下面的查询应达到同样的目的。

SELECT my_id, col2, col3 
FROM mytable 
ORDER BY my_id DESC 
LIMIT 1 

Answer 2

SELECT my_id, col2, col3 FROM mytable WHERE my_id = (select MAX(my_id) FROM mytab) 

或使用

SELECT my_id, col2, col3 FROM mytable ORDER BY my_id DESC LIMIT 1 

Answer 3

只是为了通过my_id，并采取只limit 1

SELECT my_id, col2, col3 
FROM mytable 
order by my_id desc 
limit 1 

的第一条记录的另一个但不太高性能的办法是

SELECT my_id, col2, col3 
FROM mytable 
where my_id = (select max(my_id) from mytable) 

Answer 4

子查询可以帮助你

SELECT my_id, col2, col3 FROM mytable WHERE my_id = (select MAX(my_id) FROM mytable) 

Answer 5

当你在my_id上有索引时，子查询中的索引应该更快。当你没有一个索引采取“顺序”。 （如果相关，取决于数据库大小）