Question

通过在PG 9.1上使用libpq，我试图编写查询以从具有最高索引'my_id'的行获取值：

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

这给了我错误：

错误：WHERE子句中不允许聚合...

如何正确编写此类查询？

Answer 1

如果您的目标是获取具有最高my_id值的行，则以下查询应实现相同的目标。

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

Answer 2

只需按my_id排序，只使用limit 1

的第一条记录

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

另一种性能较差的方式是

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

Answer 3

子查询可以帮助您

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

Answer 4

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 5

当my_id上有索引时，带有子查询的索引应该更快。当你没有索引时，请选择“order by”。（如果相关，则obv。取决于数据库大小）