Question

如何仅返回特定列中的条目唯一的SQL表中的行？

例如，

假设我有桌子：

FRUIT   COLOR
_____________

apple   green    
apple   red    
peach   blue
banana  red

我想编写一个PostgreSQL查询，它只返回FRUIT列中的条目唯一的行，在这种情况下意味着返回最后2行。我怎么做？感谢。

Answer 1

这是使用having子句的基本练习。阅读文档：Aggregate Functions并比较这些查询：

select fruit, count(*)
from my_table
group by 1;

 fruit  | count 
--------+-------
 banana |     1
 peach  |     1
 apple  |     2
(3 rows)


select fruit
from my_table
group by 1
having count(*) = 1;

 fruit  
--------
 banana
 peach
(2 rows)

Answer 2

如果你需要整排，

1）使用having和join：

select t.*
from
  my_table as t join (
    select fruit
    from my_table
    group by fruit
    having count(*) = 1) as f on (t.fruit = f.fruit);

2）使用窗口功能：

select *
from (
  select *, count(*) over (partition by fruit) as cnt
  from my_table) as t
where
  cnt = 1;

Answer 3

特定列的典型SQL语法是

 SELECT * FROM (
   SELECT FRUIT, Count(FRUIT) AS NUM_OF_FRUITS FROM MyTable
   GROUP BY City )
 WHERE NUM_OF_FRUITS = 1

给出桃子和香蕉但是如果你想要整行，你可以使用下面的sql语法

SELECT * FROM MyTable 
WHERE FRUIT IN  (
    SELECT * FROM (
        SELECT FRUIT, Count(FRUIT) AS NUM_OF_FRUITS FROM MyTable
        GROUP BY City )
    WHERE NUM_OF_FRUITS = 1 
)

我认为这对你的问题已经足够了。

仅返回SQL表中的行，该表在特定列中的条目是唯一的

3 个答案: