SQL获取具有相同列A但不同B的记录

Question

假设我们有下表称为meals ：

| meal  | stars |
-----------------
| steak |   1   |
| steak |   2   |
| fish  |   4   |
| fish  |   4   |
| salad |   5   |

如何获得同一顿饭但不同星星的记录？ 我需要只有不同星星的记录。

上表的结果应如下：

| meal  | stars |
-----------------
| steak |   1   |
| steak |   2   |

我尝试过以下查询：

SELECT DISTINCT t1.*
FROM meals t1
INNER JOIN meals t2 ON t1.meal = t2.meal
AND t1.stars <> t2.stars;

但它耗费了太多时间和一些明显的内存。

我桌子的实际大小是：

SELECT pg_size_pretty(pg_relation_size('table_name')); 
 pg_size_pretty 
----------------
 2295 MB

所以我需要拿出别的东西，我正在寻求你的帮助！

Answer 1

SELECT  a.*
FROM    meals a
        INNER JOIN
        (
            SELECT  meal
            FROM    meals
            GROUP   BY meal
            HAVING  COUNT(DISTINCT stars) > 1
        ) b ON a.meal = b.meal

• SQLFiddle演示

OUTPUT

╔═══════╦═══════╗
║ MEAL  ║ STARS ║
╠═══════╬═══════╣
║ steak ║     1 ║
║ steak ║     2 ║
╚═══════╩═══════╝

Answer 2

 SELECT meal,stars FROM meals
 GROUP BY meal,stars
 HAVING count(*)=1 and meal in (
    SELECT meal FROM meals
    GROUP BY meal
    HAVING count(*)>1 )

Answer 3

这应该是最快的方式：

SELECT  *
FROM    meals m
WHERE   EXISTS (SELECT FROM meals WHERE meal = m.meal AND stars <> m.stars);

“获取所有行，其中至少有一餐存在，名称相同，但星星不同。”

假设不发生NULL值。

SQL小提琴。

Answer 4

一种简单而快捷的方法来实现相同的结果

SELECT  
  m.*
FROM    
  meals m
  JOIN meals m2 ON ( m2.meal = m.meal AND m2.stars<>m.stars)