如何在SQL中的一个字段中选择无重复的记录？

Question

我有一个包含3列的表,如下所示:

+------------+---------------+-------+  
| Country_id | country_title | State |  
+------------+---------------+-------+    

此表中有许多记录.他们中的一些state人和其他人没有.现在,想象一下这些记录:

1 | Canada  | Alberta  
2 |  Canada | British  Columbia  
3 | Canada  | Manitoba  
4 | China   |

我需要没有任何重复的国名.其实我需要他们id和title,什么是最好的SQL命令,使这个？我DISTINCT在下面的表格中使用但是我无法获得适当的结果.

SELECT DISTINCT title,id FROM tbl_countries ORDER BY title

我想要的结果是这样的:

1, Canada  
4, China

Answer 1

试试这个:

SELECT MIN(id) AS id, title
FROM tbl_countries
GROUP BY title

Answer 2

DISTINCT是关键字
对我来说你的查询是正确的

试着先尝试这样做

SELECT DISTINCT title,id FROM tbl_countries

稍后您可以尝试订购.

Answer 3

对于使用DISTINCT关键字,您可以像这样使用它:

SELECT DISTINCT 
    (SELECT min(ti.Country_id) 
     FROM tbl_countries ti 
     WHERE t.country_title = ti.country_title) As Country_id
    , country_title
FROM 
    tbl_countries t

对于使用ROW_NUMBER(),您可以像这样使用它:

SELECT 
    Country_id, country_title 
FROM (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY country_title ORDER BY Country_id) As rn
    FROM tbl_countries) t
WHERE rn = 1

使用时LEFT JOIN,您可以使用:

SELECT t1.Country_id, t1.country_title
FROM tbl_countries t1
    LEFT OUTER JOIN
    tbl_countries t2 ON t1.country_title = t2.country_title AND t1.Country_id > t2.Country_id
WHERE
    t2.country_title IS NULL

使用EXISTS,您可以尝试:

SELECT t1.Country_id, t1.country_title
FROM tbl_countries t1   
WHERE
    NOT EXISTS (SELECT * 
                FROM tbl_countries t2 
                WHERE t1.country_title = t2.country_title AND t1.Country_id > t2.Country_id)