MySQL-如何选择对应列值的第一个重复行 [英] MySQL-How to select first duplicate row of corresponding column value
问题描述
我有一个包含三列(A,B,C)的表.所有列的数据类型均为varchar().在这里,我只选择那些初始行,它们是A列的第一个重复值.像A列具有不同的值(1,2,3),因此初始行在列中的值为1需要检索.还有一件事,我的数据集中没有时间戳列.
I have a table that contains three columns(A,B,C). All columns are of datatype varchar().Here i want to select only those initial rows which is first duplicate value of column A. Like column A has distinct value (1,2,3) so the initial row that has value 1 in column A need to be retrieved.one more thing i don't have a timestamp column in my dataset.
For value 1 in column A, i need to retrieve row that has 1,2,1 value for column (A,B,C)
For value 2 in column A, i need to retrieve row that has 2,4,1 value for column (A,B,C)
For value 3 in column A, i need to retrieve row that has 3,1,1 value for column (A,B,C)
A|B|C
-----
1|2|1
3|1|2
3|3|1
2|4|1
3|1|2
3|1|4
1|2|3
Now i want a Output Table as describe below.
A|B|C
-----
1|2|1
3|1|2
2|4|1
推荐答案
请考虑以下内容.为简单起见,我使用的是auto_incrementing id,但是唯一的时间戳也可以正常工作...
Consider the following. For simplicity, I'm using an auto_incrementing id but a unique timestamp would work just as well...
DROP TABLE IF EXISTS my_table;
CREATE TABLE my_table
(id INT NOT NULL AUTO_INCREMENT PRIMARY KEY
,A INT NOT NULL
,B INT NOT NULL
,C INT NOT NULL
,UNIQUE(a,b,c)
);
INSERT INTO my_table (a,b,c) VALUES
(1 ,2 ,1),
(1 ,1 ,2),
(2 ,3 ,1),
(2 ,4 ,1),
(3 ,1 ,2),
(3 ,1 ,4),
(3 ,2 ,3);
SELECT x.* FROM my_table x ORDER BY id;
+----+---+---+---+
| id | A | B | C |
+----+---+---+---+
| 1 | 1 | 2 | 1 |
| 2 | 1 | 1 | 2 |
| 3 | 2 | 3 | 1 |
| 4 | 2 | 4 | 1 |
| 5 | 3 | 1 | 2 |
| 6 | 3 | 1 | 4 |
| 7 | 3 | 2 | 3 |
+----+---+---+---+
SELECT x.*
FROM my_table x
JOIN
( SELECT a
, MIN(id) min_id
FROM my_table
GROUP
BY a
) y
ON y.a = x.a
AND y.min_id = x.id
ORDER
BY id;
+----+---+---+---+
| id | A | B | C |
+----+---+---+---+
| 1 | 1 | 2 | 1 |
| 3 | 2 | 3 | 1 |
| 5 | 3 | 1 | 2 |
+----+---+---+---+
有关此部分的更多信息,请参见手册( http://dev.mysql.com/doc/refman/5.1/en/example-maximum-column-group-row.html ).
For more information on this soution, see the manual (http://dev.mysql.com/doc/refman/5.1/en/example-maximum-column-group-row.html).
这篇关于MySQL-如何选择对应列值的第一个重复行的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!