MySQL将多行连接为列 [英] MySQL Join Multiple Rows as Columns

问题描述

说我在MySQL数据库中有两个表.

Say I have two tables in a MySQL Database.

表1:

ID    Name
1     Jim
2     Bob

表2:

ID    Place    Race_Number
1     2nd      1
1     3rd      2
1     4th      3
2     1st      1
2     2nd      2
2     2nd      3

从数据库中选择行时，是否有任何方法可以将第二个表中的行作为列连接到第一个表中?目前，我正在使用SELECT * FROM Table1 NATURAL JOIN Table2.

When selecting rows from the database, is there any way to join rows from the second table as columns to the first table? Currently I am using SELECT * FROM Table1 NATURAL JOIN Table2.

这将输出:

ID   Name    Place    Race_Number
1    Jim     2nd      1
1    Jim     3rd      2
1    Jim     4th      3
2    Bob     1st      1
2    Bob     2nd      2
2    Bob     2nd      3

当前，我正在PHP脚本中对此进行排序，以将其排序为一个数组.这很痛苦，因为我必须查看ID，看看它们是否相同，然后进行相应的排序.我觉得有一种方法可以在MySQL中正确执行此操作，而不必在PHP中将其分类为数组.每个ID在第二张表中可以有无限多个条目.

Currently I am sorting through this in my PHP script to sort it into an array. This is a pain, as I have to look at the IDs and see if they're the same and then sort accordingly. I feel like there is a way to do this right in MySQL, without having to sort it into an array in the PHP. There can be an unlimited number of entries in the second table for each ID.

直接从MySQL查询获得的期望结果是:

The desired result right from the MySQL query is:

ID    Name    Race1    Race2    Race3
1     Jim     2nd      3rd      4th
2     Bob     1st      2nd      2nd

我无法在表格中为Race1，Race2等创建列，因为每个ID可以有无限数量的比赛.

I can't make columns for Race1, Race2 etc in the table themselves because there can be an unlimited number of races for each ID.

感谢您的帮助！

推荐答案

INNER JOIN即可满足您的需求. MySQL没有PIVOT函数，您仍然可以使用CASE和MAX()函数对其进行仿真.

An INNER JOIN will suffice your needs. MySQL has no PIVOT function by you can still simulate it using CASE and MAX() function.

SELECT  a.ID, a.NAME,
        MAX(CASE WHEN b.Race_Number = 1 THEN b.Place ELSE NULL END) Race1,
        MAX(CASE WHEN b.Race_Number = 2 THEN b.Place ELSE NULL END) Race2,
        MAX(CASE WHEN b.Race_Number = 3 THEN b.Place ELSE NULL END) Race3
FROM    Table1 a
        INNER JOIN Table2 b
            ON a.ID = b.ID
GROUP   BY a.ID, a.Name

但是，如果您不知道RACE的数目，那么DYNAMIC SQL则更可取.

But if you have unknown number of RACE, then a DYNAMIC SQL is much more preferred.

SET @sql = NULL;
SELECT
    GROUP_CONCAT(DISTINCT
    CONCAT('MAX(CASE WHEN b.Race_Number = ', Race_Number,
      ' THEN b.Place END) AS ', CONCAT('`Race', Race_Number, '`'))
    ) INTO @sql
FROM Table2;

SET @sql = CONCAT('SELECT s.Student_name, ', @sql, ' 
                   FROM Table1 a
                   LEFT JOIN Table2 b 
                        ON ON a.ID = b.ID
                   GROUP   BY a.ID, a.Name');

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

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