MySQL Join Multiple Rows as Columns
Say I have two tables in a MySQL Database.
Table 1:
ID Name
1 Jim
2 Bob
Table 2:
ID Place Race_Number
1 2nd 1
1 3rd 2
1 4th 3
2 1st 1
2 2nd 2
2 2nd 3
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
.
This outputs:
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
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.
The desired result right from the MySQL query is:
ID Name Race1 Race2 Race3
1 Jim 2nd 3rd 4th
2 Bob 1st 2nd 2nd
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.
Thanks for any help!
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
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;