MySQL - Help me construct this query - mysql

Question

I have a relational DB that I can't think of how to form this query.
Here's the info
Table1
id name
1 Mike
Table2
id table_1_id value setting
1 1 something setting1
2 1 something2 setting2
2 1 something3 setting3
Currently, this is my sql query
SELECT * FROM Table1
JOIN Table2 on Table2.table_1_id = Table1.id
What this outputs is something like this
id name table_1_id value setting
1 Mike 1 something1 setting1
1 Mike 1 something2 setting2
1 Mike 1 something3 setting3
Is it possible to construct this in such a way to return these results so I can export it to a CSV file?
id name table_1_id something1 something2 something3
1 Mike 1 setting1 setting2 setting3

Answer 1

SELECT
Table1.*,
something1Table.setting AS something1,
something2Table.setting AS something2,
something3Table.setting AS something3
FROM Table1
JOIN Table2 AS something1Table ON something1Table.table_1_id = Table1.id AND something1Table.value = 'something'
JOIN Table2 AS something2Table ON something2Table.table_1_id = Table1.id AND something2Table.value = 'something2'
JOIN Table2 AS something3Table ON something3Table.table_1_id = Table1.id AND something3Table.value = 'something3'

Answer 2

You need a conditional aggregation:
select table1.id, table1.name,
max(case when value = 'something1' then setting end) as setting1,
max(case when value = 'something2' then setting end) as setting2,
max(case when value = 'something3' then setting end) as setting3
from table1 join
table2
on table1.id = table2.id
group by table1.id, table1.name

Answer 3

This type of data transformation is known an a pivot but MySQL does not have a pivot function. So you will want to replicate it using an aggregate function with a CASE expression.
If you know the the number of values ahead of time, then you can hard-code your query similar to this:
select t1.id,
t1.name,
max(case when t2.value = 'something' then t2.setting end) as setting1,
max(case when t2.value = 'something2' then t2.setting end) as setting2,
max(case when t2.value = 'something3' then t2.setting end) as setting3
from table1 t1
left join table2 t2
on t1.id = t2.table_1_id
group by t1.id, t1.name;
See SQL Fiddle with Demo
But if you have an unknown number of values that you want to transform into columns, then you can use a prepared statement to generate dynamic sql.
The query would be similar to this:
SET #sql = NULL;
SELECT
GROUP_CONCAT(DISTINCT
CONCAT(
'max(case when t2.value = ''',
value,
''' then t2.setting end) AS `',
value, '`'
)
) INTO #sql
FROM table2;
SET #sql = CONCAT('SELECT t1.id,
t1.name, ', #sql, '
FROM table1 t1
left join table2 t2
on t1.id = t2.table_1_id
group by t1.id, t1.name');
PREPARE stmt FROM #sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
See SQL Fiddle with Demo
The result of both versions is:
| ID | NAME | SOMETHING | SOMETHING2 | SOMETHING3 |
---------------------------------------------------
| 1 | Mike | setting1 | setting2 | setting3 |

Answer 4

GROUP_CONCAT may be of use. It doesn't give you exactly what you want, because it would put the concatenated values into a single field. But depending on what you're actually trying to accomplish, perhaps you can work around that. The advantage of the GROUP_CONCAT is that it can handle any number of table2 rows per table1 row, whereas the conditional aggregation above hardwires having three entries (which may well be what you want).
SELECT table1.*,
GROUP_CONCAT(value) AS value_group,
GROUP_CONCAT(setting) AS setting_group
FROM table1
INNER JOIN table2
ON table2.table_1_id = table1.id
returns
id,person,value_group,setting_group
1,Mike,"something1,something2,something3","setting1,setting2,setting3"