Question

I’ve got a table that looks like:

Table 1 ->

+----+--------+--------+
| id |  name  | author |
+----+--------+--------+
| 1  |  First |   Me   |
| 2  | Second |  You   |
+----+--------+--------+

Table 2 ->

+-----+------------+-----------+------------+
| mid |  table1_id |    key    |    value   |
+-----+------------+-----------+------------+
|  1  |      1     |   desc    |   hello    |
|  2  |      1     | begin_day |   monday   |
|  3  |      1     | end_day   |   tuesday  |
|  4  |      2     |   desc    |   goodbye  |
|  5  |      2     | begin_day |  wednesday |
|  6  |      2     | end_day   |  friday    |
+-----+------------+-----------+------------+

The relationship here is that the id in table 1 corresponds to the table1_id in table 2.

The output that I’m trying to get is…

+----+---------+---------+-------------+-----------+-----------+
| id |  name   |  author |     desc    | begin_day |  end_day  |
+----+---------+---------+-------------+-----------+-----------+
|  1 |  First  |   Me    |    hello    |   monday  |  tuesday  |
|  1 |  Second |   You   |   goodbye   | wednesday |  friday   |
+----+---------+---------+-------------+-----------+-----------+

I’ve tried several different join statements — all a variation of the below. I’m not that well versed in MySQL queries, however.

SELECT * FROM table_1 LEFT JOIN table_2 on table_1.id = table_2.table1_id

Which produces…

+----+----------+----------+----------+------------+-----------+
| id |    mid   |   name   |  author  |    key     |   value   |
+----+----------+----------+----------+------------+-----------+
| 1  |     1    |   First  |    Me    |    desc    |   hello   |
| 1  |     2    |   First  |    Me    |  begin_day |   monday  |
| 1  |     3    |   First  |    Me    |   end_day  |   tuesday |
| 2  |     4    |   Second |   You    |    desc    |  goodbye  |
| 2  |     5    |   Second |   You    |  begin_day |  wednesday|
| 2  |     6    |   Second |   You    |   end_day  |  friday   | 

Obviously, iterating over this join statement produces 6 results, 1 for each row in table 2 that matches the id in table 1. How can I avoid this with a proper query statement?

Thank you in advance.

Answer 1

You can use a case statement if you know all of the columns you will be getting, as follows:

Select distinct table_1.*, 
case when table_2.key='desc' then value end as desc, 
case when table_2.key='begin_day' then value end as begin_day, 
case when table_2.key='end_day' then value end as end_day
  FROM table_1 LEFT JOIN table_2 on table_1.id = table_2.table1_id 

Hope this helps!