Mysql table joins using a where clause - mysql

Question

I am trying to run a MySQL query against a database and join 3 tables together.
I have 3 tables in the database;
Table containing the important data
+----+-------+----------------------+
| id | name | value |
+----+-------+----------------------+
| 1 | data1 | First piece of data |
| 2 | data2 | Second piece of data |
| 3 | data3 | Third piece of data |
+----+-------+----------------------+
Table containing flag_id to flag_name
+----+-------+-------------+
| id | name | description |
+----+-------+-------------+
| 1 | flag1 | NULL |
| 2 | flag2 | NULL |
| 3 | flag3 | NULL |
| 4 | flag4 | NULL |
+----+-------+-------------+
Table containing the one-to-many data to flag mapping
+----+---------+--------+---------+
| id | type_id | status | data_id |
+----+---------+--------+---------+
| 1 | 1 | 0 | 1 |
| 2 | 2 | 0 | 1 |
| 3 | 4 | 1 | 1 |
| 4 | 2 | 0 | 2 |
| 5 | 3 | 0 | 2 |
| 6 | 4 | 1 | 2 |
| 7 | 3 | 0 | 3 |
| 8 | 4 | 1 | 3 |
+----+---------+--------+---------+
I want to be able to have a single query that will show me each named flag as a column for each piece of data. In searching the web, I found an example doing it using this query:
select d.id, d.name, d.value,
MAX(CASE WHEN f.type_id = 1 THEN f.status ELSE NULL END) as flag1,
MAX(CASE WHEN f.type_id = 2 THEN f.status ELSE NULL END) as flag2,
MAX(CASE WHEN f.type_id = 3 THEN f.status ELSE NULL END) as flag3,
MAX(CASE WHEN f.type_id = 4 THEN f.status ELSE NULL END) as flag4
from data d
inner join flags f on f.data_id = d.id
group by d.id
Which gives the result I want.
+----+-------+----------------------+-------+-------+-------+-------+
| id | name | value | flag1 | flag2 | flag3 | flag4 |
+----+-------+----------------------+-------+-------+-------+-------+
| 1 | data1 | First piece of data | 0 | 0 | NULL | 1 |
| 2 | data2 | Second piece of data | NULL | 0 | 0 | 1 |
| 3 | data3 | Third piece of data | NULL | NULL | 0 | 1 |
+----+-------+----------------------+-------+-------+-------+-------+
The problem is that I need to add a where clause to filter on specific flags, but I get the error that 'flag4' is an Unknown column, which is because an alias cannot be used in a where clause.
How do I accomplish this, preferably in a single query that I can use with a 'where' clause?

Answer 1

Instead of use where, use having.
The flag columns are the result of an aggregating operation, so, if you want to apply a selection criteria on them, you have to use having. Example:
select d.id, d.name, d.value,
MAX(CASE WHEN f.type_id = 1 THEN f.status ELSE NULL END) as flag1,
MAX(CASE WHEN f.type_id = 2 THEN f.status ELSE NULL END) as flag2,
MAX(CASE WHEN f.type_id = 3 THEN f.status ELSE NULL END) as flag3,
MAX(CASE WHEN f.type_id = 4 THEN f.status ELSE NULL END) as flag4
from data d
inner join flags f on f.data_id = d.id
group by d.id
havig flag1 = 0