Join multiple tables and search parameters - php

Question

I have database with 2 tables, 1 for storing customers Id and for customer informations. The second table is ordered with key/value because I need to store undefined values without needing to alter customers table.
Table structure
table customers:
=================
id | status
=================
table customers_info:
=======================================
id | id_customer | key | value
=======================================
Content example:
table customers:
=================
id | status
1 | 1
2 | 1
3 | 1
==================
table customers_info
=======================================
id | id_customer | key | value
1 | 1 | name| Doe
2 | 1 | age | 25
3 | 1 | city| NY
4 | 2 | name| Smith
5 | 2 | age | 26
6 | 3 | age | 30
=======================================
I simply query my tables with
SELECT ci.id_customer AS CustomerId,
MAX(CASE WHEN ci.key = 'name' THEN ci.value ELSE '' END) AS CustomerName,
MAX(CASE WHEN ci.key = 'age' THEN ci.value ELSE '' END) AS CustomerAge,
MAX(CASE WHEN ci.key = 'city' THEN ci.value ELSE '' END) AS CustomerCity
FROM customers_info ci
GROUP BY ci.id_customer
ORDER BY ci.id_customer;
I can get all customers, but cannot query with age for parameter for example, I tried try search every customer with age 25 but with this query I can't get all values for the customer...
SELECT ci.id_customer AS CustomerId,
MAX(CASE WHEN ci.key = 'name' THEN ci.value ELSE '' END) AS CustomerName,
MAX(CASE WHEN ci.key = 'age' THEN ci.value ELSE '' END) AS CustomerAge,
MAX(CASE WHEN ci.key = 'city' THEN ci.value ELSE '' END) AS CustomerCity
FROM customers_info ci
LEFT JOIN customers_info on customers_info.age = '25'
GROUP BY ci.id_customer
ORDER BY ci.id_customer;
I would like to get
=======================================
id_customer | name | age | city
1 | Doe | 25 | NY
=======================================

Answer 1

I am not sure that the way you are doing this the best way (I am not sure I would use maximums to do this, nor do I think I would create the tables in the way you are doing it - since in this case you are performing several extra operations in order to have the key value table that you are using) - but here is my go -
First, my create statements:
create database CustomerLinkQuestion;
use CustomerLinkQuestion;
create table customers (
id int,
status int
);
create table customer_info (
id int,
id_customer int,
k varchar(255),
v varchar(255)
);
For whatever reason (and this may be because I don't use MySQL all that much) key and value didn't work (probably reserved) so I used k and v for your key and value segments.
The core concept is that you are going to have to use a sub-query of some sort or a view (a view would be better) -
select * From (
SELECT ci.id_customer AS CustomerId,
MAX(CASE WHEN ci.k = 'name' THEN ci.v ELSE '' END) AS CustomerName,
MAX(CASE WHEN ci.k = 'age' THEN ci.v ELSE '' END) AS CustomerAge,
MAX(CASE WHEN ci.k = 'city' THEN ci.v ELSE '' END) AS CustomerCity
FROM customer_info ci
GROUP BY ci.id_customer
ORDER BY ci.id_customer) as sub
where sub.CustomerAge = 25
the inside (the part in parenthesis and called sub) is exactly what you posted (except - for whatever reason on my test server I used "customer" and not "customers"). Then the sub-query, then the where clause cutting down the sub-query.
Again - I would strongly suggest you look at your schema and design since it seems like you are trying to create a code table for something that it simply doesn't make sense. You will end up with a fixed number of fields in the customer table, each tied to a specific customer. When you are doing what you are doing here, you are performing more database operations that are simply not necessary.
Best of luck!
Aleksei