Tricky SQL query... how to get all data in a single query?

Question

create table employee_definition (def_id number, def_name varchar(50));
insert into employee_definition values (100, 'EMAIL');
insert into employee_definition values (200, 'MOBILE_PHONE');
insert into employee_definition values (300, 'HOME_PHONE');
SQL> select * from employee_definition;
    DEF_ID DEF_NAME
       100 EMAIL
       200 MOBILE_PHONE
       300 HOME_PHONE
create table employee_data (def_id number, def_value varchar(20), emp_id number);
insert into employee_data values (100, '[email protected]', 123);
insert into employee_data values (200, '01232222', 123);
insert into employee_data values (300, '5555', 123);
insert into employee_data values (100, '[email protected]', 666);
insert into employee_data values (200, '888', 666);
insert into employee_data values (300, '999', 666);
insert into employee_data values (300, '444', 777);
SQL> select * from employee_data;
    DEF_ID DEF_VALUE                EMP_ID
       100 [email protected]              123
       200 01232222                    123
       300 5555                        123
       100 [email protected]              666
       200 888                         666
       300 999                         666
       300 999                         777
7 rows selected.I'm supposed to create a SQL that will return me the email, mobile_phone, and home_phone for a set of employees. The result will be something like this:
EMPLOYEE ID | HOME_PHONE | MOBILE_PHONE | EMAIL
123         |  5555  |    01232222      | [email protected]
666         |  999  |    888      | [email protected]
777         |  444  |    null     | nullThe thing I'm finding difficulty here is that the same column is used to store different values, based on the value in employee_definition table (something like a key/value pair). If I do:
SQL> select emp_id, def_value as email from employee_data, employee_definition
  2  where employee_data.def_id = employee_definition.def_id
  3  and employee_definition.def_name = 'EMAIL';
    EMP_ID EMAIL
       123 [email protected]
       666 [email protected]'s partially ok.. I'm just getting the definition for 'EMAIL'. But how can I get all the values in a single query, knowing that the column stores different values based on def_name?

Answer 1

Oh no, not again.
Entity attribute models always seem like a great idea to people who have been in the profession for five minutes and lack any kind of fundamental knowledge.
It staggers me that someone with 2,345 posts still believes "you need a 'detail table' for [storing multiple telephone numbers]"
"A person can have multiple telephone numbers" is not an excuse to build a tired person_attribute table. Niether is the bizarre proposal by someone with over 4k posts who should know better in an earlier post that EAV models are necessary to support temporal fidelity.
Taken to it's logical conclusion, EAV modelling leads to just two application tables. THINGS and THING_ATTRIBUTES. And when you consider that a THING_ATTRIBUTE is also a THING, why not roll those two tables up into one also? Hmmm, what does THINGS and THING_ATTRIBUTES look like? I know, TABLES and COLUMNS. Who would've guessed? SQL already provides the completely flexible extensible attribute model the advocates of EAV proscribe. But it also has data types, physical data independence, constraints and an efficient query language which EAV does not.
EAV modelling errodes the semantics of the attributes which are bundled into the "attribute" table.
There is no point in storing 12 different phone numbers with implied functional dependency to unconstrained and often repeating notional attributes like "MOBILE", "LANDLINE", "WORK", err, "WORK2", err, "MOBILE2", err, ... when this phone type attribute has no semantic value. When you want to call someone, you invariably want to retrive the prefered_phone_number which may depend on a time of day, or a call context.
These things need to be modelled properly (i.e normalised to BCNF) within the context of the database.