Question

I have a set of three tables:

 Dining_Tables; +--------------------+--------------+------+-----+---------+-------+ | Field              | Type         | Null | Key | Default | Extra | +--------------------+--------------+------+-----+---------+-------+ | dining_table       | int(11)      | NO   | PRI | NULL    |       | | bus_boy            | varchar(35)  | NO   |     | NULL    |       | | waiter             | varchar(35)  | NO   |     | NULL    |       | | server             | varchar(35)  | NO   |     | NULL    |       | +--------------------+--------------+------+-----+---------+-------+  Poker_Tables; +--------------------+--------------+------+-----+---------+-------+ | Field              | Type         | Null | Key | Default | Extra | +--------------------+--------------+------+-----+---------+-------+ | poker_table        | int(11)      | NO   | PRI | NULL    |       | | dealer             | varchar(35)  | NO   |     | NULL    |       | | pit_boss           | varchar(35)  | NO   |     | NULL    |       | +--------------------+--------------+------+-----+---------+-------+  Computer_Tables; +--------------------+--------------+------+-----+---------+-------+ | Field              | Type         | Null | Key | Default | Extra | +--------------------+--------------+------+-----+---------+-------+ | computer_table     | int(11)      | NO   | PRI | NULL    |       | | programmer         | varchar(35)  | NO   |     | NULL    |       | +--------------------+--------------+------+-----+---------+-------+ 

Each of these rows has globally unique table id associated with it: (dining_table, poker_table, computer_table) The other columns store the First/Last name of the person fulfilling the roll.

In my model, a single individual can be doing multiple things. For example, Joe Smith could be simultanously sitting at a computer_table as a programmer, sitting at a poker_table as a dealer, and waiting on a dining_table as a waiter.

My question is this: I want a query that will allow me to retrieve all the table_ids for a given person. Specifically, the query would then return a list of table_ids that Joe Smith is currently at.

I could do something along these lines:

 select dining_table from Dining_Tables where        bus_boy = 'Joe Smith' or       waiter = 'Joe Smith' or        server = 'Joe Smith';  select poker_table from Poker_Tables where        dealer = 'Joe Smith' or        pit_boss = 'Joe Smith';  select computer_table from Computer_Tables where        programmer = 'Joe Smith'; 

However, that’s three seperate queries and I’d really prefer to avoid doing that if possible. Can I do it in one query using joins?

Answer 1

If you want to return it in a single query you could use a UNION statement that would join those three queries into one.

select dining_table as Table from Dining_Tables where        bus_boy = 'Joe Smith' or       waiter = 'Joe Smith' or        server = 'Joe Smith' UNION select poker_table as Table from Poker_Tables where        dealer = 'Joe Smith' or        pit_boss = 'Joe Smith' UNION select computer_table as Table from Computer_Tables where        programmer = 'Joe Smith' 

Although it is three statements, it would be returned as a single result set.

If you separated your ‘jobs’ into a separate table, it would make querying a lot easier.

I could imagine something along the lines of:

Tables, TableTypes, Persons, Roles, TableRoles

TableTypes ID    TypeName ----------------------- 1     Dining 2     Poker 3     Computer  Tables ID    TableTypeID ---------------------------- 1     1 2     1 3     2 4     3 5     2 6     3  Persons ID    Name --------------------------- 1     Joe Smith 2     Mary Belcher 3     Norma Grey   Roles ID    RoleName ----------------------------- 1     Bus Boy 2     Waiter 3     Server 4     Dealer 5     Pit Boss 6     Programmer   TableRoles TableID    RoleID    PersonID ----------------------------- 1          1         1 //etc //etc 

This would make all of your querying very simple and straightforward and also makes it scalable (so you can add more roles to each table without mucking your database)